MEIOSIS

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Introduction

Asexual reproduction:• offspring are

genetically identical • involves:

– mitosis (in eukaryotes)

– binary fission (in prokaryotes)

Sexual reproduction:• offspring are

genetically different• parents produce

gametes or sex cells • fuse during

fertilisation: – bring characteristics

from each parent

Living organisms reproduce. Reproduction may be: asexual (without sex) sexual (with sex)

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Q. What do the terms haploid and diploid mean?

• haploid • diploid

Q. Which of the cells are haploid?

Q. Which of the cells are diploid?

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Meiosis

Egg n=23

Sperm n=23

Meiosis

Fusion of gametes

Zygote n=46

Mitosis leading to growth

Mitosis leading to growth

Mitosis

Q. What do the terms haploid and diploid mean?• haploid

– contains a single set of chromosomes – in humans, n=23

• diploid– contains two sets of chromosomes – in humans, 2n = 46

Q. Which of the cells are haploid?– the sex cells or gametes – sperm and egg

Q. Which of the cells are diploid? – all of the cells that are not gametes! – zygote, embryo and body cells

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• Meiosis halves the chromosome number in the gametes so that when fertilisation happens, the normal diploid number is restored. KEY POINT Without meiosis the chromosome number would double in successive generations.

• Meiosis also has a key role introducing genetic variation into gametes and so into the offspring of sexually reproducing organisms.

• Variation is also introduced by the mixing of genetic information from the two parents and from mutation.

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Q. Why is genetic variation so important?

• Natural selection operates on variation.

• Some variants are adapted and so survive.

• Without variation, evolution would not occur.

• Changing conditions might result in extinctions of poorly adapted organisms.

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Mitosis

• In the Cells Exchange and Transport Unit you studied cell division (AS)

• There are two types of cell division: – mitosis and meiosis

• However, you will only have studied mitosis in any depth.

• You may still need to know mitosis for your A2 exam!

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• Mitosis is a continuous sequence, but is divided into four stages:1. prophase2. metaphase3. anaphase4. telophase

• Essentially:– chromatids are separated by contraction of

spindle fibres – chromatids are pulled to opposite poles of the cell– the cell then divides

• Each chromatid contains identical genetic information so each daughter cell also contains identical information.

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Early Prophase During prophase the

chromosomes become more distinct: they coil up shorten thicken take up stain more

intensely condense

The centriole divides. Nucleolus becomes less

prominent

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Late Prophase • The chromosomes have

become more distinct and are seen to consist of two chromatids joined by a centromere.

• The centrioles migrate to opposite poles of the cell.

• The nucleolus continues to shrink and disappears.

• The nuclear envelope disintegrates.

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Metaphase

Each centriole is at a pole.

Centrioles grow/produce spindle fibres.

Spindle fibres attach to the centromere of the chromosomes.

Each centromere is attached to both poles.

Chromosomes pulled to the metaphase plate or equator.

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Anaphase Spindle fibres

contract: the centromere

divides chromatids

(daughter chromosomes) are pulled to opposite poles of the cell

Pulled centromere first.

Each half of the cell receives one chromatid from each chromosome.

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Telophase Chromatids reach the

poles of the spindle: they begin to uncoil they become less

distinct Nuclear envelope starts

to reform.Q. What are the chromatids known as when they reach the poles of the spindle?

Daughter chromosomes

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Cytokinesis The cell divides! In animal cells:

starts by constriction from the edges of the cell (invagination)

In plant cells: a cell wall is laid down

Daughter cells have the same chromosome number and genetic make-up as each other and the parent cell – DNA replication precedes mitosis.

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Q. Why is mitosis biologically important?• During mitosis two genetically identical

daughter cells are produced whose nuclei contain the same number of chromosomes as the parent cell. Allowing:1. growth

All cells of the organism are genetically identical.

2. repairReplacement cells are the same as those they are replacing (genuine parts capable of doing an identical job vs cheap imitation that gives out quickly!).

3. asexual reproductionOffspring are identical and able to colonise quickly.

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Mitosis – The following slides show stages in Allium

Prophase (early)• Chromosomes coil and

condense.• Nuclear envelope is present.• Nucleolus is evident.

Prophase (late) Chromosome clearly visible as

two chromatids joined at the centromere.

Nuclear envelope disappears. Nucleolus disappears.

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Metaphase• Spindle forms – some fibres

attach to the centromeres, others run from pole to pole.

• Chromosomes are pulled to the equator of the cell (metaphase plate) by contraction of the fibres.

• Centromere splits and cell then enters anaphase.

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Anaphase• Chromatids move to

opposite poles of the cell.• They are pulled

centromere first by the contracting spindle fibres.

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Telophase• Chromatids (now often

called daughter chromosomes) reach the poles of the spindle.

• Nuclear envelope reforms.• Nucleolus reforms.• Cell moves into cytokinesis

or cell division.

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Cytokinesis• As shown here, in plant

cells a cell wall is laid down in the position of the metaphase plate.

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Interphase• This stage comes between

successive cell divisions. It is not really part of mitosis, but mitosis couldn’t happen without it.

• DNA replication occurs (allowing for the double-stranded chromosome which later divides).

• Cellular structures are made (subsequently divided between the two daughter cells).

• A significant proportion of time is spent checking genetic information.

Mitosis in Allium

Telophase

Prophase

Interphase

Metaphase

Anaphase

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Mitosis – The following slides show stages in whitefish

Prophase• Chromosomes coil and condense.• Nuclear envelope is present.• Nucleolus is evident.• The centriole

replicates/separates.

In late prophase Chromosome seen as two chromatids

joined at the centromere. Nuclear envelope disappears. Nucleolus disappears.

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Metaphase• The centrioles reach poles

and produce spindle fibres.• Spindle forms – some fibres

attach to the centromeres, others run from pole to pole.

• Chromosomes are pulled to the equator of the cell (metaphase plate) by contraction of the fibres.

• So-called asters are visible• Centromere splits and cell

then enters anaphase.

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Anaphase• Chromatids move to

opposite poles of the cell.• They are pulled

centromere first by the contracting spindle fibres.

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Telophase• Chromatids (now often

called daughter chromosomes) reach the poles of the spindle.

• Nuclear envelope reforms.• Nucleolus reforms.• Cell moves into cytokinesis

or cell division.

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Cytokinesis• The cell membrane

invaginates or constricts. • This eventually pinches

the cell into two.

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…Mitosis

Interphase• This stage comes between

successive cell divisions. It is not really part of mitosis, but mitosis couldn’t happen without it.

• DNA replication occurs.• Cellular structures are made. • A significant proportion of

time is spent checking genetic information.

• As seen here cytokinesis merges into interphase with chromosomes disappearing.

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Mitosis in Whitefish

Metaphase

Telophase –Cytokinesis

Anaphase

Prophase

Cytokinesis – Interphase

Meiosis

• Meiosis involves two divisions: • During Meiosis I:

– genetic variation is introduced • ‘crossing over’ • ‘independent assortment’

– chromosome number is halved

• During Meiosis II: – chromosomes (pairs of chromatids) are

split– four haploid cells are produced

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Early Prophase I

Homologous chromosomes pair up: pairing is called synapsis paired chromosomes are called

bivalents or tetrads

The centriole divides and starts migrating to opposite poles of the cell.

Nucleolus is present, but less prominent.

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Late Prophase I

• Crossing over (chiasma formation) may occur:– especially in the longer

chromosomes– adjacent chromatids can

break and reconnect with another chromatid

– introduces genetic variation

• The nuclear envelope and the nucleolus disappear.

• At the end of Prophase I the spindle is formed.

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Metaphase I Bivalents (tetrads)

line up at the metaphase plate.

Each bivalent is attached to a spindle fibre from opposite poles of the cell: like in a tug of war, the

paired chromosomes are pulled to the metaphase plate or equator

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Anaphase I

Contraction of spindle fibres separate whole chromosomes. Centromeres do not

divide.

Note: orientation of one

bivalent is independent of the orientation of other bivalents

during anaphase either the maternal or paternal chromosome can pass into either cell

2n possible combinations (where n = haploid number)

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Telophase I

Chromosomes reach the poles of the spindle.

Following telophase: animal cells:

usually divide nucleolus and

nuclear envelope reform

cytokinesis follows plant cells:

often go straight into the second meiotic division

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Prophase II and Metaphase II

The centriole divides, nucleolus and nuclear envelope disappear.

Spindle fibres attach to the centromeres: one to each pole pull chromosome to the

metaphase plate or equator of the cell

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Anaphase II The centromere divides:

contraction of spindle fibres pull the chromatids to opposite poles of the cell

the chromatids are now sometimes called daughter chromosomes

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Telophase II The daughter

chromosomes: reach the opposite poles of the

spindle start to decondense nuclear envelope and nucleolus

reform

Cytokinesis follows and results in the formation of four haploid daughter cells.

Note: half the chromosome number variation due to chiasma/cross-

overs

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What are the key markers for each of the stages of meiosis?

Prophase I

Metaphase I

Anaphase I

Telophase I

Prophase II

Metaphase II

Anaphase II

Telophase II

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Prophase IHomologous chromosomes pair (bivalents present); chiasma (cross-overs) present; nucleolus disappears; envelope disintegrates. Centrioles.

Metaphase IBivalents line up at equator; chromosomes attached to only one pole; behaviour is independent.

Anaphase IHomologous chromosomes separate; centromeres intact; chromosome number halved.

Telophase IChromosomes arrive at poles. De-condense, nucleus reforms (detail)

Prophase II Chromosomes condense, envelope disappears.

Metaphase II Chromosomes at equator, attached to each pole.

Anaphase II Chromatids separate; centromeres break

Telophase IIFour haploid cells produces by cytokinesis. Chromosomes de-condense, nucleus reforms (detail)

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What are the differences between mitosis and meiosis?

Mitosis Meiosis

Prophase:Chromosome pairing

Prophase:Chiasma/cross-over

Attachment of spindle fibres

Number of divisions

Cells produced

Role

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Mitosis Meiosis

Prophase:Chromosome pairing

Chromosomes unpaired

Pairing to form bivalents or tetrads

Prophase:Chiasma/cross-over

No Yes, exchanges DNA between chromosomes

Attachment of spindle fibres

Each centromere attached to each pole

Meiosis I: centromere attached to one pole.Meiosis II: centromere attached to both poles.

Number of nuclear divisions

One Two

Cells produced Two diploid (2n) cells Four haploid (n) cells

Role Growth, repair, asexual reproduction. Daughter cells identical.

Gamete production. Daughter cells different – variation and half chromosome number.

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Meiosis in a Nutshell• During meiosis dissimilar daughter cells are

produced. The nuclei contain half the number of chromosomes as the parent cell.

• This halving prevents the chromosome number doubling in each generation of organisms during sexual reproduction.

• Thus in humans: – somatic (body) cells have 46 chromosomes – gametes have 23 chromosomes

• Fertilisation: – restores the diploid number in the zygote– introduces variation (genes from both mother and

father!)