9. mitosis and meiosis

cell cycle order

G1 (gap phase 1)

S (synthesise phase)

G2

M (mitosis phase: nuclear division + cytokinesis)

Which are the 3 main stages of cell cycle

Interphase, nuclear division, cytokinesis

Which is the longest phase in cell cycle

Interphase (G1,S,G2)

What happens during interphase

• cell produces many materials and organelles required for carrying out all its functions

• Cell replicates its DNA (S phase) to prepare nuclear division

What happens during nuclear division

Mitosis or meiosis

What happens during cytokinesis

Division of cytoplasmic contents into 2 daughter cells

What happens during G1

Intensive cellular synthesis

• organelle synthesis

• RNA synthesis

• Proteins synthesis

What happens during S phase

Semi conservative DNA replication (DNA in cell doubles)

What happens during G2 phase

Intensive cellular synthesis

• organelle synthesis

• Synthesise of spindle proteins

• Microtubules begin to form

What is the difference between G1 and G2

• G1 occurs after mitosis before S, G2 before mitosis after S

• G1 RNA synthesis and protein synthesis VS spindle proteins synthesis & microtubules begin to form

What happens by the end of interphase

• organelles duplicated

• DNA duplicated

• Nucleus still bound by nuclear envelope

• Single centrosome replicated to form 2 centrosomes (each 1 pair of centrioles in animals)

Are there centrioles in higher plant cells

No. Only microtubule organising Center

What are the 3 checkpoints in cell cycle

G1 checkpoint, G2 checkpoint, M (metaphase)/ SAC (spindle assembly checkpoint)

importance of cell cycle check points (TCM)

• provide sufficient TIME for cellular time for cellular activities to occur within a phase

• Enable cell to ensure important processes are COMPLETED PROPERLY

• PREVENT transmission of MUTATIONS to daughter cells

What form does chromosome take when it is not dividing

Chromatin; dispersed, uncondensed form as a ass of long thin thread like fibres

What is chromatin

Complex of DNA and histone proteins . DNA winds around an octamer formed by 8 histone proteins forming a nucleoside. (Chromatin made of many nucleosome slinked tgt)

How does chromatin become chromosome

Chromatin condensed by coiling/folding may times upon itself results in a chromosome which appears thicker shorter and more visible

Is chromatin visible on microscope?

Only heterochromatin. Euchromatin is not visible

When is chromosome found (in stages of mitosis and meiosis)

• late prophase

• Metaphase

• Anaphase

• Early telophase

(Everything except early of first stage and late of last stage)

What is a nucleosome

Formed when DNA wraps around an octamer of 8 histone proteins

When are chromosomes visible under a microscope

Cell division: Late prophase, metaphase, anaphase, early telophase (of Mitosis and Meiosis)

When does chromatin start to condense during cell cycle

DNA replication in interphase, start condensing in prophase and becomes fully compact in anaphase

What are sister chromatids

Identical copies of DNA molecule formed after DNA molecule

Are sister chromatids always identical

No. In meiosis, crossing over can occur making them non identical

What holds sister chromatids together

They are held together at centromere

1 chromatid =

1 DNA molecule

1 chromosome =

1 centrosomes

What is ploidy

The number of complete sets of chromosomes in a cell

What is a haploid

A nucleus/cell/organism that contains only 1 complete set of chromosomes (one copy of each chromosome)

What is the ploidy of gametes

Haploid

What is diploid

nucleus/cell/organism with 2 complete sets of chromosomes (2 copied of each chromosome, maternal or paternal)

Are somatic cells diploid of haploid

Diploid

What is the diploid number of humans

46 chromosomes

Describe fertilisation process using Ploidy terms

Fusion of haploid sperm and haploid egg during fertilisation results in the formation of diploid zygote

What are homologous chromosomes

Chromosomes having

1. Same size

2. Same shape

3. Centromere position

4. Staining position

5. Position of gene (gene loci)

What is an allele

Alternative forms of a gene, occupy same locus on a pair of chromosomes

A homologous chromosomes identical

No. Same genes at corresponding loci, diff alleles at same locus → occupy same loci so code for same hair characteristics e.g.

What is the definition of mitosis

A form of nuclear division in eukaryotic cells which produces two daughter nuclei containing identical sets of chromosomes as the parental cell nucleus/ begins after interphase, (during M phase) occurs before cytokinesis

What are the 4 stages of mitosis

Prophase (early+late)

Metaphase

Anaphase

Telophase

Describe what happens during early prophase

Things to talk about:

DNA + nucleus

Spindle fibre + centrioles + centrosome

(E stands for easier)

Chromatin: in the process of condensing into a chromosome

Nucleolus: Begins to disappear & nuclear envelope begins to disintegrate

Centrioles: move to opposite poles.

Spindle fibres form.

Describe what happens during late prophase

(P for P(b)ao le)

DNA: chromatin dully condensed in chromosome. Each chromosome appears as 2 sister chromatid joined together at centromere.

Nucleus: nucleolus gradually disappears, nuclear envelope disintegrates

Centromere: Centrioles migrate to opposite poles of cell (for M to occur)

Spindle: Mitotic spindle continues to form. Spindle fibres extend from each pole towards kinetochore & metaphase plate. Kinetochore microtubules attach to centromeres of the chromosomes + move chromosomes to required location

asters extend from centrosome

What is a centromere

Tandem repeat sequence on the region of chromosome that holds sister chromatids together.

What is kinetochore?

Specialised protein complex called kinetochore, located at centromere. Sites where spindle fibres attach.

What are asters

Radial arrays of shorter microtubules that extend from the centrosome

Describe what happens during metaphase

DNA: Chromosomes are aligned at the equator/metaphase plate in a single row.

Centrioles: located at poles of cell nucleus

Spindle completely formed. Kinetochore microtubules attached to centromeres of the chromosomes.

Describe what happens during Anaphase

(Anna separated from her sister Elsa)

DNA: daughter chromosomes led by their centrosomes resulting in the characteristic V shape of chromosomes see in anaphase → Separated sister chromatid (daughter chromosome) pulled to opposite poles by shortening spindle fibre/kinetochore microtubules

Centromere: divide (chromosome number doubles)

Spindle Fibre (non kinetochore MT): elongate and slide in opposite direction due to action of motor protein. 2 poles move further away

Name & Describe the differences between the the 2 spindle fibres in anaphase

Kinetochore Microtubules:

• shortened by daughter chromosomes

• Attach to kinetochore

• Pull sister chromatids apart towards opposite poles

• Pull chromosomes inwards

Non Kinetochore microtubules:

• elongate and slide in opposite direction due to motor protein

• Attach to MT from opposite pole, not chromosome

• Push poles apart to elongate the cell

• Pull spindle poles outwards

Describe what happens during telophase of mitosis

DNA: daughter chromosomes reach poles of cell, decondense and lengthen into chromatin. Chromosome appear diffused, not clearly visible

Nuclear membrane reforms around the chromatin at each pole and nucleolus reappears.

Spindle Fibres disintegrates

Cleavage furrow (animal)/Cell plate (plant) start to form

Cytokinesis (after mitosis)

Decides cytoplasm of parental cell into 2 daughter cells

Describe the differences between Cytokenesis in animal and plant cells

Animal: CSM invaginate towards (previously metaphase), form cleavage furrow due to contracting microfilaments. Deepens until parent cell pinched into 2, produce 2 cells.

Plant (higher plant cell): series of vesicles from GA containing (pectin) move to MP and fuse to form cell plate/ membranes of vesicles form CSM of daughter cells. Cell plate fuses with parent cell wall and CSM, separating

Why is mitosis important? (GGRRA)

1. Maintaining genetic stability (chromomes in 2 daughter cells genetically identical→ does not introduce genetic variation, maintain genetic stability.) (will not result in rejection by body immune system, recognise self vs non self cells)

2. Growth (increase in number of cells or size of cells)

3. Regeneration and cell replacement (damaged cells replaced by cells that are genetically identical to original cells)

4. Asexual reproduction (single parent produces offspring genetically identical to self)

What is meiosis?