cell cycle and division Flashcards
what is the cell cycle?
phases a cell passes through to produce daughter cells by cell division
4 events of the cell cycle
cell division signals
DNA replication
DNA segregation
cytokinesis
cell division in prokaryotes
binary fission
- replication of entire, single celled organism
cell division signals usually external factors
eg nutrient concentration and env conditions
DNA replication in prokaryotes
replicate one circular chromosome
ori - where replication starts
ter - where replication ends
DNA segregation in prokaryotes
when replication complete, ori regions move to opposite ends of the cell
daughter chromosomes are segregated
cytokinesis in prokaryotes
cell membrane pinches in
protein fibres form a ring
new cell wall materials synthesised
= separation of 2 cells
cell division in eukaryotes
mitosis or meiosis
DNA replication in eukaryotes
more complex
have more than one chromosome
replication starts at many origins on each
limited to one part of the cell cycle
interphase
nucleus visible
cells functions occur
3 subphases
G1, S, G2
G1 phase (interphase)
chromosomes are single (unreplicated)
duration variable
ends at G1 to S transition when commitment made to DNA replication and cell division
S phase (interphase)
DNA replicates
sister chromatids remind together until mitosis
G2 phase (interphase)
cell prepares for mitosis
eg synthesises structures that move the chromatids
M phase
includes mitosis and cytokinesis
mitosis in eukaryotes
leads to production of 2 nuclei
genetically identical to each other and parent
subdivided into prophase, pro metaphase, metaphase, anaphase and telophase
basis of asexual reproduction
single celled organism reproduces itself with each cycle
meiosis
occurs in sexual reproduction
- creates gametes
offspring not genetically identical
2 nuclear divisions occur
but DNA only replicated once
chromosome number reduced from 2n to n
ensures each haploid products has a complete set of chromosomes
how does meiosis create genetic diversity?
raw material of evolution
through recombination
- crossing over = exchange of genetic material between non-sister chromatids
independent assortment of alleles
- matter of chance how homologous chromosomes line up in anaphase 1 and which go to which daughter cell = random combination
errors in meiosis
results in inherited disorders
can be visualised and detected on a karyotype
chromosome number
chromosome structural rearrangements
abnormalities in chromosome number
due to nondisjunction
- pairs of homologous or sister chromosomes fail to separate during meiosis
different outcome depending on if occurs in meiosis 1 or 2
result of nondisjunction in meiosis 1
2 gametes lack that chromosome
2 gametes have 2 copies of the chromosome
result of nondisjunction in meiosis 2
1 gamete lacks the chromosome
2 normal gametes
1 gamete with 2 copies of the chromosome
euploid
having appropriate number of chromosomes
aneuploid
having wrong number of chromosomes
monosomy
loss of one chromosome
usually fatal to zygote
trisomy
having an extra chromosome
if it occurs in small chromosomes it can be viable
chromosome structural rearrangements
can be partial duplications, deletions, inversions and translocations that occur during crossing over
can also occur due to errors in DNA repair
not all chromosome rearrangements result in disease
some can contribute to genetic variation
sexual life cycles
evolution has generated many different versions
all involve meiosis to produce haploid cells
meiosis and fertilisation alternate
haploid cells alternate with diploid cells
relative length varies between organisms
diploid advantages
can repair DNA damage using other chromosome
recessive mutations can be masked
more rapid evolution possible
greater genetic diversity of immune gene alleles
haploid advantages
more efficient cell cycle allows faster division
more rapid growth
survival in resource poor conditions
alternation of generations (heterogamy)
alternate between sexual and asexual reproduction
but both forms are diploid
