Cell Cycle and Cell Division Flashcards
why is cell division important? (4)
evolution of life
cell repair / enhancement
genetic variation
growth
what are sister and homologous chromosomes?
sister chromatids are identical copies of same chromatid (i.e both from mother / father)
homologous chromosome: a pair of chromosoemes, each derived from one parent
the process of cell divison is broadly divided into which phases?
- interphase: DNA replication
composed of: G1 phase (cell growth), S phase (DNA synthesis occurs) and G2 phase (cell growth)
- mitotic phase (M phase): nucleus divides
3. cytokinesis: cytoplasm divided into two daughter cells
CELL IS MOSTLY IN INTERPHASE
Explain each phase of a bit more :)
G0 phase: cells outside of cycle and have stopped dividing. can return to G1 phase.
G1 phase: normal growth phase. prep for DNA synthesis
S phase: DNA synthesised and duplicated
G2 phase: cell prepares for cell division
M phase: proper cell division
what are the G phases?
gap / growth phases:
- cell undergoes normal function.
- NOT growing or replicating
- allows cell time to monitor the env to check conditons correct for replication
- act as regulatory phases / checkpoints: indicate of cell should continue dividing or undergo apoptosis
Explain the S phase
Synthesis Phase
DNA duplicated: 23 -> 46
explain the M phase
(each chromosome has doubled)
Mitotic phase:
Made of 4 stages:
Prophase, Metaphase, Anaphase, Telophase
then have cytokinesis (splitting of cytoplasm into 2)
Explain what occurs in prophase
- nucleolous dissapears (dont need DNA that codes for ribosomes cell division)
- chromatin condenses, linked by centromere
- cytoplasmic microtubules dissasemble (and form mitotic spindle)
- mitotic spindle forms at two centrosomes (at opposing ends of cell)
explain what occurs in metaphase
pro-metaphase
- nuclear envelope dissolves
- microtubules form a radiating array
- microtubules attach to the centromere of the chromosomes at the kinetochore (part
- centrosome moves towards opposing ends of cell
Metaphase
- chromosomes are aligned by mitotic spindle towards the centre of the spindle called the metaphase plate
how do kinetochores form
one microtubule from one side attaches to kinetochore on one side of one sister chromatin. another microtubule attaches to other side of other kinetochore on other sister chromatid
explain anaphase
Anaphase:
- mitotic spindle microtubules start to retract
- pulls sister chromatids apart through kinetochores
- spindles move further apart from the poles
MOST CRITICAL PART OF MITOSIS WHICH ENSURES EQUAL DISTRIBUTION OF CHROMOSOMES BETWEEN CELLS
what is cohesin?
What is non-disjunction?
cohesin: protein complex that regulates seperation of sister chromatids during cell division. hold chromatids together till anaphase
non-dysjunction: one sister chromatin wont let go of other sister chromatin -> both get carried to one side = get one extra / less chromosome
chesionpathies - leads to extra / less chromosomes due to non-dysjunction. monosomies . trisomies e.g. D/S
explain telophase and cytokinesis
Telophase
- chromosomes reach poles of mitotic spindle
- mitotic spindle dissapears
- new nuclear envelope
cytokinesis:
- actin and myosin form contractile ring - pinches cytoplasm until splits (after forming a cleavage furrow)
-
how many phases are there in meiosis? explain
what does error in meisois lead to?
meiosis I: (2N to N). (PMAT I) homologous chromosomes are separated.
meiosis II: (PMAT II). sister chromatids are seperated
errors in meisosis: aneuploidy - leading cause of miscarriage
explain meiosis prophase I
(same as in mitosis but..)
- homologous chromosomes align at the centre of the cell (insead of sister like in mitosis)
- homologous chromosomes are joined at points called chiasmata
- crossing over / recombination can occur at chiasma
-
- explain process of crossing over / recombination. when does it occur?
- explain what independent assortment. when does it occur?
Crossing over
MAIN SOURCE OF GENETIC VARIATION BETWEEN GENERATIONS = zygote
- essentially a balanced translocation
- occurs at chiasmata
- can occur in prophase or metaphase
Independent assortment
- sister chromatids during metaphase II, are randomly allocated from one side to the other
explain metaphase and anaphase I (for meiosis)
Metaphase
- same as mitosis
- Homologous chromosomes move to centre of cell and face opposite ends of spindle (randomly = source of variation)
Anaphase
- same as mitosis
- sister chromatids remain attached at this stage so each pole gets two copies of the same chromosome
explain telophase and cytokinesis I (for meisois)
very similar to telophase and cytokinesis of mitosis. BUT can have:
cells can undergo interkensis / interphase II: no DNA replication occurs during this stage
what is difference between meiosis I and meiosis II? (general)
during anaphase in meisois II, get seperation of sister chromatids rather than homologous chromsomes
meisois II: more similar to mitosis
when can non-disjunction occur?
meisois I or II
(also in mitosis)
when does
which is most common non-disjunction disease?
trisomy 21 - Down Syndrome
how is the cell cycle controlled? name and explain the checkpoints
- very tightly regulated (if escape leads to cancer)
G1 checkpoint: end of G1. controls if cell enters S phase. is environment favourable? checks for growth factors, nutrients, cell size and DNA damage
G2 checkpoint: end of G2. environment favourable? is all DNA replicated? checks for cell size, DNA damage and DNA replication
Metaphase checkpoint: are all chromsomes attached to spindle?
what happens if cell fails checkpoint?
cell cycle can be stopped (senesence) or die by apoptosis
which proteins regulate cell divison?
- Cyclins (cylin A,B, D) - allow cell cycle to continue
- cyclin dependent kinases (CDK) add P groups to cyclin -> activates the cyclins
3. cyclin dependent kinases inhibitors (CDKIs)
depending on if CDK or CDKI is more prominant - regulates cell cycle
when is each cyclin actived?
- G1 checkpoint passed: activates cyclin D (regulates early G1 phase) and cyclin E (regulates early G1 phase and triggers S phase)
- S checkpoint passed: activates cyclin A (cyclin A acitvates DNA replication in S phase and movement into G2 phase)
- G2 / M checkpoint passed: activates cyclin B (takes cells into mitosis)
what is p53? why important?
P53 - guardian of the genome!
- blocks progression of cell cycle by of synthesis p21 (a CDKI) which inhibits CDK2 ( which inhibits activation of cyclin E and A)
- is an important tumour suppressor gene
explain retinoblastoma
(dont need to know in detail)
mechanism:
- retinoblastoma protein (Rb): tumour suppressor. generally switched on by p53/p21 & switched off by cyclin E/CDK
- Rb prevents DNA syn by binding E2F protein and preventing from going into S phase.
- mutation leaves uncontrolled cell division
who?
- young children
how does chemotherapy work? (3)
prevents cancer cell cycle from dividing:
a) most are anti-mitotic (prevents mitotic spindle forming). not selective - kills normall cells
b) biologicals - selective drug that target proteins essential for growth (e.g. CDKs). monoclonal antibodies
c) anti-hormonal drugs - block production of hormones of their receptors.
anti-mitotic drugs?
- anti microtubule agents. inhibit assembly
- vinca alkaloids and taxanes are two main groups.
- (need to know other mitotic blockers?)
