Chromosomes, the Cell Cycle and an Intro to its Regulation Flashcards
In eukaryotic cells, what is a chromatin?
chromosomal DNA in a complex wtih RNA and proteins
Histones
highly alkaline proteins which package and order DNA into structural units called nucleosomes
chief protein of chromatin, acting as spools around which DNA winds, and playing a role in gene regulation.
What makes up a nucleosome?
2 loops of DNA double helix wrapped around a cluster of 8 histones
How are chromosomes formed?
supercoiling and condensation of chromatin
What are genes in relation to chromosomes?
specialised functional sites arranged along the choromosomes
Forms of chromatin
in interphase nuclei:
heterochromatin
euchromatin
Heterochromatin
more condensely and densely staining
tends to be found near nuclear envelope
represents genes that are switched off
two types: constitutive and facultative
Euchromatin
less condensed and lightly staining
more central location
represents genes that are switched on
How are tissues of the body divided in relation to cell turnover?
- continuously renewing: epidermis of skin, intestinal epithelium, blood-forming tissues
- conditionally renewing: liver, kidney, endocrine glands
- static/non-proliferative: cardiac, nerve cells
Cell cycle
G1 phase - S phase - G2 - M phase - cytokinesis - G1/G0
G0 = doing function, no division
Mitosis
prophase - prometaphase - metaphase - anaphase - telophase
G1-S phase transition importance
control point to deermine whether cells enter S phase
S phase
replication of DNA and RNA and protein
G2 phase
final prep for mitosis
M phase
division of 2 sister chromatids of each chromosome into separate nucleoli and division of the parent cell into 2 daughter cells
Prophase
chromatin condenses to form visible chromosomes
each chromosome comprises 2 sister chromatids joined at the centromere
centrioles duplicate
centrioles move apart to form 2 poles and act as microtubule-organising centres
microtubules form the mitotic spindle apparatus between the centrioles
Prometaphase
nucleoli regress and nuclear membranes disassemble
chromosomes move towards the equator and interact with microtubules
each pair of sister chromatids has an attachment site - kinetochore, which attaches to spindle
Metaphase
central region of cell forms a metaphase plate
chromosomes arrange themselves on this plate, 2 sister chromatids joined at centromeres
spindle consists of microtubules running from pole to pole or pole to chromosome
latter microtubules run to centromeres of sister chromatids and attach at kinetochores
Anaphase
sister chromatids of each pair move apart to opposite poles of the cell - elongation of polar microtubules, shortened kinetochore microtubule
these chromosomes are pulled along by the microtubules
Telophase
chromosomes reach poles and start to become less condensed
nuclear membrane reassembles
a cleavage furrow begins around the equator
spindle disappears
nucleoli reappear
Cytokinesis
actin-myosin belt
cells separate to form 2 daughter cells
Interphase
G0, G1, S and G2
How is the cell cycle regulated?
cyclins and cyclin-dependent kinases
What are cyclins and CDK?
regulatory proteins
determine cell’s progress through the cell cycle
coordinate cell’s entry into the next phase of the ceel cycle
How do cyclin and CDKs regulate the cell cycle?
CDKs activated when they bind to cyclin
activated CDKs cause phosphorylation to activate or inactivate target proteins
these proteins coordinate cell’s entry into next phase
Clinical relevance of cell cycle regulation
inhibitors of cyclin/CDK complexes: 2 families of genes prevent the progression of the cell cycle
can be used as anticancer agents - tumour suppressors
Difference in cell cycle of cells undergoing mitosis and meiosis
in S phase of meiosis, DNA is duplicated (4c), therefore there must be 2 separate reduction divisions to achieve 1c amount in gamete
Meiosis I
prophase, metaphase, anaphase, telophase
Prophase I
recombination and pairing
Leptotene - chromatin condenses -> chromosomes, chromomeres visible
Zygotene - homologous chromosomes pair to form bivalents, split into sister chromatids, synapsis develop between sister chromatids of homo pairs
Pachytene - synapsis completed and chiasma formed, where genetic info is exchanged
Diplotene - chromatids held together by chiasmata and centromeres
Diakinesis - chiasmata appear to move towards the ends of chromatids
Metaphase I
each bivalent has 2 centromeres and arrange themselves by chance on opposite sides of the metaphase plate
independent assortment results in genetic variation
Anaphase I
chromosomes move to opposite poles of cell
each pole mix of maternal and paternal chromosomes - source of genetiv variation
Telophase I
each daughter cell has half diploid number of chromosomes
nuclear envelope may reassemble before prophase II of the 2nd meiotic division
Meiosis II
similar to mitosis
centromeres split at kinetochores and sister chromatids move to opposite poles
Gametogenesis and meiosis
4 daughter cells formed at end of meiosis II in spermatogenesis but not in oogenesis
oogenesis: ovum and 3 polar bodies
Genetic variation in meiosis
pachytene of prophase I: genetic info exchanged bewtween homo pairs of chromosomes at chiasmata
anaphase I: each pole has mixture of maternal and paternal chromosomes
fertilisation
Monosomy
individual missing chromosome from a pair
eg. Turner’s syndrome (XO)
Trisomy
individual has more than 2 chromosomes of a pair
eg. Down’s syndrome (trisomy 21), Patau’s syndrome (trisomy 13), Edward’s syndrome (trisomy 18), Kleinfelter’s (XXY)
How does aneuploidy occur?
homo chromosomes in meiosis I do not separate properly, one has 24, other has 22 = non-disjunction
