Cell division Flashcards
mitosis
somatic cell division, in which the nucleus also divides
produces 2 daughter cells with the same number of chromosomes (23 pairs)
separation of chromatids, one of which has been replicated from the other in the S phase
how long does mitosis last?
1-2 hours, continuous process
what are the phases in mitosis?
prophase
prometaphase
metaphase
anaphase
telophase
prophase
chromosomes condense and mitotic spindle begins to form. 2 centrioles form in each cell, from which microtubules radiate as they move towards opposite ends of the cell
prometaphase
nuclear membrane begins to disintegrate, allowing chromosomes to spread around the cell
each chromosome becomes attached at its centromere to a microtubule of the mitotic spindle
metaphase
chromosomes become aligned along the equatorial plane, each one attached to the centriole
maximally contracted and most easily visible
chromatids separate longitudinally but are attached at the centromere (not undergone division yet)
anaphase
centromere of each chromosome divides longitudinally and 2 daughter chromatids separate to opposite poles
telophase
chromatids are independent chromosomes consisting of a single double helix
two groups of daughter chromosomes become enveloped in a new nuclear membrane
cytokinesis - nuclear membrane separates
what does each daughter cell contain?
complete diploid chromosome complement
interphase
period between successive mitoses
how long does interphase last in rapidly dividing cells?
16-24 hours
components of interphase
G1, G2, S, G0
G1 phase
chromosomes become thin and extended
variation in length - responsible for variation in generation time between different cell populations
cells that have stopped dividing usually arrest in this phase and enter G0
S phase
when DNA replication occurs and chromatin of each chromosome is replicated
formation of 2 chromatids
DNA replication occurs at multiple points on a chromosome
sex chromatin
Barr body
one of the X chromosomes is always late in replicating - visualised during interphase in female somatic cells
cells from buccal mucosa - buccal smear
when does meiosis occur?
final stage of gamete formation
how does meiosis differ from mitosis?
mitosis results in each daughter cell having a diploid chromosome complement (46)
in meiosis the diploid count is halved so each mature gamete receives a haploid complement (23)
mitosis occurs in somatic cells and in early cell divisions in gamete formation. meiosis occurs only in final division of gamete maturation
mitosis is a 1 step process, meiosis has 2
what is meiosis sometimes referred to?
reduction division - chromosome number is halved
prophase 1
chromosomes enter phase already split longitudinally into 2 chromatids joined at the centromere
homologous chromosomes pair and exchange of homologous segments occurs between non-sister chromatids
recombination
crossing over
exchange of homologous segments between chromatids occurs
prophase 1 in the male
pairing occurs between homologous segments of the X and Y chromosomes at the tip of their short arms - this portion of each chromosome is the pseudoautosomal region
five stages of prophase 1
leptotene
zygotene
pachytene
diplotene
diakinesis
leptotene
chromosomes become visible as they start to condense
zygotene
homologous chromosmes align directly opposite eachother (synapsis) and are held together at points along their length by filamentous structures (synaptonemal complexes)
synapsis and synaptonemal complexes
homologous chromosomes aligning directly opposite eachother
filamentous structures holding chromosomes together
pachytene
each pair of homologous chromosomes (bivalent) becomes tightly coiled. crossing over occurs
bivalent
pair of homologous chromosomes
diplotene
homologous recombinant chromosomes begin to separate but remain attached at points where crossing over has occurred (chiasmata)
chiasmata
points on chromosomes where recombination has occured. small, medium and large chromosomes have 1, 2 adn 3 chiasmata respectively - approx. 40 recombination events per meiosis per gamete
diakinesis
separation of homologous chromosome pairs proceeds as chromosomes become maximally condensed
metaphase 1
nuclear membrane disappears and chromosomes align along equatorial plane, attached to the spindle
anaphase 1
chromosomes separate to opposite poles of the cell as the spindle contracts
telophase 1
each set of haploid chromosomes has separated completely to opposite ends of the cell, and cleaves into 2 new daughter gametes (secondary spermatocytes or oocytes)
meiosis 2
each chromosome becomes aligned along the equatorial plane and splits longitudinally, forming 2 new daughter gametes - spermatids or ova
consequences of meiosis
facilitates halving of the diploid number of chromosomes so each child receives half of its chromosome complement from each parent
potential for genetic diversity
how does meiosis have potential to generate genetic diversity?
when the bivalents separate, they do so independently of each other - each gamete receives a selection of parental chromosomes
likelihood that 2 gametes contain same chromosomes is 1 in 8 million
chromatid contains portions of DNA derived from both parental homologous chromosomes
what is gene shuffling?
dispersion of DNA into different gametes
clinical relevance of mitosis
detecting chromosomal abnormalities
categorising tumours as benign or malignant
grading malignant tumors
clinical relevance of the mitotic spindle
taxol
vinca alkaloids - vinblastine, vincristine
clinical relevance of anaphase
colchicine like drugs
clinical relevance of spindle poles
ispinesib
