Chapter 7

Chromosomes and Cell division

Chromosomes, DNA and genes

Chromosomes are present in the nucleus. When the cell is at rest, the chromosomes are invisible( 不能見到 ). When the cell begins to divide, the chromosomes can absorb a special stain (Aceto-carmine or Aceto-orcein) and can be seen under microscope.  

Inside the chromosome are DNA (Deoxyribonucleic acid) and histones molecules. Each DNA molecule contains a number of genes which carry hereditory(遺傳 ) information

Homologous Chromosomes

It consists of

-Paternal chromosome: inherited from the male parent.

- Maternal chromosome: inherited from the female parent.

Which of the following statements is/ are true?

If the statement is false, try to amend the statement.Homologous chromosomes exit in a diploid cell. True

Homologous pair is separated during mitosis.

False! They are separated during meiosis.

X and Y chromosomes are a pair of homologous chromosomes.True

Homologous chromosomes have identical genotype.

False

In human body, sex chromosomes are found in gametes only.

False! All cells in the human body contain sex chromosomes.

The gene for insulin production can be found in the cells of pancreas only.

False! The gene can be found in all body cells. But the gene is expressed in the cells of pancreas, but suppressed in the other body cells.

1 2 3

4 5

Arrange the above stages in correct sequence.

1 2 3

4 5

Chromosome

When the cell is not dividing, the chromosomes are not visible.

At this stage, chromosome is called chromatin.

splitting

Chromatid

centromere

A pair of homologous chromosomes

Sister chromatids

centromere

A splitted chromosome

A pair of homologous chromosomes

Maternal chromosome

Paternal chromosome

Appearance of Chromosome when a cell is dividing

Cell division = Nuclear division + cytoplasmic cleavage

Two types of nuclear division: Mitosis and meiosis

MitosisMitosis

Mitosis consists of four phases:

Prophase

Metaphase

Anaphase

Telophase

Between two cell divisions, there is a phase called interphase.

Interphase

The chromatins are unidentified. During interphase, the following events occur:

[ DNA duplication takes place just before mitosis.

[Respiratory rate is very high for ATP synthesis. ATP is essential for the various stages of mitosis especially anaphase at which separation of sister chromatids occur.

Prophase

The chromosomes become demonstrable under staining technique. The chromatids become shorter, thicker and disentangled.

The nuclear membrane dissolves and the nucleoli disappear.

The two pairs of centrioles move to the opposite poles of the cell.

Each chromosomes are splitted up longitudinally into two thread like sister chromatids joining at the centromere. The sister chromatids have identical genotype.

Spindle of fibres are formed from the centrioles.

1

2

3 4

The chromosomes move to and line up along the equatorial region of the cell.

Metaphase

Anaphase

[1]The spindle fibres contract.

[2]The centromeres are splitted up.

[3]The sister chromatids are pulled by the shortening spindle fibres to the opposite poles of the cell.

Telophase

[1]The chromosomes become longer, thinner, entangled, fainter and finally lose their identity.

[2]The nuclear membranes are formed. Each encloses the separated chromatids. Thus, two new nuclei are formed.

It is followed by cytoplasmic cleavage in animal cell.

Cytoplasmic cleavage

Special features of cell division in plant cell

After telophase, a series of flattened vacuoles made by Golgi bodies, appear in the middle of the cell instead of cytoplasmic cleavage.

The membranes of these vacuoles fuse to form a cell plate, a double membrane unit.

This grows outward to join the plasma membrane.

Cell wall materials are laid down between the two membranes of the cell plate.

Significance of mitosis

[1]It enables replication of genetic materials so that the daughter cells have the same genotype as the parent cell. This results in genetic stability within population of cells.

[2]It takes place in somatic cells for production of new cells leading to growth, replacement of worn‑out tissues and regeneration.

[3]It is responsible for asexual reproduction of some species. e.g. Binary fission of Amoeba, vegetative propagation of angiosperms.

The mammalian and angiospermic parts where mitosis takes place are as follows:

[1]In mammalian body, with the exception of nerve cells, most somatic cells have the capacity of mitosis.

[2]In angiosperm, mitosis takes place at

(a) cambium: mainly for formation of secondary xylem and secondary phloem.

(b) cork cambium: for formation of secondary cortex.

(c) apical meristems in both shoot and root apexes: for growth in length of both parts.

 

MeiosisMeiosis

Meiosis consists of two consecutive divisions:

[1] Meiosis I (or 1st meiotic division)

[2] Meiosis II (or 2nd meiotic division)

Interphase exist with identical diagnostic features to those of mitosis.

The chromosomes become condensed, shorter and thicker and come into identity.

Pairing of homologous chromosomes forming bivalents.

Each chromosome is splitted longitudinally to form 2 sister chromatids joining at the centromeres.

Chromatids from homologous chromosomes may adhere at some points called chiasmata. This process is called crossing over, resulting in formation of non‑parental chromosomes.

The bivalents migrate to and line up along the equatorial plane of the cell.

Members of homologous chromosomes (i.e. non‑sister chromatids) move to opposite poles of the cell leaded by spindle fibres contraction. The manner of migration of paternal and maternal chromosomes are in random. This is known as independent assortment of maternal and paternal chromosomes. This results in formation of different variety of gametes which will be formed at the end of meiosis II. In anaphase I, sister chromatids of the same chromosomes do not separate and they go towards the same pole.

Anaphase II

It involves separation of sister chromatids.

Significance of meiosis

[1] Meiosis results in formation of haploid gametes, each of which contain half the chromosome number of the parent cells which are diploid. After fertilization with another haploid gamete, it gives rise to a diploid zygote, therefore restoring the chromosome number of the parent cell.

Therefore, meiosis ensures that the chromosome number remains constant throughout generations. 

[2] Variation occurs during meiosis due to

(a) Crossing over in prophase I results in formation of non-parental chromosomes.

(b) Independent assortment in parental and maternal chromosomes results in formation of a great variety of gametes.

Subjected to natural selection, disadvantageous variations are eliminated. The advantageous variations are adaptive to change in environment and is therefore able to survive and reproduce. Through a number of generations, a species which is completely different from the ancestor may form.

This is the basis of evolution.