Meiosis
• 2-stage cell division in gonads of sexually reproducing eukaryotic organisms
• Genetically unique gametes – 4 products with one set of chromosomes
• Gametes have one set of chromosomes (haploid – n= 23), so in fertilisation, the
offspring has full set of chromosomes (diploid – 2n =
46)
• Testes and ovaries. Sperm and egg
• Anthers. Pollen and egg
• Yeast as well. Normal stage is haploid instead of
diploid. N + n = 2n, and then meiosis
• Reduces chromosome number and creates variation
• Products of meiosis → mature gametes. In females, only one of the 4 products forms
a mature egg
• Pairing – homologous chromosomes. Maternal and paternal
• Prophase of meiosis – homologous chromosomes pair.
• Metaphase – paired chromosomes come to equator of cell and move apart in
anaphase
• Meiosis 2 is the same as mitosis as sister chromatids come apart
Mitosis Meiosis
One cell division (2 products) Two cell divisions (4 products)
Homologous chromosomes don’t pair Homologous chromosomes pair and recombine
Daughter cells identical to parent cell Daughter cells are unique
Occurs in somatic tissue Occurs in specialised tissue called gonads
Centromeres separate Whole chromosomes separate in M1, centromeres separate in M2
DNA replication prior to cell division DNA replication only prior to M1
, • Prophase divided into 5 subsections
Prophase
1. Leptotene = “thin thread”. Chromatin starts to condense and the chromosomes first
become visible as long, thin threads
2. Zygotene = “paired thread”. Homologous chromosomes start to find each other.
Pairing of homologous chromosomes begin.
3. Pachtyene = “thick thread”. Chromosomes fully aligned all along their length. Pairing
is complete. Crossing over occurs.
4. Diplotene = “double thread”. Sister chromatids can be seen. Chromosomes become
even shorter and thicker and may be able to distinguish individual chromatids and
sites of crossing over.
5. Diakinesis = “moving apart”. Homologous chromosomes start to move apart but are
still held together at sites of crossing over
Crossing Over
• Reciprocal exchange of genetic material between parental chromosomes (non-sister
chromatids)
• S phase – replication into
sister chromatids.
• Chiasma – physical site
where two chromosomes
have crossed over
• 2-stage cell division in gonads of sexually reproducing eukaryotic organisms
• Genetically unique gametes – 4 products with one set of chromosomes
• Gametes have one set of chromosomes (haploid – n= 23), so in fertilisation, the
offspring has full set of chromosomes (diploid – 2n =
46)
• Testes and ovaries. Sperm and egg
• Anthers. Pollen and egg
• Yeast as well. Normal stage is haploid instead of
diploid. N + n = 2n, and then meiosis
• Reduces chromosome number and creates variation
• Products of meiosis → mature gametes. In females, only one of the 4 products forms
a mature egg
• Pairing – homologous chromosomes. Maternal and paternal
• Prophase of meiosis – homologous chromosomes pair.
• Metaphase – paired chromosomes come to equator of cell and move apart in
anaphase
• Meiosis 2 is the same as mitosis as sister chromatids come apart
Mitosis Meiosis
One cell division (2 products) Two cell divisions (4 products)
Homologous chromosomes don’t pair Homologous chromosomes pair and recombine
Daughter cells identical to parent cell Daughter cells are unique
Occurs in somatic tissue Occurs in specialised tissue called gonads
Centromeres separate Whole chromosomes separate in M1, centromeres separate in M2
DNA replication prior to cell division DNA replication only prior to M1
, • Prophase divided into 5 subsections
Prophase
1. Leptotene = “thin thread”. Chromatin starts to condense and the chromosomes first
become visible as long, thin threads
2. Zygotene = “paired thread”. Homologous chromosomes start to find each other.
Pairing of homologous chromosomes begin.
3. Pachtyene = “thick thread”. Chromosomes fully aligned all along their length. Pairing
is complete. Crossing over occurs.
4. Diplotene = “double thread”. Sister chromatids can be seen. Chromosomes become
even shorter and thicker and may be able to distinguish individual chromatids and
sites of crossing over.
5. Diakinesis = “moving apart”. Homologous chromosomes start to move apart but are
still held together at sites of crossing over
Crossing Over
• Reciprocal exchange of genetic material between parental chromosomes (non-sister
chromatids)
• S phase – replication into
sister chromatids.
• Chiasma – physical site
where two chromosomes
have crossed over
