ALL CELLULAR DIVISION

What does the cell cycle describe

The sequence of events which occurs between one cell division and the next. It also shows the time spent at each stage

The length of cell cycle depends on?

Organism, type of cell, temperature, water, food, oxygen supplies

Stages which make up the cell cycle

1. Interphase

   1. Contains G1, S and G2

2. Nuclear Division (Mitosis)

   1. Prophase, metaphase, anaphase and telophase

3. Cell division

   1. Leads to new daughter cells

Interphase

G1 (Gap1) Phase

• very metabolically active

• cell grows

• produces rna and ribosomes needed for protein synthesis

• produces new organelles

S (Synthesis) Phase

• DNA replication occurs

• histones are produced so newly synthesized DNA molecules can be wrapped around them 

• replication increases number of DNA molecules but NOT THE NUMBER OF CHROMSOMES

G2 (Gap 2) Phase

• mitochondria and chloroplasts replicate

• more protein synthesis

• Duplicated DNA is proofed and DNA damage is repaired

• centrosomes and spindle fibers are formed

Centrosomes

• non membrane bound regions located near nucleus - mitotic center

• initiate development of spindle fibres

• in animal cells they are made up of centrioles arranged at right angles and responsible for orientating spindle fibres during division

• centrosomes are made of microtubules

Spindle Fibres

• colchicine is used to arrest inhibit spindle formation arresting cells in metaphase stage

• made of microtubules

• form during mitosis and meiosis by centromere

Nuclear Division can happen through

1. M phase of the cell cycle (Mitosis)

2. Meiosis NOT PART OF THE CELL CYCLE 

What is mitosis and what does it produce

Mitosis is the mechanism used to divide a parent cell into two identical daughter cells

daughter cells are genetically identical to parent cell 

Where does mitosis occur

occurs in the m-phase of the cell cycle (takes up only 10% of cell cycle and occurs in all somatic cells 

Why does mitosis occur

• growth of an organism

• asexual reproduction

• regeneration of body parts 

• replacement/repair of damaged somatic cells 

Stages of Mitosis

• prophase

• metaphase

• anaphase

• telophase

Prophase

• DNA is condensed from chromatin into chromosomes

• nuclear membrane disintegrates

• nucleolus shrinks and disappears

• centrioles move to opposite poles of cell

Metaphase

• spindle fibres from each centriole attach to each sister chromatid at the kinetochore

• drag and line up the chromosomes to the middle/equator of cell

Anaphase 

• pull chromatids apart by shortening of spindle fibres 

• sister chromatids migrate to the opposite ends of the cell

• mitochondria supply energy needed for this phase

Telophase

• sister chromatids reach opposite poles

• uncoil and form chromatin

• spindle fibres disintegrate

• nuclear membranes form + nucleoli reappear

What is cytokinesis

• the division of the cytoplasm

Where does cytokinesis come in

• follows telophase and precedes the G1 phase of interphase

Cytokinesis in animal cells 

• cell membrane begins to invaginate at the equator during telophase

• microfilaments draw the cell surface membrane to form a furrow

• this pinches the cell into two at the cleavage furrow 

Cytokinesis in plant cells 

• sugar filled vesicles from the golig are lined up at the equator by microtubules. 

• collection of microtubules + vesicles is a phragmoplast (serves as a scaffolding) 

• vesicles fuse together to form the cell plate

• sugars remain in the space between membranes and form the middle lamella + new cell wall 

Preparation of a Temporary Microscopic Slide showing Mitosis

1. cut off 1cm of the growing root tip

2. transfer root tip to hot hcl and leave it to hydrolyze for 5mins to dissolve the middle lamellae

3. Wash + Dry

4. Transfer onto a clean microscope slide

5. Stain with toludine blue - and leave the stain to set for 2 minutes

6. Break up the tissues using a glass rod to spread it as thinly as possible

7. Blot off excess stain

8. Add a drop of water and cover with a coverslip and squash gently

9. View under the microscope

Meiosis

• the process by which a cell nucleus divides to produce daughter nuclei each containing half the number of chromosomes of the original nucleus 

meiosis is sometimes referred to as…

reduction division bcs it reduces the number of chromosomes

Prophase I

• genetic material condenses into chromosomes

• nuclear membrane disintegrates

• centrioles move to opposite poles of the cell 

• spindle fibres form from asters 

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• homologous chromosomes pair up to form bivalents - process called SYNAPSIS

• homologous chromosomes join at several points called chiasmata

• crossing over occurs at each chiasma where genetic material is exchanged between non-sister chromatids

Metaphase I

• centromere of chromosomes is attached to spindle fibres

• bivalents line up at the equator through spindle fibres

• one homolgous chromosomes faces 1 pole and the other faces the opposite one

  • THIS ALIGNMENT IS COMPLETELY RANDOM

Anaphase I

• homologous chromosomes are pulled apart by spindle fibres

• separates chromosomes into two haploid sets

• independent assortment occurs - what chromosomes go to which side is random

Telophase I and Cytokinesis

• nuclear membrane reforms + nucleolus

• homologous chromosomes reached the opposite poles

• spindle fibers disappear

• cleavage of cytoplasm may occur

Prophase II

• chromosomes condense

• centrioles duplicate and move to opposite poles of cell

• nuclear membrane disintegrates

• spindle fibres appear

• NO PAIRING OF CHROMOSOMES AND CHIASMATA DONT DEVELOP

Metaphase II 

• spindle fibres at each centriole attach to each sister chromatid 

• spindle fibres drag the sister chromatids and line them up at the center

• chromosomes are oriented at random with respect to one another 

Anaphase II

• sister chromatids are pulled apart by spindle fibres

• sister chromatids go to each side randomly

Telophase II and Cytokinesis

• Sister chromatids reach opposite poles

• spindle fibres disintegrate

• nuclear membrane reformes

• cleavage of cytolasm/cell walls in plants

• 4 NEW HALPOID DAUGHTER CELLS R PRODUCED ALL GENETICALLY DIFFERENT TO EACH OTHER

How does meiosis generate genetic variation 

• crossing over and synapsis 

• random alignment in M1 AND M2

• independent assortment in A1 and A2

Crossing over and Synapsis

alleles are exchanged at chiasmata

produces new combinations of genes on chromosomes

each time meiosis occurs chiasmata form at any point on the chromosomes - such that an infinite number of diff combination of genes can be produced

always equal amounts crossed over - chromosome stay the same length and have the same genes

Random Alignment in M1/M2

• which homologous chromosome is facing which side is purely up to chance

  • maternal or paternal could face either pole

• random orientation of sister chromatids

Independent assortment

• it is pure chance as to which chromosome or sister chromatid get’s into which gamete

• eg not all maternal chromosomes will go into 1 gamete, you could have 40% mom 60% dad - literally any combo 

Significance of Meiosis in Sexual Reproduction

• keeps the number of chromosomes constant which each generation

• if it was 2n - then the new gen would be 4n and the no of chromosomes would double w every generation

  • bcs mitosis keeps the number of chromosomes constant

Other sources of generation genetic variation 

• random fertilization

  • it is pure chance as to which egg fertilizes which sperm

  • since each gamete is unique a unique zygote is ALWAYS created

• mutations 

  • occur during dna replication/cell division 

  • responsible for changes in gene pool

  • driving force of evolution