Cell Cycle, Cell Division and Structure of Chromosomes | ICSE Class 10 Biology Notes

This note covers the ICSE Class 10 Biology topics Structure of Chromosome and Cell Cycle and Cell Division: chromatin, chromatids, the centromere, genes and the structure of DNA; the phases of the cell cycle; the stages of mitosis in animal and plant cells and how they differ; meiosis as a reduction division; homologous chromosomes; and the significance of, and differences between, mitosis and meiosis.

What is a chromosome made of?

The nucleus of a cell that is not dividing (the interphase nucleus) contains a loose and indistinct network of nucleoprotein fibres called chromatin.

The name was given by Flemming to the material of the nucleus that is stained by basic dyes. During cell division this chromatin condenses into distinct, structured bodies called chromosomes, which is why chromosomes are visible only in dividing cells.

Chromatin contains DNA, some basic proteins called histones, some non-histone proteins, and RNA. A single human cell has a thread of DNA about two metres long, distributed among its forty-six (twenty-three pairs of) chromosomes. This is far longer than the nucleus itself, so the DNA has to be packed very tightly.

How DNA is packed into a chromosome

1. Histones are positively charged proteins, rich in the basic amino acids lysine and arginine. Eight histone molecules form a unit called the histone octamer.
2. The negatively charged DNA is wrapped around the positively charged histone octamer to form a nucleosome. A typical nucleosome contains 200 base pairs of DNA.
3. Nucleosomes are the repeating units of chromatin. Under the electron microscope they appear as a "beads-on-string" structure.
4. The beads-on-string chromatin is packed into chromatin fibres, which are further coiled and condensed at the metaphase stage of cell division to form chromosomes. This higher-level packing needs non-histone chromosomal (NHC) proteins.

A large sphere labelled "core of histone molecules" (the histone octamer) with a DNA strand wrapped around it. An H1 histone is shown as a bar lying across the DNA where it enters and leaves the core. A companion electron micrograph shows chromatin as beads on a string.

Loosely packed chromatin that stains light is called euchromatin and is transcriptionally active. Densely packed chromatin that stains dark is heterochromatin and is inactive.

What are chromatids, the centromere and the types of chromosome?

By metaphase, each chromosome is made up of two identical sister chromatids, formed when the DNA replicated during interphase. The two chromatids are held together at a primary constriction called the centromere. On the sides of the centromere are small disc-shaped structures called kinetochores, which are the sites where spindle fibres attach during cell division.

Chromosomes are classified into four types by the position of the centromere:

Some chromosomes also have a non-staining secondary constriction at a constant location, which gives the appearance of a small fragment called the satellite.

Four chromosomes are drawn side by side, each with a centromere marked as an orange dot.

The first has the centromere in the middle, with a satellite on a secondary constriction at the top.

The next ones have the centromere progressively nearer one end, with the short arm and long arm labelled, until the last has the centromere almost at the tip.

Note: A chromatid is one half of a duplicated chromosome. Before the S phase a chromosome has one chromatid; after it, two sister chromatids joined at the centromere. They separate only at anaphase, when each becomes a daughter chromosome.

What is DNA, and how is a gene carried on it?

DNA (deoxyribonucleic acid) is the genetic material carried on chromosomes. It is a long polymer of units called nucleotides. Each nucleotide has three parts: a nitrogenous base, a pentose sugar (deoxyribose in DNA) and a phosphate group. The sugar and phosphate groups form the backbone of each chain.

DNA has four nitrogenous bases. Adenine (A) and guanine (G) are purines; cytosine (C) and thymine (T) are pyrimidines. In RNA, uracil takes the place of thymine. DNA was first identified as an acidic substance in the nucleus by Friedrich Meischer in 1869, who named it "nuclein".

The double helix

In 1953 James Watson and Francis Crick proposed the double helix model of DNA, based on X-ray diffraction data from Maurice Wilkins and Rosalind Franklin, and on Erwin Chargaff's observation that in double-stranded DNA the ratios of adenine to thymine and of guanine to cytosine are constant and equal one. Its main features are:

1. DNA is made of two polynucleotide chains. The sugar-phosphate backbone lies outside, and the bases project inside.
2. The two chains have anti-parallel polarity: if one runs 5'→3', the other runs 3'→5'.
3. The bases of the two strands pair through hydrogen bonds. Adenine pairs with thymine through two hydrogen bonds, and guanine with cytosine through three, so a purine always lies opposite a pyrimidine.
4. The two chains coil in a right-handed fashion. One turn of the helix (the pitch) is 3.4 nm long and holds about 10 base pairs, so adjacent base pairs are about 0.34 nm apart.
5. The planes of the base pairs stack over one another, which, together with the hydrogen bonds, keeps the helix stable.

Because A always pairs with T and G with C, the two strands are complementary: if the sequence of one strand is known, the other can be predicted. When each strand acts as a template for a new strand, the two daughter DNA molecules are identical to the parent.

A gene is the functional unit of inheritance. Genes are segments of DNA located on the chromosomes, and each carries the information needed to express a particular trait.

What are homologous chromosomes, and what do haploid and diploid mean?

Human body cells contain 46 chromosomes in 23 pairs. The two chromosomes of a pair are homologous chromosomes: one comes from the mother and one from the father, they are similar in length and in the position of the centromere, and they carry genes for the same characters at the same positions.

The two alleles of a gene pair lie at matching sites on the two homologous chromosomes.

A cell with both members of every pair has the diploid number of chromosomes, written 2n. A cell with only one member of each pair, as in a gamete, has the haploid number, written n.

In humans, 2n = 46 and n = 23. Fertilisation of two haploid gametes restores the diploid number in the zygote.

Pairing of homologous chromosomes happens only in meiosis, during the zygotene stage, and the process is called synapsis. It does not happen in mitosis, where each chromosome behaves independently.

What is the cell cycle, and what happens in interphase?

The cell cycle is the sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells.

Cell growth, in terms of cytoplasm, is continuous, but DNA is synthesised only during one specific stage. The cell cycle has two basic phases: interphase and the M phase (mitosis phase).

Human cells in culture divide about once every 24 hours, though the duration varies between organisms and cell types; yeast can complete a cycle in about 90 minutes. Of a human cell's 24-hour cycle, cell division proper lasts only about an hour, and interphase takes more than 95 per cent of the time.

Interphase is sometimes called the resting phase, but the cell is actively preparing for division through growth and DNA replication. For example, an onion root-tip cell with 16 chromosomes has 16 chromosomes at G₁, after the S phase and after M phase; only the DNA content changes, from 2C to 4C and back.

A circle divided into wedges. The largest wedge is G₁, followed by S and G₂, which together form interphase.

A smaller M phase wedge is split into prophase, metaphase, anaphase, telophase and cytokinesis, with a black outer band labelled M Phase. A red arrow on the rim shows the direction of the cycle.

Two daughter cells are drawn leaving the circle after cytokinesis, and one cell leaves the cycle at G₁ into G₀, shown by a circular arrow.

What happens in prophase and metaphase of mitosis?

Mitosis is called equational division because the parent and daughter cells have the same number of chromosomes. For convenience its nuclear division (karyokinesis) is divided into four stages: prophase, metaphase, anaphase and telophase. Cell division is a progressive process, though, and there are no clear-cut lines between the stages.

Prophase

• Chromosomal material condenses to form compact mitotic chromosomes, each seen to be made of two chromatids attached at the centromere.
• In animal cells, the centrosome, which duplicated during interphase, begins to move towards opposite poles. Each centrosome radiates microtubules called asters, and the two asters with the spindle fibres form the mitotic apparatus.
• By the end of prophase, the Golgi complex, endoplasmic reticulum, nucleolus and nuclear envelope are no longer seen.

Metaphase

• The complete disintegration of the nuclear envelope marks the start of metaphase, and the chromosomes spread through the cytoplasm.
• Condensation is complete, so this is the stage at which the morphology of chromosomes is most easily studied.
• Spindle fibres attach to the kinetochores. One chromatid of each chromosome is connected to fibres from one pole, and its sister to fibres from the opposite pole.
• All the chromosomes come to lie at the equator. This plane of alignment is the metaphase plate.

Early prophase shows a round nucleus with long, thin, tangled chromosomes. Late prophase shows shorter, thicker chromosomes, each with two chromatids.

Transition to metaphase shows the nuclear envelope gone and spindle fibres running from two poles to the chromosomes. Metaphase shows the chromosomes lined up across the middle of the cell, attached to fibres from both poles.

What happens in anaphase, telophase and cytokinesis?

Anaphase

At the onset of anaphase each chromosome at the metaphase plate splits, and the two daughter chromatids, now called daughter chromosomes, begin moving to opposite poles. The centromere of each leads towards the pole, with the arms trailing behind.

The key events are that centromeres split and chromatids separate, and the chromatids move to opposite poles.

Telophase

• The chromosomes reach their poles, decondense and lose their individuality as discrete elements.
• A nuclear envelope forms around the chromosome cluster at each pole, giving two daughter nuclei.
• The nucleolus, Golgi complex and endoplasmic reticulum reform.

Cytokinesis

Cytokinesis is the division of the cytoplasm that completes cell division. Organelles such as mitochondria and plastids are distributed between the two daughter cells. In some organisms karyokinesis is not followed by cytokinesis, which gives a multinucleate condition called a syncytium, as in the liquid endosperm of coconut.

Anaphase shows two groups of chromosomes being pulled towards opposite poles of an elongated cell.

Telophase shows a cell pinching in at the middle, with a new nucleus forming at each end. The last panel shows the two separate daughter cells in interphase.

How does mitosis differ in animal and plant cells?

The stages of mitosis are the same in both, but animal and plant cells differ in their structure. Animal cells have centrioles, which are absent in almost all plant cells, while plant cells have a relatively inextensible cell wall. These two facts explain most of the differences.

The cell plate is a simple precursor of the new cell wall. It represents the middle lamella between the walls of the two adjacent daughter cells. Plant cells need this method because a rigid cell wall cannot be pinched in by a furrow.

In animals, mitosis is seen only in diploid somatic cells, with a few exceptions such as male honey bees, whose haploid cells divide by mitosis. Plants can show mitosis in both haploid and diploid cells.

Why is mitosis significant?

Mitosis usually produces two daughter cells with the same number of chromosomes and an identical genetic complement to the parent. Its significance lies in the following:

1. Growth: the growth of multicellular organisms is due to mitosis. A single-celled zygote grows into a body of millions of cells by repeated mitotic divisions.
2. Nucleo-cytoplasmic ratio: cell growth disturbs the ratio between the nucleus and the cytoplasm, so the cell must divide to restore it.
3. Repair and replacement: the cells of the upper layer of the epidermis, the lining of the gut and blood cells are constantly replaced by mitosis, which also helps in cell repair after injury.
4. Continuous growth in plants: mitotic divisions in the apical and lateral meristems allow plants to grow throughout their life.
5. Genetic stability: since each daughter cell receives an exact copy of every chromosome, the chromosome number and genetic information stay constant in all the body cells.

Mitosis is also the basis of asexual reproduction in single-celled eukaryotes such as Amoeba, where one cell dividing into two produces two new individuals.

What is meiosis, and why is it called reduction division?

Sexual reproduction involves the fusion of two gametes, each with a complete haploid set of chromosomes. Gametes are formed from specialised diploid cells by a kind of division that halves the chromosome number. This is meiosis, the reduction division.

Meiosis produces the haploid phase in the life cycle of sexually reproducing organisms, and fertilisation restores the diploid phase. It occurs during gametogenesis in plants and animals.

Key features of meiosis

1. Meiosis has two sequential cycles of nuclear and cell division, meiosis I and meiosis II, but only one cycle of DNA replication.
2. Meiosis I begins after the chromosomes have replicated in the S phase to form identical sister chromatids.
3. Homologous chromosomes pair, and crossing over, the exchange of genetic material between non-sister chromatids of homologous chromosomes, takes place.
4. In meiosis I the homologous chromosomes separate, so each daughter cell gets only one chromosome of each pair. This is where the number is halved.
5. Meiosis II resembles a normal mitosis: the sister chromatids separate.
6. Four haploid cells are formed at the end of meiosis II.

Meiosis keeps the chromosome number of each species constant across generations, even though the process itself halves it. It also increases genetic variability in the population from one generation to the next, through crossing over and the independent assortment of chromosomes, and variation is very important for evolution.

How do mitosis and meiosis differ?

Note: Keep the two anaphases apart. In anaphase of mitosis the centromere splits and chromatids move apart. In anaphase I of meiosis the centromeres do not split: whole homologous chromosomes, each still with both its chromatids, move to opposite poles.

Taken together, mitosis keeps the number of chromosomes the same in every body cell, while meiosis halves it in the gametes so that fertilisation can restore it. Without meiosis the chromosome number would double in every generation.

Glossary

• Chromatin: The loose network of DNA, histones, non-histone proteins and RNA in the nucleus of a non-dividing cell.
• Nucleosome: The unit of chromatin formed by about 200 base pairs of DNA wrapped around a histone octamer.
• Centromere: The primary constriction of a chromosome, which holds its two sister chromatids together.
• Kinetochore: A disc-shaped structure on the side of the centromere, where spindle fibres attach during division.
• Gene: The functional unit of inheritance, a segment of DNA on a chromosome that carries information for a trait.
• Homologous chromosomes: A pair of similar chromosomes, one from each parent, that carry genes for the same characters.
• Interphase: The phase between two M phases in which the cell grows and replicates its DNA, divided into G₁, S and G₂.
• Karyokinesis: Division of the nucleus, which in mitosis has four stages: prophase, metaphase, anaphase and telophase.
• Cytokinesis: Division of the cytoplasm, by furrowing in animal cells and by a cell plate in plant cells.
• Metaphase plate: The plane at the equator of the spindle where the chromosomes line up during metaphase.
• Crossing over: The exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis I.
• Reduction division: Another name for meiosis, because it halves the chromosome number to form haploid cells.

Common errors and misconceptions

• Misconception: Interphase is a resting phase in which nothing happens. Correct: The cell grows and replicates its DNA during interphase, which takes more than 95 per cent of the cycle.
• Misconception: The chromosome number doubles in the S phase. Correct: The DNA content doubles from 2C to 4C, but the chromosome number stays 2n, since each chromosome now has two chromatids.
• Misconception: Chromatin and chromosomes are different substances. Correct: They are the same material. Chromatin is the loose form in a non-dividing nucleus; chromosomes are its condensed form during division.
• Misconception: Plant cells divide by forming a furrow. Correct: Plant cells have a rigid wall, so cytokinesis is by a cell plate growing from the centre outwards.
• Misconception: Homologous chromosomes pair during mitosis. Correct: Pairing (synapsis) happens only in prophase I of meiosis.
• Misconception: Meiosis replicates DNA twice. Correct: Meiosis has two divisions but only one round of DNA replication, before meiosis I.
• Misconception: Adenine pairs with guanine. Correct: Adenine pairs with thymine by two hydrogen bonds, and guanine with cytosine by three.

Exam-style questions with model answers

Q1. Name the stage of the cell cycle at which (a) DNA replicates, (b) chromosomes line up at the equator. [2 marks]

1. (a) DNA replication takes place in the S (synthesis) phase of interphase, when the DNA content doubles from 2C to 4C.
2. (b) The chromosomes line up at the equator, on the metaphase plate, during metaphase of mitosis.

Q2. What is a centromere? Name the chromosome in which it lies in the middle. [2 marks]

1. The centromere is the primary constriction of a chromosome, which holds its two sister chromatids together and bears the kinetochores to which spindle fibres attach.
2. A chromosome with the centromere in the middle, giving two equal arms, is a metacentric chromosome.

Q3. Give three differences between mitosis in an animal cell and in a plant cell. [3 marks]

1. Animal cells have centrioles that move to opposite poles and form asters; almost all plant cells lack centrioles, so no asters form.
2. In animal cells the spindle arises from the centrioles; in plant cells it forms without them.
3. Cytokinesis in animal cells is by a furrow in the plasma membrane that deepens and divides the cell; in plant cells a cell plate forms in the centre and grows outwards to the lateral walls.

Q4. State three features of the double helix model of DNA. [3 marks]

1. DNA has two polynucleotide chains with a sugar-phosphate backbone on the outside and the bases inside; the chains run in opposite directions (anti-parallel).
2. Adenine pairs with thymine through two hydrogen bonds and guanine with cytosine through three, so the strands are complementary.
3. The chains coil in a right-handed helix with a pitch of 3.4 nm and about 10 base pairs per turn.

Q5. Describe the stages of mitosis in an animal cell. [5 marks]

1. Prophase: chromatin condenses into chromosomes, each with two chromatids joined at the centromere. The duplicated centrosomes move to opposite poles and form asters. The nucleolus and nuclear envelope disappear.
2. Metaphase: spindle fibres attach to the kinetochores, and the chromosomes line up at the equator on the metaphase plate. Chromosome morphology is easiest to study now.
3. Anaphase: the centromeres split and the sister chromatids, now daughter chromosomes, move to opposite poles, centromere first.
4. Telophase: the chromosomes decondense at the poles, and a nuclear envelope, nucleolus, Golgi complex and endoplasmic reticulum reform, giving two daughter nuclei.
5. Cytokinesis: a furrow in the plasma membrane deepens and divides the cytoplasm, forming two daughter cells with the same chromosome number as the parent.

Q6. Why is meiosis called reduction division? Give four differences between mitosis and meiosis, and state its significance. [5 marks]

1. Meiosis is called reduction division because a diploid cell forms haploid cells: the separation of homologous chromosomes in meiosis I halves the chromosome number.
2. Mitosis has one division and gives two daughter cells; meiosis has two divisions, with one DNA replication, and gives four.
3. Mitosis keeps the chromosome number the same; meiosis halves it. Homologous chromosomes pair and cross over only in meiosis.
4. Mitotic daughter cells are genetically identical; meiotic daughter cells differ from the parent and from each other.
5. Meiosis keeps the chromosome number of a species constant across generations, since fertilisation restores the diploid number, and it increases genetic variation, which is important for evolution.

Key takeaways

• Chromatin is DNA wrapped around histone octamers as nucleosomes; it condenses into chromosomes only during cell division.
• A duplicated chromosome has two sister chromatids joined at the centromere, where kinetochores bind spindle fibres.
• Chromosomes are metacentric, sub-metacentric, acrocentric or telocentric according to where the centromere lies.
• DNA is a right-handed double helix of two anti-parallel chains with A paired to T and G paired to C; a gene is its functional unit of inheritance.
• The cell cycle is interphase (G₁, S, G₂) followed by the M phase; DNA doubles in S, but the chromosome number does not change.
• Mitosis has prophase, metaphase, anaphase and telophase, then cytokinesis by a furrow in animal cells or a cell plate in plant cells.
• Mitosis gives two identical diploid cells for growth and repair; meiosis gives four haploid, genetically varied cells for gametes.
• Homologous chromosomes pair and cross over only in meiosis I, and their separation in anaphase I halves the chromosome number, which is why meiosis is the reduction division.

Test yourself

How long does a typical human cell cycle take in culture?

About 24 hours, of which the actual cell division lasts only about an hour.

What is the G₀ stage?

A quiescent stage entered from G₁ by cells that stop dividing; they stay metabolically active but do not proliferate unless required.

At which stage is the morphology of chromosomes best studied, and why?

At metaphase, because condensation is complete and the chromosomes are spread out and lined up at the equator.

What is a cell plate?

The precursor of the new cell wall in a dividing plant cell; it forms in the centre and represents the middle lamella.

How many chromosomes does a human gamete have?

A human gamete is haploid and has 23 chromosomes, one from each homologous pair.

What is a syncytium? Give an example.

A multinucleate condition formed when nuclear division is not followed by cytokinesis, as in the liquid endosperm of coconut.

How many hydrogen bonds join guanine and cytosine?

Guanine and cytosine are joined by three hydrogen bonds; adenine and thymine are joined by two.

Why is variation produced by meiosis important?

Crossing over and the independent assortment of chromosomes create new gene combinations, and this variation is very important for evolution.