CELL DIVISION

EXIST BTW DIVISIONS

EXIST AFTER DNA REPLICATION, BEFORE NUCLEAR DIVISION

• Each chromosomes consists of two identical DNA molecule
• Held at the centromere
• Each DNA molecule > sister chromatid (only referred to as this after undergone DNA replication)

• Specific proteins bind
• Form kinetochore complex
• Site of attachment of spindle fibres

contain units of inheritence (gene) arranged along it

specific seq. of nucleotides along a DNA molecule that codes for a specific seq. of A.A in a polypeptide chain

• found at particular position of chromosome
• location > locus (plural LOCI)

2 identical sister chromatid > 1 chromosome

2 chromosome > 1 homologous chromosome

• Chromosome pairs that are
• i) similiar in length, gene position, and centromere location
• ii) derived from separate parents
• iii) pair up during synapsis in prophase I form bivalents

occurence of homologous pair where one chromosome is inherited from each parent

Alleles

Ploidy

1 set of chromosomes

2 sets of chromosomes

More than 2 sets of chromosomes

Increase genetic variation - mixture of characteristics from both parents

Presence of backup - if a gene on 1 chromosome is faulty, the gene on 2nd chromosome can provide back up

• Interphase
• M phase

period of synthesis and growth

• G1 - intensive cellular synthesis
• S phase - semi conservative DNA replication
• G2 - cellular synthesis

Karyokinesis PMAT - nuclear division

Cytokinesis - cytoplasmic division with equal distribution of cytoplasm and organelles into each daugther cell

• Interphase
• Mitosis
• Cytokinesis

process of nuclear division in eukaryotic cells

• Prophase
• Metaphase
• Anaphase
• Telophase

• Conserves chromosome no.
• By equally allocating replicated chromosomes to each of the daughter nuclei

Intense synthesis and growth in cytolasm & nucleus

• G1 & G2
• growth and replication of organelles

• S phase - DNA REPLICATION
• loosely coiled threads of chromatin replicated via Semiconservative DNA replication

Longest stage in M phase

Condensation of chromatin into visible chromosomes > sister chromatids joined at centromere

Centrioles move to 2 opp. poles of cells

Aster (microtubules,radiate from centrioles)

Spindle fibres formed

Nuclear envelope and nucleolus disintegrate

Chromosomes aligh themselves singly along equatorial plane of cell, right angle to spindle axis

Spindle fibres attached to kinetochore complex on both sides of centromere

Pole to pole spindle fibres formed

Shortest stage where each centreomere divides into 2

Spindle fibres pull separated chromosomes to opp. poles of cell

Pole to pole spindle fibres lengthen push poles apart

chromosomes reach poles of cell uncoil and lengthen to form chromatin again

spindle fibres disintegrate

nuclear envelope reforms arnd chromatin at each pole

nucleoli reappear

cell membrane invaginates at equatorial plane, form cleavage furrow

cleavage furrow extends inwards until they meet, separate two cells

spindle fibres disappear except at equatorial plane (phragmoplast region)

organells attracted to region

golgi produces vesicles containing cellulose, fuse to form cell plate

cell plate extends outwards to parent wall at periphery, form new cell wall separate 2 daughter cells

Maintaining genetic stability - same hereditary info (genetically identical)

Growth in no. of cells - no. of cells increases by mitosis

Replacement of cells - cells are constantly dying and replaced by mitosis

Regeneration - by mitosis

Asexual Reproduction - mitosis is the basis of AR

Production of stem cells - allow for renewal and proliferation of stem cells while maintaing undifferentiated state

*CANCER - uncontrolled mitosis responsible for formation of tumours

• Interphase
• Meiosis
• Cytokinesis

Forming 4 daughter cells with half the number of chromosomes of the parent

• Single replication of DNA in parent cell
• Followed by 2 nuclear division and 2 cytokineses

PMAT

Reduction Division

Formation of gametes

• G1 and G2 growth and replication
• S phase DNA replication

Separation of homologous chromosomes

Chromatin condense to form visible chromosomes

Each chromosome > 2 sister chromatids

Homologous chromosomes pair up, form bivalents (synapsis)

Joined at several pts along their length (chiasmata)

Centrioles move to 2 opp. poles of cell

Spindle fibres formed

Nuclear envelope & nucleolus disintegrate

sites of exchange of genes btw non-sister chromatids of homologous chromosomes (crossing over)

Breakage and reunion btw 2 non-sister chromatids at each site

Allleles from 1 chromosome may swap with corresponding alleles from th other chromosome > lead to new combinations of alleles in resulting chromatids

Bivalents arrange themselves at equator of spindle

Spindle fibres attached to one side of kinetochore complex on centromere of each homologous chromosomes

Pole to pole spindle fibres formed

Centromeres do not divide (different from mitosis)

Spindle fibres pull homologous chromosomes, towards opp poles of cell

Homologous chromosomes separate into 2 haploid set (1 at each end of spindle)

Pole to pole spindle lengthen to push pole apart

Arrival of homologous chromosomes at opp poles (each still composed of 2 sister chromatids)

Crossing over causes sister chromatids to be not genetically identical

Chromatids uncoil, nuclear envelope reforms

Spindle fibres disintergrate

Nucleoli reappear

• Homologous chromosome separated
• Cleavage occurs

No further DNA replication occurs

REPLICATION OF CENTRIOLES OCCUR

Separation of sister chromatids (may not be identical anymore due to crossing over)

Chromatids shorten and thicken

Centrioles move to opp. poles of cells

Spindle fibres appear

Arranged at right angles to first spindle axis of meiosis I

Nucleoli and nuclear envelopes disintegrate

Chromosomes align themselves singly along equatorial plane

Spindle fibres attached to kinetochore complex on both sides of centromere

Pole to pole spindle fibres formed

Centromeres divide into 2

Spindle fibres pull chromatids to opp poles of cell

Pole to pole spindle fibres lengthen to push poles apart

Chromosomes uncoil lengthen decondense to form chromatin

Spindle fibre disintegrate

Nucleoli reappear

Nuclear envelopes reform arnd each nucleus

Half the no. of chromosomes of original parent cell

Subsequent cleavage produce 4 daughter cells

Reduction Division - no. of sets of chromosomes halved to haploid no.

Fertilization - fusion of 2 haploid gametes > diploid zygote

Meiosis occurs to form haploid gametes

Fusion of gametes will result in doubling of chromosomes if meiosis did not occur hence reduction division necessary prior to fertilization in sexual reproduction

• Gametes are formed - new combinations of alleles can occur via
• i) Crossing Over (prophase I)
• ii) Independent Assortment (metaphase I and II)

Crossing over segments of non sister chromatids occurs at chiasmata - leads to formation of new combinations of alleles on chromosomes of gametes - recombinant chromosomes that contains genes from both parents

• Bivalents orientate themselves at the equator of spindle RANDOMLY
• They line up independently of one another

Hence undergo independent separation during anaphase I

Different combinations of maternal and paternal chromosomes may result in different gametes being formed

• 2n = 46
• n = 23

Number of variations = 2ⁿ = 2²³

Orientation of pairs of chromatids is random and determines which chromosomes migrate to opp poles during ANAPHASE II

• Fusion of gametes completely random
• Random Fertilization - genetic variation