Cell division Flashcards
Human cells are diverse….
Sperm cell, SMooth muscle cell, Red blood cell, Nerve cell, epthelial cell
A cell has three possible destinies:
Live and function without dividing
* grow and divide
* die
Various signals tell a cell which path to take
Homeostasis is maintained when there is balance
between cell proliferation and cell death
The cell theory states that..
all organisms are made of cells and all cells come from pre-existing cells
Somatic cell division
Mitosis –diploid (2n) to diploid (2n)
Reproductive cell division
Meiosis -diploid (2n) to haploid (1n)
Why do somatic cells divide?
Growth and development, tissue renewal.
Results in two daughter cells that are genetically and
functionally identical to the parent cell.
Do all somatic cells divide?
Many, but not all, some a lot more than others!
What are somatic cells doing most of the time?
Going about their functions, within interphase
The eukaryotic cell cycle - Interphase
G1: Growth or Gap Phase 1
most cellular activities are occurring here
Duration variable – cell type specific
S: Synthesis of DNA
DNA replication occurs strands are separated at the
hydrogen bonds holding the nucleotides together new
strand of DNA is synthesized opposite each of the old
strands.
G2 : Growth or Gap Phase 2
checks for correct DNA synthesis prepares for the Mitotic
Phase (synthesis of the proteins and enzymes required,
gathering of reactants), replication of centrosomes is
completed
The eukaryotic cell cycle - Interphase
Objective
One
Most nerve cells are in..
G0
Mitotic phase =
mitosis + cytokinesis
Prophase Early Late
Metaphase
Anaphase Early Late
Telophase and
cytokinesis
In some textbooks you will see prometaphase (it is shown here as late prophase)
Interphase mitosis
Uncondensed chromosomes
Prophase mitosis
Early; two sister chromatids
Late- condensed chromoseoms + spindle tracks- fragments of nuclear enevlope
Metaphase mitosis
condensed chromosomes align at equator
anaphase mitosis
seprated chromosomes
telophase mitosis
nuclear enevlope forming
cleavage furrow
What is a sister chromatid?
Human diploid cells have 46 chromosomes, 23 from each parent
During interphase, DNA replicates
Two identical chromatids per chromosome
These are called sister chromatids
During anaphase, sister chromatids separate
before the nuclear envelope reforms in
telophase
Daughter cells are “identical” to parent cell
During prophase, DNA condenses
During metaphase, chromosomes “line” up
mitosis result
Result:
two daughter cells that are genetically identical to the parent cell
mitotic cell cycle checkpoints
G1 check
M check
G2 check
multiple signals are required to pass…
G1 & M checkpoint
G1 checkpoints:
- Is the DNA undamaged?
- Is cell size and nutrition OK?
- Appropriate signals present?
If not - may exit to G0
M checkpoint
are all chromosomes
attached to spindles?
if not stop singal is received
Meosis occurs in the..
gonads (ovaries and testes)
Meosis produces gametes which are
haploid (a single set of 23 chromosomes
fertilisation then restores the diploid number of chromosomes (2n)
produces cells genetically different from the parent cell
There are two stages of Meiosis
Mesosis I
Meosis II
Meosis I
Prophase I (synapsis and crossing over, tetrads form)
Metaphase I (pairs of homologous chromosomes)
Anaphase I (sister chromatids remain attached)
Telophase I
Meiosis I – separates homologous chromosomes
Synapsis: two sister chromatids
of each pair of homologous
chromosomes pair up
The 4 chromatids are called a
tetrad
Non-sister chromatids within
these tetrads may then cross
over: recombination
Meiosis II – separates sister chromatid
very similar to
mitosis,
except not
preceded by DNA
replication
Mitosis versus Meiosis
Mitosis- occurs in both haploid and diploid cells
Meosis- occurs only in diploid cells
Mitosis; DNA repication occurs in interpahse before mitosis begind
Meosis; Ocurrs during interphase before Meosis I but not meosis II
Number of divsions
Meosis: 2
Mitosis- 1
Synapsis of homologus chromosomes
Meosis- occurs in prophase I along with crossing over between non sister chromatids ; resulting chiasmata hold pairs together due to sister chromatid coehession
Mitosis; no
Number of daughter cells & genetic composition
Meosis- 4 haploid genetically diff from parent cells and eachother
Mitosis- 2 genetically identical to parent cells with same umber of chromsomes
Sources of variation
Sources of genetic variation as a result of meiosis:
* independent assortment at metaphase I (223 >8 million possible combinations)
* crossing over at prophase 1 (~1-3 crossover events per pair)
* Fusion between two gametes (> 223 times 2 23 combinations!!
