cell division -- topic 6

Growth

a permanent increase in the size of all living beings, accompanied by cell division and differentiation to form tissues and organs

growth occurs where in plants and animals

plants: tip of shoots and tip of roots

animal : throughout the body

cell cycle

the order of events that occurs between one cell division and the next

cell cycle consists of

1. interphase (G1 , S , G2) —> non dividing stage

2. Nuclear division (either mitosis or meiosis) —> separation of DNA into the nucleus of daughter cells

3. Cytoplasmic division of cytokinesis —> separation of organelles and cytoplasm to daughter cells

at interphase chromosomes appear as long thin threads called

chromatin. Each chromatin thread replicates other produce 2 identical chromatin threads joined at a point called the centromere

2 chromatin threads attached to the same centromere are called

sister chromatids

interphase —G1

• Longest

• Cell size , protein synthesis and energy increases

interphase — S

• DNA duplicated but chromosome number remains the same

interphase — G2

• shortest

• organelles replicate , protein synthesis and energy increases

• important to check for mistake / mistakes corrected at this stage

Mitosis

a nuclear division that produces daughter nuclei containing the same number of chromosomes as the parent nucleus. the daughter cells are genetically identical to the [arent cell

chromosomes are made up of

deoxyribonucleic acid (DNA)— stores all the information required to grow and carry out vital activities. The. information is stored as sections of DNA called genes

importance of mitosis

1. growth

2. repair of damaged tissues

3. replacement of old, worn out cells

4. asexual reproduction in plants

importance of genetically identical daughter cells

1. zygote divides to form embryo after fertilisation (the cells must be genetically identical for normal development)

2. an error during DNA replication = transmitted to daughter cells and affect the genes and proteins produced

3. replication errors could be because of uncontrolled division of cells (cancer)

mitosis 4 stages

1. prophase

2. metaphase

3. anaphase

4. telophase

5. cytokinesis

• telophase + cytokinesis occur at same time

prophase In mitosis

1. chromatin threads condense, coil and shorten to form chromosomes which is made of 2 sister chromatids attached to the centromere

2. in animal cells , pairs of centriole migrate to the opposite poles of the cell

3. the nucleolus disappears + nuclear envelope disintegrates

4. spindle fibres being to assemble and extend

metaphase — mitosis

1. chromosomes migrate to and line up singly along the equatorial/metaphase plate

2. the centromere of each chromosome is attached to spindle fibres

anaphase—mitosis

1. each centromere divide

2. the spindle fibres shorten and chromatids are pulled apart to opposite poles

3. sister chromatids that are separated are known as daughter chromosomes

telophase — mitosis

1. chromosomes uncoil and lengthen to form chromatin threads

2. spindle fibres disassemble

3. nuclear envelope reforms

4. nucleolus reapers

final stage of mitosis

cytokinesis—mitosis

division of cytoplasm

in animals : cleavage furrows appear in the cytoplasm between 2 nuclei and deepens to produce 2 daughter cells

in plants : a cell plate is formed to divide the 2 daughter cells instead

meiosis

reduction , nuclear division that produces haploid daughter nuclei containing single set of unpaired chromosomes and half the number of chromosomes as the parent nucleus

• occurs in reproductive organs

• contributes to genetic variation

meiosis notes

1. Begins with diploid cell 

2. Only produce gametes : genetically different haploid cells 

3. Has 2 cell division stages : meiosis 1 and 2

4.  Meiosis 1 produces 2 haploid cells each containing one chromosome form the homologous pairs 

5. Meiosis 2 separates sister chromatids to produce 4 haploid gametes

what happens to haploid male and female gametes

the haploid (n) male and female gametes will fuse with each other during fertilisation to produce a diploid (2n ) zygote

• haploid (n) male gamete : have only one set of chromosome (23)

• meiosis reduces chromosome number from 2n → n to produce haploid gametes. (goes backwards)

prophase I — meiosis

1. chromatin threads condense, coil and shorten to form chromosomes

2. each chromosome comprises two sister chromatids attached to the centromere

3. pairs of centriole migrate to the opposite poles of the cell

4. nucleolus disappears

5. the nuclear envelope disintegrates

6. spindle fibres begin to assemble and extend

1-6 : mitosis also have

additionally , in meiosis , homologous (have the same size and shape) chromosomes pair up . Corresponding alleles (version of a gene) between non sister chromatids are exchanged during crossing over

Metaphase I — meiosis

1. independent assortment occurs , where pairs of homologous chromosomes migrate to and line up randomly along the equatorial/metaphase plate (2 by 2)

2. the centromere of each homologous chromosome is attached to spindle fibres

anaphase I — meiosis

1. the shortening of spindle fibres causes each pair of homologous chromosomes to separate

2. each homologous chromosome is pulled to the opposite poles of the cell

In Anaphase I, sister chromatids stay together because only the homologous chromosomes separate. The centromeres do not split.

Telophase I — meiosis

1. chromosomes uncoil and lengthen to form chromatin threads

2. spindle fibres disassemble

3. nuclear envelope reforms

4. nucleolus reappears

Cytokinesis — meiosis

cleavage furrows appear in the cytoplasm between 2 nuclei + deepens and the two haploid daughter cells are produced

mitosis vs meiosis — mitosis

1. homologous chromosomes do not pair up in prophase

2. no crossing over in prophase

3. chromosomes align singly in metaphase

4. no independent assortment of chromosomes in metaphase

5. sister chromatids separate in anaphase

6. 2 genetically identical daughter cells are produced

7. daughter cells contain same number of chromosomes as that in parent cell

mitosis vs meiosis — meiosis

1. homologous chromosome pair up in prophase 1

2. crossing over in prophase 1

3. random assortment — pairs of homologous chromosomes align in metaphase 1

4. independent assortment of chromosomes in metaphase 1

5. homologous chromosomes separate in anaphase 1

6. 4 genetically different daughter cells are produced

7. daughter cells are haploid and contain half the chromosome number as that in the parent cell

difference between prophase I and prophase II in meiosis

• Similar : chromatin that contains 2 uncoiled spread out chromosomes one from each parent. Each chromosome binds with their corresponding homologous chromosome through synapsis forming tetrad (4 sister chromatids) . Nuclear membrane disappears , centriole move to opposite ends of the cell , spindle fibres bend out from centriole 

• However, Prophase II in meiosis occurs in haploid cell , no parking of homologous chromosomes and there is no crossing over involved.