MEIOSIS

Introduction Asexual reproduction: offspring are genetically identical Living organisms reproduce. Reproduction may be: asexual (without sex) sexual (with sex) 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 2

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?  3

Mitosis leading to growth Mitosis leading to growth Meiosis Egg n=23 Sperm n=23 Fusion of gametes Zygote n=46 Mitosis leading to growth Mitosis leading to growth Mitosis 4

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 5

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. 6

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. 7

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! 8

Mitosis is a continuous sequence, but is divided into four stages: prophase metaphase anaphase 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. 9

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 Early Prophase 10

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. 11

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. Metaphase 12

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. Anaphase 13

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 Telophase 14

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. Cytokinesis 15

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: growth All cells of the organism are genetically identical. repair Replacement cells are the same as those they are replacing (genuine parts capable of doing an identical job vs cheap imitation that gives out quickly!). asexual reproduction Offspring are identical and able to colonise quickly. 16

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. 17

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. 18

Anaphase Chromatids move to opposite poles of the cell. They are pulled centromere first by the contracting spindle fibres. 19

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. 20

Cytokinesis As shown here, in plant cells a cell wall is laid down in the position of the metaphase plate. 21

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. 22

Mitosis in Allium Prophase Metaphase Interphase Anaphase Telophase 23

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. 24

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. 25

Anaphase Chromatids move to opposite poles of the cell. They are pulled centromere first by the contracting spindle fibres. 26

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. 27

Cytokinesis The cell membrane invaginates or constricts. This eventually pinches the cell into two. 28

…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. 29

Mitosis in Whitefish Prophase Metaphase Cytokinesis – Interphase Anaphase Telophase –Cytokinesis 30

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 31

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. Early Prophase I 32

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. Late Prophase I 33

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 Metaphase I 34

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) Anaphase I 35

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 Telophase I 36

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 37

The centromere divides: 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 38

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 39

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 40

Chromosomes arrive at poles. De-condense, nucleus reforms (detail) Prophase I Homologous chromosomes pair (bivalents present); chiasma (cross-overs) present; nucleolus disappears; envelope disintegrates. Centrioles. Metaphase I Bivalents line up at equator; chromosomes attached to only one pole; behaviour is independent. Anaphase I Homologous chromosomes separate; centromeres intact; chromosome number halved. Telophase I Chromosomes 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 II Four haploid cells produces by cytokinesis. Chromosomes de-condense, nucleus reforms (detail) 41

What are the differences between mitosis and meiosis? Prophase: Chromosome pairing Chiasma/cross-over Attachment of spindle fibres Number of divisions Cells produced Role 42

Mitosis Meiosis Prophase: Chromosomes unpaired Chromosome pairing Chromosomes unpaired Pairing to form bivalents or tetrads 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. 43

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!) 44