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?

A form of nuclear division that occurs in sexually reproducing organisms. Each parental cell produces 4 haploid genetically diff daughter nuclei via meiosis.

Meiosis is also known as

Reduction division. As daughter cells have half as many chromosomes as parents.

What is the difference between meiosis 1 and meiosis 2

M1: involves pairing of homologous chromosomes and their subsequent separation into 2 daughter cells (which reduces the chromosome number by half)

M2: involves separation of the 2 sister chromatids

What happens during meiosis prophase 1 (early)

Nucleolus behinds to disappear and nuclear envelope begins to disintegrate

Chromatin coils shortens and thickens into a condensed chromosome

**homologues (homologous chromosomes) PAIR UP via the process of SYNOPSIS to form BIVALENTS. Independent of spindle fibre)

Centrioles begin to migrate to opposite poles of the cell.

Spindle formation begins

What are homologues

Homologous chromosomes. One homologues (chromosome) comes from father and other from mother

Explain the difference between bivalent and tetrads

Bivalent: 2 homologous chromosomes (4 chromatids) formed during synapsis in early prophase 1

Tetrads: 4 chromatids within bivalent (each Tetrads 2 chromosomes each with 2 chromatids) formed in crossing over of late prophase 1

What occurs during late prophase 1 of meiosis?

Nucleolus disappear and nuclear envelope disintegrated

Centrioles continue to migrate to opposite poles of the cell

Spindle fibres begin to attach to kinetochores of each homologues

CROSSING OVER between non sister chromatids of homologous chromosomes

Explain what happens during crossing over between non sister chromatids of homologous chromosomes

At chiasmata (site where non sister chromatids of homologous chromosomes break and rejoin)

• exchange of equivalent portion of genetic material or alleles between non sister chromatids of homologous chromosomes.

• Results in new combination of alleles on chromosomes → diversity and variation

Crossing over occurs between…

Non sister chromatids of homologous chromosomes

Describe what happens during metaphase 1

Spindle completely formed, spindle fibre (KCMT) attach to centromere of each homologues

Tetrads/homologous pairs move to the metaphase plate with help of KCMT. Each homologues is attached from the pole it will be pulled towards in anaphase.

Independent assortment of homologous chromosomes and their occurs at this stage.

What occurs during meiosis anaphase 1?

Spindle fibres (non KCMT) elongate and slide in the opposite direction due to the actions of motor proteins. This cause the 2 poles to move further apart.

Homologues separate to opposite poles. Each homologues is pulled by a shortening KCMT (attaches to centromere) towards one of the poles.

What happens during telophase 1 of meiosis?

Nuclear envelope starts to reform around each group of chromosomes, nucleolus reforms.

Chromosomes each consisting of 2 sister chromatids reach opposite poles. Each pole has a haploid set of chromosomes. Chromosomes sometimes deco dense into chromatin but no replication of DNA takes place.

Spindle fires disintegrate

Cleavage furrow begins to form

At the end of meiosis 1, the nuclei are…

Haploid

Difference between meiosis 2 and meiosis 1

1. M2: sister chromatids held together at the centrosome vs M1: homologous pairs of chromosomes are held together at the chiasmata

2. M2: MT attach to individual kinetochores of the sister chromatids vs M1: MT attach to fused KC of the sister chromatids

3. M1: independent assortment of non homologous chromosomes occur

4. M2: random separation of sister chromatids occur

Difference between meiosis 2 and mitosis

1) ploidy of original cell: M2: haploid vs M: diploid

2) purpose: M2: create haploid gametes vs M: growth repair of asexual reproduction

3) number of cells produced M2 (total): 4 vs M: 2 diploid daughter cells

4) genetic identity: M2: genetic all identical (crossing over + random separation) vs M: genetic all identical

5) process: M2: part of sexual reproduction bs M: asexual reproduction/somatic cell division

significance of meiosis

1. Formation of haploid gametes in sexual reproduction: produce haploid gametes for sexual reproduction → restore ploidy level, ensures chromosome number in each species is kept constant every generation

2. Genetic variation: allows new combinations of alleles in gametes → genetic variation : 1) crossing over of non sister chromatids of homologous chromosomes during prophase 1 results in new combinations of alleles on chromatids. 2) independent assortment of homologous chromosomes: random arrangement of homologous chromosomes along metaphase plate at M1 3) random arrangement + separation of genetically no identical sister chromatids during M2 and A2 respectively

Why is variation important

• when environmental conditions change certain individuals in population will be better adapted to change than others, favourable characteristics that allow them to survive better

What are the 2 forms of chromosomal aberrations

1. Variation in chromosomal structure

2. Variation in chromosomal number

Explain variation in chromosomal structure

Alteration in several gene loci occurs by DDI T

Delete and duplication: occurs during CROSSING, non sister chromatids of homologous chromosomes may break and rejoin at incorrect places such that 1 chromatid may give up more genes than it receives

Inversion and translocation: result in DISEASE even though amount of chromosomal material REMAINS THE SAME. alters phenotype as gene expression can be influenced by its new location among neighbouring genes.

What are the 2 types of variation in chromosomal number

1. Aneuploidy

2. Polyploidy

What is anueploidy

Condition where the cell does not have a chromosome number that is a multiple of the haploid number. Chromosomes present in EXTRA or FEWER copies than in normal cells. (Triplicate=trisomic, missing 1=monosomic). Is a result of non-disjunction

What is non-disjunction

Failure of homologous chromosomes or sister chromatids to separate properly during nuclear division (mitosis/meiosis)

Explain the difference between non-disjunction in meiosis and mitosis

Meiosis: mitosis will subsequently transmit anormaly to all embryonic cells

Mitosis: aneuploid condition passed on to a large number of cells where the severity of effects is more pronounced

What is an example of aneuploidy

Down syndrome. Extra chromosome 21, non-disjunction during meiosis 1

What is polyploidy

Condition where there are 3 or more times this haploid number of chromosomes in nucleus. Results from 1) non disjunction of ENTIRE chromosome sets in mitosis or 2) non dysfunction of ENTIRE chromosome sets in meiosis

Why so some species gave odd sets of ploidy numbers?

Chromosomes cannot pair up as homologues during prophase 1 of meiosis and form visible gametes. Organisms may survive but are infertile unless they propagate vegetatively

What are the 4 principals of bioethics

1. Respect for autonomy/dignity of persons

2. Non Maleficent, minimising harm

3. Beneficence (maximising benefit)

4. Justice fairness

Formula of possible combinations of chromosomes in gametes

2^n (n is number of chromosome pairs)