mitosis & meiosis lesson 3

Cells in living things do not last forever, for they…

• Wear out after some time• Get damaged (through cuts, by ultraviolet

radiation or by hazardous environmental pollutants)

• Grow old naturally and die

Importance of new cells produced are genetically identical to their parent cells:

• Continue with the specific cell functions of their parent cells within a particular tissue

• Avoid disrupting the stable internal environment of life or its processes

• Produce offspring that have the complete functions of an adult organism (in asexual reproduction) to ensure the survival of that species

Significance of mitosis

• Nucleus contains chromosomes.• Each chromosome consists of a long DNA

molecule which carries genes in a linear sequence

• Gene determines the individual characteristics of an organism

Significance of mitosis

• The characteristic number of chromosomes is referred to as the chromosomal number of the species

• Exp: Onion cell – 16 chromosomes• Exp: Fruit fly - 8 chromosomes

Human genetics = 46 chromosome (2n)23 pairs of chromosome

Significance of mitosis

• Somatic cells have two sets of chromosomes, one set inherited from each parent.

• Each cell contains a diploid number of chromosomes (2n)

• In humans, each set consist of 23 chromosomes.

• Typical human somatic cell, 46 chromosomes arranged in 23 pairs or 2n = 46

Significance of mitosis• The two

chromosomes in each pair have the same structural features and are referred to as homologous chromosomes

Significance of mitosis

• Gametes contain only one set of unpaired chromosomes, or haploid number of chromosomes (n)

Cell Cycle

Uncontrolled Mitosis in Living Things

• Cancer• Cancerous cell - tumour

Application of mitosis

• Cloning

Application of mitosis

Tissue culture

MEIOSIS

Human genetics = 46 chromosome (2n)23 pairs of chromosome

Stages of Meiosis

• Meiosis I– Prophase I– Metaphase I– Anaphase I– Telophase I

• Meiosis II– Prophase II– Metaphase II– Anaphase II– Telophase II

Prophase I

• The homologous chromosomes come together to form bivalents through synapsis process

Prophase I

• Each bivalent is visible under the microscope as a four-part structure called a tetrad

Prophase I

• A tetrad consists of two homologous chromosomes, each made up of two sister chromatids

Prophase I

• Non-sister chromatids exchange segments of DNA in a process known as crossing over. This results a new combination of genes on a chromosome

Prophase I

• The points a at which segments of chromatids cross over are called chiasmata.

Metaphase I

• Chromosomes are lined up side by side as tetrads on the metaphase plate. The chromosomes are still in homologous pairs

Metaphase I

• One chromosome of each pair is attached to the spindle fibre from one pole while its homologue is attached to the fibre from the opposite pole

Metaphase I

• The centromere does not divide

Anaphase I

• The spindle fibres pull the homologous chromosomes away from one another and move them to the opposite poles of the cell. Each chromosome still consist of two sister chromatids which move as a single unit.

Anaphase I

• Although the cell started with four chromosomes, only two chromosomes (each with two sister chromatids) move towards each pole.

Telophase I

• The chromosomes arrive at the poles. Each pole now has a haploid nucleus because it contains only one set of chromosomes

Telophase I

• The spindle fibres disappear. The nuclear membrane reappears to surround each set of chromosomes. The nucleolus then reappears in each nucleus

Prophase II

• The nuclear membranes of the daughter cells disintegrate again. The spindle fibres re-form in each daughter cell

Metaphase II

• The chromosomes, each still made up of sister chromatids, are positioned randomly on the metaphase plate with the sister chromatids of each chromosome pointing towards the opposite poles.

Metaphase II

• Each sister chromatid is attached to the spindle fibres at the centromere

Anaphase II

• The centromeres of the sister chromatids finally separate, and the sister chromatids of each chromosome are now individual chromosomes.

• The chromosomes move towards the opposite poles of the cell.

Telophase II

• Finally, the nucleoli and nuclear membranes re-form. The spindle fibres break down.

• Cytokinesis follows and four haploid daughter cells are formed, each containing half the number of chromosomes and is genetically different from the parent diploid cell.

Telophase II

• These haploid cells will develop into gametes.

Significance of Meiosis

(1) Crossing over in bivalent- Produces new combinations of genes in both

chromosomes

Significance of Meiosis

(2) Reduction and fusion of gametes- Meiosis produces haploid gametes- In sexual reproduction, a male gamete

fertilizes a haploid female gamete to produce a normal diploid zygote

Significance in Mitosis

(3) Independent (random assortment)- During metaphase I, homologous pairs of

chromosomes align at the equator- It is by chance which “way round” each pair

lies, before these homologous pairs of chromosomes separate into two different daughter cells.

APPRECIATING THE MOVEMENT OF CHROMOSOMES DURING MITOSIS AND MEIOSIS

Mutation

• Mutation is a change in structure, arrangement or quantity of the DNA in the chromosome

• May be caused by:– Mistakes in the replication of DNA– Damage to the DNA by radioactive and

carcinogenic substance– Disruption to the orderly movement of

chromosomes during cell division

In Mitosis

• If the functions of these genes are disrupted due to mutation, cancers may form.

• Somatic mutations are not transmitted to the offspring, but may cause body cells to malfunction

• Cancers are caused by somatic mutation

In Meiosis

• Meiosis involves an orderly movement and reduction (in meiosis I) of a diploid cell to two haploid cells that subsequently divide (in meiosis II) to form four haploid gametes

• Since these are gametes, so any mistakes – caused by disorderly movement of chromosomes during meiosis --- are inherited by the offspring.

Example: non-disjunction or improper segregation (separation) of chromosome

• During anaphase I, certain homologous chromosomes fail to segregate, resulting in the production of gametes with either an extra chromosome (n+1) or a missing chromosome (n-1)

• If this abnormal gametes unites with a normal gamete, an abnormal zygote will be produced.

Down’s syndrome or mongolism

• 3 copies of chromosomes number 21, instead of the normal 2 chromosomes

• This means a down syndrome patient has (2n+1 = 47) 47 chromosomes instead of the normal (2n=46) chromosomes