Meiosis Flashcards
Define haploid cell.
They contain one set of chromosomes and are also known as gametes.
Define meiosis.
DNA replication where four haploid cells are produced from the parent diploid cell.
Define diploid cell.
They contain two sets of chromosomes and are also known as zygotes.
Define ploidy.
The number of chromosome sets in a cell.
Define aneuploidy.
Aneuploid cells have an abnormal number of chromosomes.
Define non-disjunction.
A failure of the homologous chromosome pairs to separate at meiosis.
How does aneuploidy occur?
It results from non-disjunction.
What can increase the likelihood of non-disjunction?
Chromosome abnormalities such as translocations.
What four stages occur in meiosis I?
- Prophase I.
- Metaphase I.
- Anaphase I.
- Telophase I.
What four stages occur in meiosis II?
- Prophase II.
- Metaphase II.
- Anaphase II.
- Telophase II.
What occurs when DNA enters prophase I?
- Homologous chromosomes are in a 2n cell.
- Each chromosome is replicated in S phase.
- Each chromosome is two sister chromatids in G2 phase.
- The sister chromatids are joined at the centromere.
- The chromosomes start condensing.
What three things occur in synapsis?
- Condensing homologous chromosomes meet on envelope during prophase I.
- Protein-rich synaptonemal complex built between non-sister chromatids.
- Tetrad remains until anaphase I.
Define crossing over.
Where condensed non-sisters exchange DNA.
What four stages lead to crossing over?
- Enzymes in recombination nodules break/rejoin homologous chromosomes.
- Each chromatid has few crossovers with a non-sister.
- Synaptonemal complex breaks down after crossover.
- Homologous chromosomes stay as a tetrad linked at chiasmata.
Define chiasmata.
An exchange point.
Summarise prophase I into nine stages.
- Chromosomes keep condensing.
- The homologous chromosomes align during synapsis for pairing.
- Crossing over occurs for exchange of DNA.
- Tetrad (bivalent) is then maintained.
- Centromeres hold sisters together.
- Chiasmata hold non-sister together.
- The nuclear envelope breaks up.
- Centrosomes move to the poles.
- Spindle formation occurs.
Define synapsis.
Pairing of homologs to form a tetrad.
State the six stages of metaphase I.
- Spindle formation is complete.
- Terminal chiasmata hold non-sisters together.
- Outward-facing kinetochores of sisters function as one.
- Kinetochores attach to microtubules from opposite poles.
- Tetrad lines up on metaphase plate.
- Orientation of tetrad on plate is random.
State four steps of the anaphase I phase.
- Microtubules shorten, breaking chiasmata.
- Homologous chromosomes move to opposite poles.
- Each pole gains:
- One of each homologous pair.
- One complete chromosome set. - Each pole is haploid.
Define random orientation.
Pole may get maternal or paternal homologous chromosomes.
Define independent assortment.
Random alignment of homologous on metaphase plate.
What is the equation for independent assortment?
2^n.
State the five stages of telophase I.
- There is one set of chromosomes per pole.
- The spindle breaks down.
- An envelope forms round daughter nuclei.
- Cytokinesis usually follows.
- Each daughter cell/nucleus is haploid but each chromosome is still two non-identical sister chromatids (crossing over).
Explain what meiosis II is in comparison to meiosis I.
It is the brief interphase after meiosis I, but no S phase occurs.
Describe each stage of meiosis II.
Prophase II - Nuclear envelope breaks down and the spindle forms.
Metaphase II - Chromosomes attach to microtubules and align.
Anaphase II - Microtubules shorten, centromeres split and sister chromatids part.
Telophase II - Envelope enclose four unique haploid nuclei.
What are the three sources of genetic variation?
- Crossing over during prophase I - Produces recombinant chromosmes.
- Independent assortment during metaphase I.
- Random fertilisation of unique haploid gametes during sexual reproduction.
