4.3 Meiosis Flashcards
How many nucleur divisions occur before meiosis
2
What is meiosis always preceded by, and what effect does this have
- DNA Replication
- 2 sister chromatids held together by the centromere
Describe what happens during prophase I of meiosis
- Chromosomes condense and become visible
- Homologous chromosomes attach to each other at chiasmata, forming bivalents
- Spindle fibres begin to form
- Crossing over
When does Crossing-Over occur
Prophase I
Describe how crossing-over during Prophase I results in gentic variation
- Bivalents form from homologous chromosomes associating with each other
- They are joined at the chiasmata
- Lengths of non-sister chromatids are exchanged
- They twist and break off
- They recombine one another non-sister chromatid
- Results in new combinations of alleles on the chromatids
- And therefore genetic variation
Describe what happens during metaphase I of meiosis
- Homologous pairs of chromosomes line up at equator of cell
- Spindle fibres attach to centromeres
- Independant segregation of homologous chromosomes
When does independant segregation of homologous chromosomes occur
Metaphase I
Describe how independant segregation of homologous chromosomes results in genetic variation
- Maternal and paternal chromosomes can be at either side of the equator
* Maternal and paternal chromosomes are re-shuffled in any order - Independant for each pair
* Different combination of alleles
Describe what happens during anaphase I of meiosis
- Homologous pairs of chromosomes seperate as i ndividual chromosomes are pulled to poles of cell
- Spindle fibres contract and shortern
Describe what happens during telophase I of meiosis
- Chromosomes uncoil
- Spindle fibres break down
- Daughter nuclei are haploid
- Cytokenisis occurs
Describe what happens during prophase II of meiosis
- Chromosomes condense and become visible
- Spindle fibres form
Describe what happens in metaphase II of meiosis
- Chromosomes line up at equator of cell
- Spindle fibres attach to centromeres
Describe what happens during anaphase II of meiosis
- Chromosomes seperate as sister chromatids are pulled to poles of the cell
- Spindle fibres contract and shortern
Describe what happens in telophase II of meiosis
- Chromosomes uncoil
- Spindle fibres break down
- Daughter nuceli have formed
Describe the final stage of meiosis
- Cytokenisis
- 4 genetically different daughter cells
- That are haploid
Describe how random fertilisation results in genetic variation
- Each gamete is **genetically varied **
- In each fertilisation there is an equal probability of any female gamete fusing with any male gamete
What is non-disjunction
When chromosomes do not seperate during anaphase in meiosis
Equation for number of gamete chromosome combinations with independant segregation of homologous chromosomes
Equation for number of gamete chromosome combinations with independant segregation of homologous chromosomes AND random fertilisation
Define a mutation
The change in the DNA base sequence of a chromosome
When do mutations occur
Spontaneously during DNA replication
What increases the chance of a mutation
Exposure to mutagenic agents (ionising radiation, and chemicals)
Describe a substitution mutation
- Bases substituted
- Pinpoint mutation
- May have no effect, as DNA is degenerate
- Could change amino-acid produced and therefore tertiary structure
- Or introduce a stop codon
Describe a deletion/addition mutation
- Bases removed or added
- Frameshift mutation
- All triplets after mutation are changed
- Change in amino-acid production and therefore change in tertiary structure
- As DNA is read in triplets
Define a ‘homologous chromosome’
2 chromosomes that carry the same genes
