How a Very Long Thread of DNA Folds Into a Chromosome
Learn the parts of a chromosome — chromatin, chromatids, centromere and arms — and what it is made of, understand genes as stretches of DNA, follow the cell cycle through G1, S, G2 and M, and tell homologous chromosomes, haploid and diploid numbers apart with clear worked counts.
What carries the instructions that make you look like your parents?
Children often have their mother's eyes or their father's curly hair. The instructions for those features are passed on inside every cell, packed into tiny structures in the nucleus called chromosomes. Each chromosome is built around an extremely long molecule of DNA, and particular stretches of that DNA — the genes — carry the instructions for particular characteristics.
Most of the time, those instructions are spread out as a loose tangle in the nucleus. But when a cell is about to divide, the DNA coils up tightly and the chromosomes become visible under a microscope as distinct rod-shaped bodies. They must be copied exactly and shared out fairly, so that every new cell receives a complete set of instructions.
That copying and sharing follows a fixed routine called the cell cycle. A cell grows, copies its DNA, prepares, and then divides — and the new cells begin the cycle again.
This part covers:
- The structure of a chromosome — chromatin, chromatids, centromere and arms — and its chemical make-up
- Genes and DNA, and why DNA counts as the hereditary material
- The cell cycle — interphase with its G1, S and G2 phases, and the M phase
- Homologous chromosomes, and the difference between haploid and diploid numbers
Why this matters in everyday life. Every time a cut on your finger heals, a child grows taller, or hair and nails lengthen, cells are passing through the cell cycle. And DNA testing, used to establish family relationships or identify people, works because the DNA in chromosomes is inherited from parents.
One idea to carry into Part 2. The number of chromosomes in a species stays the same generation after generation. Part 2 shows how two kinds of cell division — mitosis and meiosis — achieve that, and this part supplies the vocabulary both depend on.
This page covers the first part of the ICSE Class 10 Biology chapter on the structure of chromosomes, cell cycle and cell division: chromosome structure, genes and DNA, the cell cycle, and haploid and diploid numbers.
Most of the time, those instructions are spread out as a loose tangle in the nucleus. But when a cell is about to divide, the DNA coils up tightly and the chromosomes become visible under a microscope as distinct rod-shaped bodies. They must be copied exactly and shared out fairly, so that every new cell receives a complete set of instructions.
That copying and sharing follows a fixed routine called the cell cycle. A cell grows, copies its DNA, prepares, and then divides — and the new cells begin the cycle again.
This part covers:
- The structure of a chromosome — chromatin, chromatids, centromere and arms — and its chemical make-up
- Genes and DNA, and why DNA counts as the hereditary material
- The cell cycle — interphase with its G1, S and G2 phases, and the M phase
- Homologous chromosomes, and the difference between haploid and diploid numbers
Why this matters in everyday life. Every time a cut on your finger heals, a child grows taller, or hair and nails lengthen, cells are passing through the cell cycle. And DNA testing, used to establish family relationships or identify people, works because the DNA in chromosomes is inherited from parents.
One idea to carry into Part 2. The number of chromosomes in a species stays the same generation after generation. Part 2 shows how two kinds of cell division — mitosis and meiosis — achieve that, and this part supplies the vocabulary both depend on.
This page covers the first part of the ICSE Class 10 Biology chapter on the structure of chromosomes, cell cycle and cell division: chromosome structure, genes and DNA, the cell cycle, and haploid and diploid numbers.
What is the structure of a chromosome, and what is it made of?
A chromosome is a thread-like structure in the nucleus made of DNA and proteins; when it has been copied, it consists of two identical sister chromatids joined at a constriction called the centromere, which divides it into two arms.
Chromatin. In a nucleus that is not dividing, the DNA and its proteins form a loose, fine network of threads called chromatin. When the cell prepares to divide, the chromatin coils and condenses into compact, visible chromosomes.
Parts of a chromosome, as seen during cell division:
- Chromatids — the two identical halves of a copied chromosome, lying side by side. Each is called a sister chromatid
- Centromere — the narrow constriction where the two sister chromatids are held together. Spindle fibres attach here during division
- Arms — the parts of the chromosome on either side of the centromere; their lengths depend on where the centromere lies
Chemical composition.
- DNA — deoxyribonucleic acid, the molecule that carries the genetic instructions
- Proteins — mainly histones, around which the DNA is wound
How the DNA is packed. The long DNA molecule is wrapped around groups of histone proteins, like thread around a series of beads. These beads are coiled and folded further, so that a very long molecule fits into a very small space.
Worked example — counting chromatids. A human body cell has chromosomes. How many chromatids are present before and after the DNA has been copied?
- Before copying: each chromosome is a single thread — chromatids
- After copying: each chromosome has two sister chromatids — chromatids
**The number of chromosomes is still , because sister chromatids joined at one centromere count as one chromosome.
An everyday analogy. Think of a long saree. Spread out, it is loose and hard to handle — like chromatin. Folded and pleated neatly, it becomes compact and easy to carry — like a chromosome. The cloth is the same; only its packing has changed.
The boundary case. Chromatin and chromosomes are not two different substances** — they are the same DNA and protein in two different states of packing. A question that asks where chromatin goes during division expects the answer that it condenses into chromosomes.
Chromatin. In a nucleus that is not dividing, the DNA and its proteins form a loose, fine network of threads called chromatin. When the cell prepares to divide, the chromatin coils and condenses into compact, visible chromosomes.
Parts of a chromosome, as seen during cell division:
- Chromatids — the two identical halves of a copied chromosome, lying side by side. Each is called a sister chromatid
- Centromere — the narrow constriction where the two sister chromatids are held together. Spindle fibres attach here during division
- Arms — the parts of the chromosome on either side of the centromere; their lengths depend on where the centromere lies
Chemical composition.
- DNA — deoxyribonucleic acid, the molecule that carries the genetic instructions
- Proteins — mainly histones, around which the DNA is wound
How the DNA is packed. The long DNA molecule is wrapped around groups of histone proteins, like thread around a series of beads. These beads are coiled and folded further, so that a very long molecule fits into a very small space.
Worked example — counting chromatids. A human body cell has chromosomes. How many chromatids are present before and after the DNA has been copied?
- Before copying: each chromosome is a single thread — chromatids
- After copying: each chromosome has two sister chromatids — chromatids
**The number of chromosomes is still , because sister chromatids joined at one centromere count as one chromosome.
An everyday analogy. Think of a long saree. Spread out, it is loose and hard to handle — like chromatin. Folded and pleated neatly, it becomes compact and easy to carry — like a chromosome. The cloth is the same; only its packing has changed.
The boundary case. Chromatin and chromosomes are not two different substances** — they are the same DNA and protein in two different states of packing. A question that asks where chromatin goes during division expects the answer that it condenses into chromosomes.
What is a gene, and how is it related to DNA on the chromosome?
A gene is a particular segment of DNA on a chromosome that carries the instructions for a specific characteristic, usually by coding for a protein, and genes are passed from parents to offspring because the DNA carrying them is copied and inherited.
DNA — the hereditary material. DNA is a double helix — two long strands twisted around each other like a spiral ladder.
- Each strand is a chain of units called nucleotides
- Each nucleotide contains a sugar (deoxyribose), a phosphate group and a nitrogenous base
- There are four bases: adenine (A), thymine (T), guanine (G) and cytosine (C)
- The two strands are held together by pairs of bases: A always pairs with T, and G always pairs with C
Genes. A gene is a segment of DNA with a particular sequence of bases.
- The sequence of bases in a gene is a code — the instructions for building a particular protein
- Proteins then shape characteristics, such as the pigment that colours the eyes or the enzymes that run the body's chemistry
- Each chromosome carries many genes, arranged one after another along its length
- The position of a gene on a chromosome is called its locus
Why DNA counts as the hereditary material:
- It carries the instructions for making proteins and so for characteristics
- It can be copied exactly before a cell divides, because each base pairs only with its partner
- It is passed on to daughter cells in division and to offspring through gametes
Worked example — using the base-pairing rule. In a segment of double-stranded DNA, of the bases are adenine. What percentage are thymine, guanine and cytosine?
Because A pairs only with T and G only with C, the amounts of each partner must be equal.
An everyday example. DNA tests used in courts and hospitals compare the sequences in a person's chromosomes with those of possible relatives. Because half of a child's chromosomes come from each parent, the DNA patterns match in a predictable way — which is why such tests can confirm family relationships.
Two boundary cases.
- Not all DNA consists of genes. Large stretches of the DNA in chromosomes lie between genes and do not code for proteins
- A gene is a part of DNA, and DNA is a part of a chromosome — the gene is the smallest of the three, and mixing up their order is a common error
DNA — the hereditary material. DNA is a double helix — two long strands twisted around each other like a spiral ladder.
- Each strand is a chain of units called nucleotides
- Each nucleotide contains a sugar (deoxyribose), a phosphate group and a nitrogenous base
- There are four bases: adenine (A), thymine (T), guanine (G) and cytosine (C)
- The two strands are held together by pairs of bases: A always pairs with T, and G always pairs with C
Genes. A gene is a segment of DNA with a particular sequence of bases.
- The sequence of bases in a gene is a code — the instructions for building a particular protein
- Proteins then shape characteristics, such as the pigment that colours the eyes or the enzymes that run the body's chemistry
- Each chromosome carries many genes, arranged one after another along its length
- The position of a gene on a chromosome is called its locus
Why DNA counts as the hereditary material:
- It carries the instructions for making proteins and so for characteristics
- It can be copied exactly before a cell divides, because each base pairs only with its partner
- It is passed on to daughter cells in division and to offspring through gametes
Worked example — using the base-pairing rule. In a segment of double-stranded DNA, of the bases are adenine. What percentage are thymine, guanine and cytosine?
Because A pairs only with T and G only with C, the amounts of each partner must be equal.
An everyday example. DNA tests used in courts and hospitals compare the sequences in a person's chromosomes with those of possible relatives. Because half of a child's chromosomes come from each parent, the DNA patterns match in a predictable way — which is why such tests can confirm family relationships.
Two boundary cases.
- Not all DNA consists of genes. Large stretches of the DNA in chromosomes lie between genes and do not code for proteins
- A gene is a part of DNA, and DNA is a part of a chromosome — the gene is the smallest of the three, and mixing up their order is a common error
What are the phases of the cell cycle, and what happens in the S phase?
The cell cycle is the series of events from one cell division to the next, made up of a long interphase — G1, S and G2 — followed by the short M phase of division, and in the S phase the DNA is copied.
1. Interphase — the period between two divisions, and the longest part of the cycle. It has three phases.
- G1 phase (first gap) — the cell grows, makes proteins and new organelles, and carries out its normal work
- S phase (synthesis) — the DNA is replicated: every chromosome is copied, so each now consists of two sister chromatids joined at the centromere
- G2 phase (second gap) — the cell grows further and prepares for division, making the proteins needed to build the spindle; in animal cells the centrioles have also been duplicated
2. M phase — the mitotic phase, when the cell actually divides.
- Karyokinesis — division of the nucleus, by mitosis
- Cytokinesis — division of the cytoplasm, forming two separate cells
The key event of the S phase. The amount of DNA doubles, but the number of chromosomes does not. Each chromosome simply gains a second, identical chromatid.
Worked example — DNA and chromosomes through the cycle. Suppose a cell in G1 has units of DNA and chromosomes.
- G1: DNA units; chromosomes
- After S: DNA units; chromosomes , each with two chromatids
- G2: DNA units; chromosomes
- After division: each daughter cell has DNA units and chromosomes
The cycle returns each new cell to exactly where the parent started.
How the time is shared. A typical human cell growing in a laboratory culture takes about a day to complete one cycle, and mitosis itself lasts only about an hour. Nearly all of the cycle is spent in interphase.
An everyday example. The cells lining your mouth and the lower layers of your skin are constantly passing through the cell cycle, replacing cells that are rubbed away. That is why a small cut on the inside of the cheek heals within days.
The misconception to correct. Interphase is not a resting phase. The cell is extremely busy — growing, making proteins and, above all, copying its entire DNA. It only looks quiet because the chromosomes are not yet visible.
The boundary case. Some cells leave the cycle and stop dividing, entering a non-dividing state often called G0. Most mature nerve cells stay in this state, which is one reason damaged nerves repair so poorly.
1. Interphase — the period between two divisions, and the longest part of the cycle. It has three phases.
- G1 phase (first gap) — the cell grows, makes proteins and new organelles, and carries out its normal work
- S phase (synthesis) — the DNA is replicated: every chromosome is copied, so each now consists of two sister chromatids joined at the centromere
- G2 phase (second gap) — the cell grows further and prepares for division, making the proteins needed to build the spindle; in animal cells the centrioles have also been duplicated
2. M phase — the mitotic phase, when the cell actually divides.
- Karyokinesis — division of the nucleus, by mitosis
- Cytokinesis — division of the cytoplasm, forming two separate cells
The key event of the S phase. The amount of DNA doubles, but the number of chromosomes does not. Each chromosome simply gains a second, identical chromatid.
Worked example — DNA and chromosomes through the cycle. Suppose a cell in G1 has units of DNA and chromosomes.
- G1: DNA units; chromosomes
- After S: DNA units; chromosomes , each with two chromatids
- G2: DNA units; chromosomes
- After division: each daughter cell has DNA units and chromosomes
The cycle returns each new cell to exactly where the parent started.
How the time is shared. A typical human cell growing in a laboratory culture takes about a day to complete one cycle, and mitosis itself lasts only about an hour. Nearly all of the cycle is spent in interphase.
An everyday example. The cells lining your mouth and the lower layers of your skin are constantly passing through the cell cycle, replacing cells that are rubbed away. That is why a small cut on the inside of the cheek heals within days.
The misconception to correct. Interphase is not a resting phase. The cell is extremely busy — growing, making proteins and, above all, copying its entire DNA. It only looks quiet because the chromosomes are not yet visible.
The boundary case. Some cells leave the cycle and stop dividing, entering a non-dividing state often called G0. Most mature nerve cells stay in this state, which is one reason damaged nerves repair so poorly.
What are homologous chromosomes, and how are haploid and diploid numbers different?
Homologous chromosomes are a matching pair — one from each parent — alike in size, shape and the genes they carry; a diploid cell has two sets of chromosomes (2n), while a haploid cell has one set (n).
Homologous chromosomes. A pair of chromosomes that are:
- Similar in length and shape
- Similar in the position of the centromere
- Carry genes for the same characteristics at the same positions
- One inherited from the mother and one from the father
The genes on a homologous pair are for the same characteristic but need not be identical — one chromosome may carry the instruction for brown eyes and its partner a different version.
Diploid number (2n). A cell with two complete sets of chromosomes — one set from each parent — is diploid.
- Found in: body cells, also called somatic cells
- Humans: , that is, pairs
Haploid number (n). A cell with one complete set of chromosomes is haploid.
- Found in: gametes — sperm and egg cells
- Humans:
Chromosome numbers in some other species:
- Pea: , so
- Onion: , so
How the numbers fit together at fertilisation:
Worked example 1 — from 2n to n. An animal's body cells have chromosomes. How many chromosomes are in its gametes, and how many homologous pairs are in its body cells?
Worked example 2 — sister chromatids versus homologous chromosomes. In a human body cell after the S phase, how many chromosomes, homologous pairs and chromatids are there?
- Chromosomes:
- Homologous pairs:
- Chromatids:
Sister chromatids are identical copies joined at one centromere; homologous chromosomes are two separate chromosomes, one from each parent.
Sex chromosomes. Of the pairs in humans, ** pairs are autosomes and pair is the sex chromosomes — XX in females and XY in males. The X and Y differ in size, so this pair is the one exception to the rule that homologues look alike.
An everyday example. Brothers and sisters from the same parents look similar but never identical. Each child receives one chromosome of every homologous pair from each parent, but which member of each parent's pair is passed on differs from child to child.
The boundary case. A haploid cell has one set, not half of every chromosome.** Each of its chromosomes is complete; it simply lacks the homologous partners that a diploid cell has.
Homologous chromosomes. A pair of chromosomes that are:
- Similar in length and shape
- Similar in the position of the centromere
- Carry genes for the same characteristics at the same positions
- One inherited from the mother and one from the father
The genes on a homologous pair are for the same characteristic but need not be identical — one chromosome may carry the instruction for brown eyes and its partner a different version.
Diploid number (2n). A cell with two complete sets of chromosomes — one set from each parent — is diploid.
- Found in: body cells, also called somatic cells
- Humans: , that is, pairs
Haploid number (n). A cell with one complete set of chromosomes is haploid.
- Found in: gametes — sperm and egg cells
- Humans:
Chromosome numbers in some other species:
- Pea: , so
- Onion: , so
How the numbers fit together at fertilisation:
Worked example 1 — from 2n to n. An animal's body cells have chromosomes. How many chromosomes are in its gametes, and how many homologous pairs are in its body cells?
Worked example 2 — sister chromatids versus homologous chromosomes. In a human body cell after the S phase, how many chromosomes, homologous pairs and chromatids are there?
- Chromosomes:
- Homologous pairs:
- Chromatids:
Sister chromatids are identical copies joined at one centromere; homologous chromosomes are two separate chromosomes, one from each parent.
Sex chromosomes. Of the pairs in humans, ** pairs are autosomes and pair is the sex chromosomes — XX in females and XY in males. The X and Y differ in size, so this pair is the one exception to the rule that homologues look alike.
An everyday example. Brothers and sisters from the same parents look similar but never identical. Each child receives one chromosome of every homologous pair from each parent, but which member of each parent's pair is passed on differs from child to child.
The boundary case. A haploid cell has one set, not half of every chromosome.** Each of its chromosomes is complete; it simply lacks the homologous partners that a diploid cell has.
Exam tip
What earns full marks on chromosome structure and the cell cycle?
Label every part of a chromosome diagram, keep chromosome number and DNA amount separate, and define each term with its exact distinguishing feature.
- Draw and label a chromosome with two chromatids, the centromere and both arms
- State the chemical composition: DNA and histone proteins
- Define chromatin as the uncoiled DNA–protein network of a non-dividing nucleus
- Define a gene as a segment of DNA carrying the code for a characteristic, and give its locus
- Describe DNA as a double helix with base pairs A–T and G–C
- List interphase phases in order: G1, S, G2, with one event for each
- State the key event of S phase: DNA replication, doubling the DNA but not the chromosome number
- Divide the M phase into karyokinesis and cytokinesis
- Define homologous chromosomes with all four features, including one from each parent
- Give human numbers: diploid , haploid
The misconception to name. Sister chromatids are not homologous chromosomes. Sister chromatids are identical copies made in the S phase and joined at one centromere; homologous chromosomes are separate chromosomes, one from each parent. Using the two terms interchangeably is the most common error in this chapter.
A second trap. Writing that the chromosome number doubles in the S phase. The DNA doubles; the chromosome number stays the same, because each copied chromosome is still one chromosome with two chromatids.
- Draw and label a chromosome with two chromatids, the centromere and both arms
- State the chemical composition: DNA and histone proteins
- Define chromatin as the uncoiled DNA–protein network of a non-dividing nucleus
- Define a gene as a segment of DNA carrying the code for a characteristic, and give its locus
- Describe DNA as a double helix with base pairs A–T and G–C
- List interphase phases in order: G1, S, G2, with one event for each
- State the key event of S phase: DNA replication, doubling the DNA but not the chromosome number
- Divide the M phase into karyokinesis and cytokinesis
- Define homologous chromosomes with all four features, including one from each parent
- Give human numbers: diploid , haploid
The misconception to name. Sister chromatids are not homologous chromosomes. Sister chromatids are identical copies made in the S phase and joined at one centromere; homologous chromosomes are separate chromosomes, one from each parent. Using the two terms interchangeably is the most common error in this chapter.
A second trap. Writing that the chromosome number doubles in the S phase. The DNA doubles; the chromosome number stays the same, because each copied chromosome is still one chromosome with two chromatids.
Did you know
How does roughly two metres of DNA fit inside a nucleus too small to see?
If all the DNA in the chromosomes of a single human cell were unwound and laid end to end, it would stretch roughly two metres — taller than most adults. Yet all of it fits inside a nucleus far too small to see without a microscope. The packing that makes this possible is one of the neatest pieces of engineering in biology.
The trick is coiling upon coiling.
- First, the DNA double helix wraps around small clusters of histone proteins, forming a chain that looks like beads on a string
- Next, that beaded string coils into a thicker fibre
- Then, the fibre loops and folds further
- During division, the loops pack so tightly that each chromosome becomes a compact rod visible under a microscope
Each stage shortens the DNA enormously, so the final chromosome is only a tiny fraction of the length of the DNA inside it.
Why the packing is not permanent. A gene can only be read when its DNA is unwound enough for the cell's machinery to reach it. So in interphase, when the cell is busy using its genes, much of the DNA is loosely packed as chromatin; during division, when the chromosomes must be moved without tangling, it is packed tightly. The cell loosens and tightens the coils as the job demands.
A useful comparison. Imagine trying to move a very long ball of wool from one room to another. Loose, it would snag and tangle on everything; wound tightly onto a spool, it can be carried easily. The cell winds its DNA into chromosomes for exactly that reason before dividing, and unwinds it afterwards so that the instructions can be read again.
And the proteins matter as much as the DNA. The histones are rich in positively charged building blocks, which hold on firmly to the negatively charged DNA — so chemistry, not just folding, keeps the packing stable.
The trick is coiling upon coiling.
- First, the DNA double helix wraps around small clusters of histone proteins, forming a chain that looks like beads on a string
- Next, that beaded string coils into a thicker fibre
- Then, the fibre loops and folds further
- During division, the loops pack so tightly that each chromosome becomes a compact rod visible under a microscope
Each stage shortens the DNA enormously, so the final chromosome is only a tiny fraction of the length of the DNA inside it.
Why the packing is not permanent. A gene can only be read when its DNA is unwound enough for the cell's machinery to reach it. So in interphase, when the cell is busy using its genes, much of the DNA is loosely packed as chromatin; during division, when the chromosomes must be moved without tangling, it is packed tightly. The cell loosens and tightens the coils as the job demands.
A useful comparison. Imagine trying to move a very long ball of wool from one room to another. Loose, it would snag and tangle on everything; wound tightly onto a spool, it can be carried easily. The cell winds its DNA into chromosomes for exactly that reason before dividing, and unwinds it afterwards so that the instructions can be read again.
And the proteins matter as much as the DNA. The histones are rich in positively charged building blocks, which hold on firmly to the negatively charged DNA — so chemistry, not just folding, keeps the packing stable.
Exam relevance
Why does NEET keep asking about chromosomes and the cell cycle?
This is foundation work for Class 11 Cell Cycle and Cell Division and Class 12 Molecular Basis of Inheritance and Principles of Inheritance and Variation — chapters central to NEET Biology.
Where the cell cycle leads. Class 11 Cell Cycle and Cell Division covers the phases of interphase and the M phase in detail, including G0. Questions on the event of each phase, and on the amount of DNA and number of chromosomes at each stage, are a recurring NEET type — exactly the worked example of DNA units through the cycle on this page, extended to more stages.
Where chromosome structure leads. Class 12 Molecular Basis of Inheritance describes DNA packaging — the nucleosome made of DNA wound around histone proteins, and the higher levels of folding. The beads-on-a-string picture introduced here is the starting point, and questions ask about the components and charges involved.
Where DNA structure leads. The same Class 12 chapter covers the double helix, base pairing and DNA replication. The base-pairing percentage calculation in this lesson is a standard NEET question in almost exactly that form.
Where homologous chromosomes and ploidy lead. Class 12 Principles of Inheritance and Variation explains inheritance through the behaviour of homologous chromosomes, and questions on chromosome numbers of gametes, zygotes and body cells — including sex chromosomes — appear frequently.
Question types to expect. At this level: labelled chromosome diagrams, definitions and phases. In NEET: statement-based questions on the cell cycle, counting chromosomes and DNA content at each stage, nucleosome structure, base-pairing calculations, and match-the-column items pairing phases with events.
The single trap that costs marks. Confusing DNA content with chromosome number. After the S phase, DNA content doubles while the chromosome number is unchanged, and NEET options are frequently built to catch candidates who double both.
A second trap. Treating interphase as a resting phase. It is the most active period of the cycle, and statement-based questions test this directly.
Board versus competitive emphasis. The ICSE paper marks labelled diagrams, precise definitions and the order of phases; NEET marks counts, statements and molecular detail. The transferable habit is tracking two numbers separately at every stage — chromosomes and DNA — because nearly every cell division question depends on keeping them apart.
Where the cell cycle leads. Class 11 Cell Cycle and Cell Division covers the phases of interphase and the M phase in detail, including G0. Questions on the event of each phase, and on the amount of DNA and number of chromosomes at each stage, are a recurring NEET type — exactly the worked example of DNA units through the cycle on this page, extended to more stages.
Where chromosome structure leads. Class 12 Molecular Basis of Inheritance describes DNA packaging — the nucleosome made of DNA wound around histone proteins, and the higher levels of folding. The beads-on-a-string picture introduced here is the starting point, and questions ask about the components and charges involved.
Where DNA structure leads. The same Class 12 chapter covers the double helix, base pairing and DNA replication. The base-pairing percentage calculation in this lesson is a standard NEET question in almost exactly that form.
Where homologous chromosomes and ploidy lead. Class 12 Principles of Inheritance and Variation explains inheritance through the behaviour of homologous chromosomes, and questions on chromosome numbers of gametes, zygotes and body cells — including sex chromosomes — appear frequently.
Question types to expect. At this level: labelled chromosome diagrams, definitions and phases. In NEET: statement-based questions on the cell cycle, counting chromosomes and DNA content at each stage, nucleosome structure, base-pairing calculations, and match-the-column items pairing phases with events.
The single trap that costs marks. Confusing DNA content with chromosome number. After the S phase, DNA content doubles while the chromosome number is unchanged, and NEET options are frequently built to catch candidates who double both.
A second trap. Treating interphase as a resting phase. It is the most active period of the cycle, and statement-based questions test this directly.
Board versus competitive emphasis. The ICSE paper marks labelled diagrams, precise definitions and the order of phases; NEET marks counts, statements and molecular detail. The transferable habit is tracking two numbers separately at every stage — chromosomes and DNA — because nearly every cell division question depends on keeping them apart.
Key takeaways
What must you be able to do from this part?
One structure, one hereditary molecule, one cycle and three numbers.
- Chromatin: loose DNA–protein network in a non-dividing nucleus; condenses into chromosomes before division
- Chromosome parts: two sister chromatids, joined at the centromere, with an arm on each side
- Chemical composition: DNA and histone proteins
- DNA is wound round histones like beads on a string, then coiled further
- DNA: double helix of nucleotides — sugar, phosphate and base; A pairs with T, G with C
- Gene: a segment of DNA coding for a characteristic, at a particular locus on a chromosome
- DNA is the hereditary material — it carries instructions, is copied exactly and is passed on
- ** adenine** means thymine and each of guanine and cytosine
- Cell cycle: interphase (G1, S, G2) then M phase (karyokinesis and cytokinesis)
- G1: growth; S: DNA replication; G2: preparation for division
- S phase doubles DNA but not chromosome number
- Interphase is not a resting phase; non-dividing cells such as nerve cells are in G0
- Homologous chromosomes: a matching pair, one from each parent, same size, shape, centromere position and gene positions
- Diploid (2n): two sets, in body cells — human ; haploid (n): one set, in gametes — human
- **After S phase a human cell has chromosomes, pairs and chromatids
- Humans have pairs of autosomes and one pair of sex chromosomes** — XX or XY
The sharpest self-test is one cell followed through a cycle. Start with a diploid cell of and write its chromosome number, chromatid number and relative DNA amount at G1, after S, at G2 and in each daughter cell — then check that the last row matches the first.
- Chromatin: loose DNA–protein network in a non-dividing nucleus; condenses into chromosomes before division
- Chromosome parts: two sister chromatids, joined at the centromere, with an arm on each side
- Chemical composition: DNA and histone proteins
- DNA is wound round histones like beads on a string, then coiled further
- DNA: double helix of nucleotides — sugar, phosphate and base; A pairs with T, G with C
- Gene: a segment of DNA coding for a characteristic, at a particular locus on a chromosome
- DNA is the hereditary material — it carries instructions, is copied exactly and is passed on
- ** adenine** means thymine and each of guanine and cytosine
- Cell cycle: interphase (G1, S, G2) then M phase (karyokinesis and cytokinesis)
- G1: growth; S: DNA replication; G2: preparation for division
- S phase doubles DNA but not chromosome number
- Interphase is not a resting phase; non-dividing cells such as nerve cells are in G0
- Homologous chromosomes: a matching pair, one from each parent, same size, shape, centromere position and gene positions
- Diploid (2n): two sets, in body cells — human ; haploid (n): one set, in gametes — human
- **After S phase a human cell has chromosomes, pairs and chromatids
- Humans have pairs of autosomes and one pair of sex chromosomes** — XX or XY
The sharpest self-test is one cell followed through a cycle. Start with a diploid cell of and write its chromosome number, chromatid number and relative DNA amount at G1, after S, at G2 and in each daughter cell — then check that the last row matches the first.