F215 Module 1 Cellular Control Meiosis - Natural Selection

Describe, with the aid of diagrams and photographs, the behaviour of chromosomes during meiosis, and the associated behaviour of the nuclear envelope, cell membrane and centrioles

Know the names of the main stages of meiosis

Meiosis - A reduction division. The resulting daughter cells have half the original number of chromosomes. They are haploid and can be used for sexual reproduction

Meiosis is used in sexual reproduction, where the offspring are genetically different from each other and their parents. Each parent produces special reproductive cells called gametes; gametes fuse to form a zygote

Asexual production occurs by mitosis in eukaryotes and binary fission in prokaryotes

In order for a zygote to have the correct number of chromosomes, each gamete must have the haploid number of chromosomes for that organism, e.g. 23 in human egg and sperm, becoming 46 when combined

Meiosis consists of 2x divisions - Meiosis I and Meiosis II. Each of these contains four subdivisions: Prophase, Metaphase, Anaphase and Telophase. In interphase, before Meiosis I, the DNA replicates, meaning that the cell now contains 4 copies of each chromosome rather than 2

MEIOSIS I

Prophase I, can last for days/months/years:

• Chromatin condenses + undergoes supercoiling so that chromosomes shorten and thicken. They can now take up stains and be seen with a light microscope
• Chromosomes come together in HOMOLOGOUS PAIRS to form a BIVALENT; each member of the pair has the same genes at the same loci. Each pair consists of one maternal and one paternal chromosome
• Non-sister chromatids wrap around each other and attach at chiasmata
• They may swap sections of chromatids with one another in a process called crossing over
• The nucleolus disappears and the nuclear envelope disintegrates 
• Spindle made of microtubules forms

Metaphase I:

• Bivalents line up across the equator attached to the spindle by centromeres. Chiasmata still present
• Bivalents arranged randomly with each member of a homologous pair facing opposite poles. This is called RANDOM ASSORTMENT and allows the chromosomes to segregate independently when they are pulled apart in anaphase I 

Anaphase I:

• Homologous chromosomes in each bivalent are pulled by the spindle microtubules to opposite poles
• Centromeres do NOT divide
• Chiasmata separate and lengths of chromatid that have been crossed over with the chromatid to which they have become newly attached

Telophase I:

• In most animal cells two new nuclear envelopes form - one around each set of chromosomes at each pole - and the cell divides by cytokinesis. there is a brief interphase and the chromosomes uncoil
• In most plant cells the cell goes straight from anaphase I into meiosis II

MEIOSIS II (happens at right angles to meiosis I, forming a square-ish shape)

Prophase II and Metaphase II are the same as in Meiosis I

Anaphase II:

• Centromeres DO divide and the chromatids are pulled to opposite poles by the spindle fibres. The chromatids randomly segregate

Telophase II:

• Nuclear envelopes reform around the haploid daughter nuclei
• Animals - 2x diploid cells now divide to give 4x haploid cells
• Plants - a…