GCSE comparison of mitosis and meiosis Forms body cells Growth and repair Cells produced are genetically identical One division Homologous pairs do not meet No crossing over Produces 2 cells from each division Diploid number of chromosomes Forms sex cells/gametes For sexual reproduction Cells produced are genetically different Two divisions Homologous pairs meet Crossing over Produces 4 cells from each division Haploid number of chromosomes In both the chromosomes replicate before division begins

Meiosis key words Gametes Haploid Meiosis I Meiosis II Prophase Metaphase Anaphase Telophase DNA replication Sister chromatids Centromere Chromatin Homologous pairs Bivalent Loci Maternal/paternal Chiasmata Crossing over Spindle Random assortment Fertilisation Mutation

Learning outcomes Describe, with the aid of diagrams and photographs, the behaviour of chromosomes during meiosis, the associated behaviour of the nuclear envelope, cell membrane and centrioles. (Names of the main stages are expected, but not the subdivisions of prophase) Pages 118-9

Reproduction and variation All living organisms can reproduce, this can be: Asexual reproduction Single organism divides by mitosis New organism is genetically identical to the parent Genetic variation can only be by mutation Mitosis (in eukaryotes) or binary fission (in prokaryotes). Sexual reproduction Meiosis produces haploid gametes Which fuse at fertilisation to form a diploid zygote This produces genetic variation amongst offspring

Human Life Cycle Diploid Zygote Mitosis 46 Haploid Sperm 23 Meiosis Adult 46 Haploid Egg 23 fertilisation

Eukaryotic chromosome structure Chromosomes in a non-dividing cell (unduplicated) exist as single-armed structures (each is equivalent to one of the chromatids in a metaphase chromosome prepared for cell division) It would not be visible as a coiled structure but would be unwound Metaphase (duplicated) chromosome has 2 sister chromatids. Each chromatid contains an identical copy of the genetic material. (DNA molecule)

Chromosome structure Chromatid vs. Chromosome: Remember that when two DNA molecules are joined together, each molecule is called a chromatid. When a DNA molecule (and proteins) is not attached to another one then that single molecule of DNA is not a chromatid but an unduplicated chromosome Chromatin: During certain times of the cell's life cycle the chromosomes are not visible. This is because the chromosomes are stretched out very thin to allow surfaces for the various chemical reactions that involve chromosomes to take place. When the nucleus is stained and examined, it appears uniformly coloured and the chromosomes collectively are termed chromatin.

Read the worksheet and complete the definitions Task 1: 6 minutes Read the worksheet and complete the definitions

Homologues Sexually reproducing organisms in nearly all cases have paired sets of chromosomes, one set coming from each parent (maternal/paternal) The equivalent chromosomes that form the pair are called Homologous pairs/homologues Each homologue carries an identical assortment of genes, but the version of the gene, the allele, from each parent may differ

Alleles A pair of homologous chromosomes centromere Some basic points… showing that different versions (alleles) occupy the same position (locus) on a chromosome. A pair of homologous chromosomes centromere Paternal chromosome, originating from the sperm T Locus of gene for height (dominant allele T = tall) Maternal chromosome, originating from the egg t Locus of gene for height (recessive allele t = short)

Meiosis Meiosis is a reduction division Meiosis has two divisions Resulting daughter cells have half the original number of chromosomes Daughter cells are haploid Can be used for sexual reproduction Source of genetic variation Meiosis has two divisions Meiosis I and Meiosis II Each division has 4 stages Prophase, metaphase, anaphase, telophase (PMAT x2)

An animation of Meiosis at http://www.cellsalive.com/meiosis.htm Just watch first Then again….Let’s go!!!!

Meiosis – During interphase the DNA replicates forming two sister chromatids joined at the centromere PROPHASE 1: The chromatin condenses, supercoils and become visible. The homologues pair up in a process called synapsis to form bivalents One of the chromosomes is maternal and one is paternal. The chromosomes in a bivalent may cross over at points called chiasmata where they may swap sections of DNA/alleles. The nucleolus disappears and the nuclear envelope breaks down.

Prophase 1 This swapping of genetic information is called crossing over. The centriole divides (animal cells only) and starts migrating to opposite poles of the cell. A spindle forms (protein microtubules) produced by centrioles lining up at the equator. Prophase 1 can last days, months, years!!

Meiosis I METAPHASE 1: The bivalents randomly line up on the equator. (random assortment) The spindle fibres attach to them at their centromeres. Each member of the homologous pair facing opposite poles This allows for independent assortment

Meiosis I ANAPHASE 1: The homologous chromosomes of each bivalent are pulled apart by the spindle fibres contracting, towards the poles. (independent assortment) The chiasmata separate as each homologous pair moves towards it separate pole.

Meiosis I TELOPHASE 1: In animal cells the chromosomes will de condense a little and the nuclear envelope will reform. Cytokinesis will occur and the cell will split. However most plant cells progress straight to Meiosis II 

Meiosis I Prophase I Metaphase I Anaphase I Telophase I Chromatin condenses Homologous pairs form a bivalent. Crossing over occurs Nucleolus disappears Spindle forms Metaphase I Bivalents line up on equator of cell Attached by centromere Random Anaphase I Homologous chromosome in each bivalent are pulled to opposite poles by spindle contracting Telophase I Two new nuclear envelopes form Cell divides by cytokinesis Each cell has???

Meiosis II: mitotic division PROPHASE 2: Everything from now on happens in two cells. The chromosomes condense. The centrioles replicate and the spindle starts to develop perpendicular to the previous division. The nuclear envelope disintegrates.

Meiosis II METAPHASE 2: The duplicated chromosomes randomly line up on the equator. They are attached to the spindle fibres at their centromeres.

Meiosis II ANAPHASE 2: The centromeres divide to allow the chromosomes to split. The chromatids (now called chromosomes, just to confuse things) are pulled to the poles by the contracting spindle fibres The chromatids randomly segregate

Meiosis II TELOPHASE 2: The chromosomes reach the poles and de condense. The nuclear envelope reforms, forming four separate nuclei. Cytokinesis may now occur and the cells divide. Each cell has half the number of chromosomes as the original parent cell, the cells are haploid.

Meiosis II Metaphase II Prophase II Anaphase II Telophase II Duplicated chromosomes arrange themselves on equator Attach by centromere to spindle fibres Random arrangement of chromatids Prophase II Nucleolus disappears Chromosomes condense Spindle forms Anaphase II Centromeres divide Chromatids pulled apart to opposite poles by contracting spindle Telophase II Nuclear envelopes reform around haploid nuclei Cell divides by cytokinesis 4 daughter cells formed

Let’s watch??: https://www.youtube.com/watch?v=16enC385R0w Learning outcome states diagrams and pictures: SORT in pairs! (Homework!) Recap on some definitions Homologous pairs Bivalent Loci Maternal chromosomes Paternal chromosomes

  Mitosis Meiosis No. of Divisions One Two Products Two genetically identical daughter cells Four cells, genetically different from each other & parent cells Chromosome number Maintained Halved Bivalents formed? No Yes Crossing over?