22. Cell reproduction. Haploid-Diploid cycle. Aberrations in the normal run of mitosis and meiosis. Flashcards
How do prokaryotes reproduce
Prokaryotes reproduce asexually through binary fission (a single cell is divided into 2 cells of the same size). Eukaryotes divide by mitosis/ meiosis
What are the definitions of Mitosis and Meiosis?
Mitosis - cell division where a parent cell forms 2 identical daughter cells
Meiosis - cell division where daughter cell contains half the info from parent cell
Give a brief explanation of meiosis
Meiosis
- only found in sex cells - gametes
- somatic cells CANNOT divide by meiosis
- gametes are haploid (n)
- fertilisation = when oocyte and spermatozoan fuse - becomes diploid (2n)
- mitosis of cells to make organs…
- mitosis - diploid progeny cells
- each gamete has either a maternal or paternal version of each chromosome - the choice is random for each pair
- Crossing over: happens only in meiosis for gametes, it’s the exchange of genetic material from maternal + paternal versions of each chromosome. Generates genetic diversity
Explain Mitosis interphase
Mitosis:
INTERPHASE
G1:
- growth phase - cell grows
- synthesises mRNA and proteins
S:
- synthesis phase - DNA is replicated
- (DNA packaged in chromosomes are replicated)
G2:
- Rapid cell growth + protein synthesis
- cell gets ready for mitosis
- safety gap - check if DNA has been duplicated properly
Explain the (M phase) Of mitosis:
Stages: Prophase, prometaphase, metaphase
M PHASE
Prophase:
- chromosomes condense
- mitotic spindle assembles
Prometaphase:
- nuclear envelope fragments
- chromosomes attach to microtubules
Metaphase:
- chromosomes at the equator (middle)
- chromatids attach to the mitotic spindle
Explain the (M phase) Of mitosis:
Stages: Anaphase and telophase
Explain the final stage cytokinesis
Anaphase:
- kinetochore microtubules shorten
- chromosomes separate
Telophase:
- chromosomes at the pole
- nuclear envelope reassembles
Cytokinesis:
- cleavage
- nuclear envelope completed
Explain Meiosis
stage: interphase
Meiosis:
INTERPHASE - before meiosis begins, genetic material is duplicated MEIOSIS I
Explain Meiosis
stage: Prophase I and its substages
Prophase I:
- duplicated chromatin condenses
- each chromosome consists of 2, closely associated sister chromatids
- (crossing over at the end stage)
Leptotene:
- consists of the condensing of replicated chromosomes
- each sister chromatid is attached to nuclear envelope
Zygotene:
- synapsis between homologous chromosomes begins
- after homologous pairs synapse, they are called tetrads, bivalents
Pachytene:
- synapses snap, allowing crossing-over of genetic info
- 2 sister chromatids separate from each other
- homologous chromosomes are attached
Diplotene:
- 2 homologous chromosomes begin to migrate apart - repel one another
- 2 chromosomes move apart
- chiasma move to end of chromatids
Diakinesis:
- Termination of the condensing of chromosomes
- chromosomes are at there most condensed
- nucleolus disappear
Explain Meiosis
stages: Metaphase I, Anaphase I and Telophase I
Metaphase I:
- homologous chromosomes align at the equatorial plate
Anaphase I:
- homologous pairs separate with sister chromatids remaining together
Telophase I:
- 2 daughter cells are formed with each daughter containing only 1 chromosome of the homologous pair
Explain MEIOSIS II:
Stages: Prophase II, Metaphase II, Anaphase II and Telophase II.
MEIOSIS II:
Prophase II:
- DNA does NOT replicate
Metaphase II:
- chromosomes align the equatorial plate
Anaphase II:
- centromeres divide + sister chromatids migrate separately to each pole
Telophase II:
- cell division is complete
- 2 haploid daughter cells are obtained
Aberrations during Meiosis:
1) What is Aneuploidy
Aneuploidy - addition or loss of one or two chromosomes
- Non-disjunction - chromatids don’t separate properly
- (division failure of sister chromatids)
- multipolar mitosis - 3 or more poles
Aberrations during Meiosis:
2) What is Polyploidy
Polyploidy - addition of complete haploid sets of chromosomes
- endomitosis, chromosomal replication not followed by cytokinesis
- endoreduplication, cells grow + enlarge, but don’t go through process of dividing - mitosis
- C mitosis, colchicine block mitotic anaphase (colchicine blocks mitotic spindles contracting
