Cellular reproduction Flashcards
effect of mitosis on allele frequency
none
meiosis allele frequency
fair
half gametes are A, other half a (of a heterozygote)
binary fission
the circular bacterial DNA molecule is attached by proteins to the inner membrane. DNA replication begins at origin of replication and proceeds bidirectionally around the circle. newly synthesised DNA molecule is also attached to the inner membrane. cell elongates symmetrically during replication, separating the DNA attachment sites
synthesis of cell membrane and wall at midpoint
daughter cells separate
bacterial genome organisation
highly organised-replication
highly expressed genes cluster near origin of replication
genes mainly on leading strand to prevent DNA polymerase and RNA polymerase clashing
genes with similar functions cluster and are co-expressed (operons)
karotype
portrait formed by the number of shapes of chromosomes representative of a species
chromosome structure
sister chromatids connected by centromere (gene poor domains)
telomeres are stable ends of chromosomes
kinetochore (spindle attachment)
G1 phase
size and protein content of cell increased
many regulatory proteins made and activated
G0 phase
only in cells that are not actively dividing
s phase
duplication of chromosomes
each chromosome has 2 sister chromatids
prophase
chromosomes condense
centrosomes duplicated, begin to migrate to opposite poles and radiate microtubules
prometaphase
microtubules attach to chromosomes at kinetochores, growing and shrinking to explore nucleus
nuclear membrane breaks down
kinetochores
2 protein complexes associated with centromeres (one on each side of the centromere)
each complex associated with a sister chromatid
site of microtubule attachment
metaphase
chromosomes align in a single plane that is roughly equidistant from both of the spindle poles (metaphase plate)
anaphase
sister chromatids separate
centromere divides
telophase
nuclear envelope reforms and chromosomes decondense
nuclear envelope reforms around each set of chromosomes
cytokinesis
division into 2 daughter cells
ring of actin filaments (contractile ring) forms against inner face of cell membrane at cell equator
ring contracts, pinching cytoplasm
cytokinesis in plants
phragmoplast formed during telophase in the middle of the cell. consists of overlapping microtubules that guide vesicles containing cell wall components to the middle of the cell
vesicles fuse to create a new cell wall during late anaphase/telophase called the cell plate
fuses with original cell wall
number of chromosomes vs number of DNA molecules
chromosomes-number of centromeres
DNA molecules- involves sister chromatids
meiosis 1
separation of homologous chromosome pairs
meiosis 2
separation of sister chromatids
similarity to mitosis suggests meiosis evolved from mitosis
prophase 1
synapsis: homologous chromosomes line up to form bivalents
Crossing over
chromosomes condensed
nuclear envelope broken down
meiotic spindle
crossing over
recombination
random
helps hold bivalents together
chiasma form
prometaphase 1
nuclear envelope broken down
meiotic spindles attach to kinetochores
metaphase 1
bivalents move to centre randomly (in terms of which side maternal and paternal go)
anaphase 1
2 homologous chromosomes of each bivalent separate
no splitting of centromeres
23 pairs at each pole
reductional division
telophase 1
cytoplasm divides
chromosomes dont completely decondense
nuclear envelope can briefly reappear
prophase II
chromosomes fully recondensed
nuclear envelope disappears
spindle forms
Prometaphase II
spindle attaches to kinetochores
metaphase II
chromosomes line up
anaphase II
centromere splits
chromatids to opposite poles
telophase II
chromosomes uncoil
nuclear envelope reforms
cytokinesis
equational division (cells in meiosis II have the same number of chromosomes at the beginning and end)
sources of variation
crossing over
independent segregation
gamete fusion/fertilisation
independent assortment
metaphase 1
different ways the homologous chromosomes align
effect of meiosis on allele frequencies
none, fair
it can affect frequencies of different combinations of alleles at different loci
cyclins
regulate cell cycle
activate enzymes called cyclin-dependent kinases which promote cell division
G1/S cyclic-CDK complex
active at end of G1
promotes expression of histone proteins needed for packaging the newly replicated DNA
prepares cell for S phase
S cyclin-CDK complex
initiates DNA synthesis
activates protein complexes involved in DNA replication that contain enzymes necessary for DNA synthesis.
M cyclin-CDK complex
initiates multiple events associated with mitosis
breakdown of nuclear envelope during prophase
formation of mitotic spindle
checkpoints
- DNA replication checkpoint (check for presence of unreplicated DNA at end of G2)
- DNA damage checkpoint (before S phase)
- spindle assembly checkpoint (all chromosomes attached to spindle)
DNA damage checkpoint
When DNA is damaged by radiation, a specific protein kinase is activated that phosphorylates the p53 protein.
Under normal conditions, p53 is exported from the nucleus at very low levels.
When phosphorylated, p53 is prevented from being exported and its levels in the nucleus rise.
As p53 levels rise, p53 activates the transcription of a gene that expresses a CDK inhibitor protein.
This inhibitor binds to and blocks the activity of the G1/S cyclin–CDK complex.
This arrests the cell at the G1/S transition, giving the cell time to repair the DNA damage.
