DNA and cells - MEIOSIS Flashcards
How many daughter cells does meiosis produce?
4
Definition of a gene
A length of DNA that codes for a polypeptide
Definition of locus
A position of a gene on a chromosome of DNA
Definition of an allele
One of the different forms of a particular gene
Definition of homologous chromosomes
A pair of chromosomes, one maternal and one paternal that have the same gene loci
Definition of a chromosome
Linear structures made of DNA and proteins, containing specific genes
Definition of a chromatid
One of the two strands of a chromosome that are joined together by a single centromere
Definition of bivalent
Pair of homologous chromosomes, coiled around each other and joined at multiple points
Definition of chiasmata
Points where non-sister chromatids of homologous chromosomes are joined
What happens in the first meiotic division?
- Homologous chromosomes pair up and their chromatids wrap around each other
- Portions of chromatids may be exchanged (crossing over)
What happens in the second meiotic division?
Chromatids move apart
What occurs during prophase I?
- Chromosomes condense
- Nuclear membrane breaks down
- Spindle fibres form
What occurs during metaphase I?
- Homologous chromosomes line up at the equator and bind to spindle fibres
- Independent segregation
- Crossing over
What occurs during anaphase I?
Homologous chromosomes separate - they are pulled by centromeres
What occurs during telophase I?
- Spindle fibres break down
- Nuclear envelope reforms
- Chromosomes partially unveil
How many cells are produced after cytokinesis of meiosis I?
2 daughter cells
What occurs during prophase II?
- Chromosomes condense
- Nuclear membrane breaks down
- Spindle fibres form
What occurs during metaphase II?
- Chromosomes line up at equator
Spindle fibres attach to centromere - Sister chromatids split at centromere
What occurs during anaphase II?
- Spindle fibres contract, pulling sister chromatids towards poles
- Now chromosomes again
- Centromere splits
What occurs during telophase II?
- Spindle fibres break down
- Nuclear envelope reforms
- Chromosomes partially uncoil
How many cells are produced after cytokinesis of meiosis II?
4 daughter cells
- Haploid number of chromosomes
How does meiosis increase genetic variation?
- Independent segregation of homologous chromosomes
- New combinations of maternal and paternal alleles by crossing over (genetic recombination)
What is non-disjunction?
- When the chromosomes fail to separate
- Gamete can have two of the same chromosome
How does non-disjunction cause problems and can you give an example?
- During formation, the zygote may have 3 of the same chromosome instead of 2
- Eg. Trisomy of chromosome 21 results in Down Syndrome
How would you describe homologous chromosomes?
Same genes
Same loci
Different variations of alleles
