Genetics: Genetic Diversity - Meiosis Flashcards
Does meiosis produce genetically identical or different daughter cells?
- Genetically different daughter cells
How many daughter cells are produced in meiosis?
- 4
How many chromosomes do cells produced from meiosis have?
- Daughter cells are haploids
- Have n number of chromosomes (half of 2n)
What type of cells does meiosis produce?
- Gametes
Where does meiosis occur?
- Occurs in reproductive organs
- Testes and ovaries in humans
Outline the steps of meiosis
• Meiosis I
- Interphase
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
• Meiosis II
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
- Cytokinesis
What are sister chromatids?
- A chromatid is one strand of a replicated chromosome
- Sister chromatids are identical and joined by a centromere
What is a chromosome?
- A thread like structure made up of one long DNA molecule
What is a homologous pair of chromosomes?
• Aka bivalents • Made of two chromosomes that: - Carry the same genes in the same locus - Are the same size - Carry genes that may have different alleles
• One chromosome from mum, one from dad
- Not the same as sister chromatids (these are replications of each other)
What happens during interphase?
- DNA unravels and replicates
- Produces two copies of each chromosome, called chromatids joined at a centromere
- Chromosomes are long and thin
What happens during prophase I?
- Chromosomes condense and become visible
- Homologous pairs of chromosomes come together to form a bivalent: one from mum, one from dad
- Non-sister chromatids may exchange genes - crossing over
- Chiasma are sections that cross over, site of crossing over
- Chromatids now have different combination of alleles - called recombinants
What happens during metaphase I?
- Nuclear membrane breaks
- Spindle fibres are made by the centrioles
- Bivalents move to the equator
- Centromeres attach to spindle
- Positioning of homologous pairs is random - independent segregation
What happens during anaphase I?
- Spindle fibres contract
- Homologous chromosomes are pulled apart to opposite poles of cell
What happens during telophase I?
- 2 chromosomes with 2 chromatids group together at each pole of cell
- Spindle disappears
- Nuclear envelope reforms
- Cytokinesis
- Cell cytoplasm divides producing 2 diploid cells
What happens during prophase II?
- 2 diploid cells which will divide a second time
- New spindle begins to form in each cell
What happens during metaphase II?
- Chromosomes, each comprised of 1 pair of sister chromatids line up on equator
- Sister chromatids may not be identical - recombinants
What happens during anaphase II?
- Sister chromatids pulled apart by spindle
- Move to opposite poles of cell
What happens during telophase II?
- Spindle fibres break down
- Nuclear envelope reforms around separated chromosomes
- Cytokinesis
- Produces 4 haploid cells, each with n chromosomes
Which processes that occur during meiosis result in variation?
- Crossing over
- Independent segregation
What is ‘crossing over’?
- Occurs during prophase I
- Process of non-sister chromatids in a homologous pair of chromosomes exchanging genes
- Chromatids have same genes but different combination of alleles
- Site of crossing over is at chiasmas
What is independent segregation of chromosomes?
- Occurs during metaphase I
- Homologous pairs made of one chromosome from mum, one from dad
- Homologous pairs separated in metaphase I
- Random which chromosome from each pair ends up in which daughter cell
What happens during meiosis I?
- Homologous chromosomes pair to form bivalents
- Crossing over occurs at chiasmata
- Independent segregation randomises which chromosome from each homologous pair goes into which cell
- Cell divides into 2
What happens during meiosis II?
- Sister chromatids are separated
- 2 cells each divide again
- Four haploid cells are produced
