Cell Cycle, Mitosis, Meiosis, Gametogenesis Flashcards
What is mitosis and which cells carry out mitosis?
somatic cell division
results in two daughter cells, each with chromosomes and genes identical to those of the parent cell
How many chromosomes do somatic cells have?
Somatic cells are diploid cells with 2n chromosomes = 46
What is meiosis and which cells carry it out?
occurs only in cells of the germline.
Meiosis results in the formation of reproductive cells (gametes)
How many chromosomes do gametes have?
Gametes have 23 chromosomes - one of each kind of autosome and either an X or a Y.
Gametes are haploid (n chromosomes = 23)
What is genetic recombination/crossing over?
homologous segments of DNA are exchanged between non-sister chromatids of each pair of homologous chromosomes, thus ensuring that none of the gametes produced by meiosis will be identical to another.
Provides opportunity for genetic diversity in the next generation random segregation of chromosomes received from previous generation
The stages of an actively dividing cell’s life cycle are…
G1
S
G2
M
What happens in G1 of the cell cycle?
cell growth 1
the cell grows and cytoplasmic organelles are replicated. proteins are synthesized.
duration is hours to years depending on cell type.
no DNA synthesis (until next phase which is S)
What happens in the S phase of the cell cycle?
synthesis of DNA
formation of sister chromatids joined at the centromere to form the kinetochore (protein structure that assembles on the centromere and links the chromosome to microtubules polymers)
What happens during the G2 phase of the cell cycle?
cell growth 2
preparation for mitosis
What happens during the M phase of the cell cycle?
nuclear and cytoplasmic division
mitosis
What happens during the G0 phase of cellular life?
cell cycle arrest.
Cells that do not divide include neurons and red blood cells
What steps of the cell cycle make up interphase?
G1, S, G2
During this time chromosomes resemble fine fibers and are dispersed throughout the nucleus. Individual chromosomes are not recognizable.
What are G1 chromosomes made of?
one strand of DNA
This is before S phase
What are G2 chromosomes made of?
two strands of DNA (sister chromatids)
There are checkpoints along the way in the cell cycle. What are these checkpoints responsible for?
Determine the timing of each step in mitosis. Can halt the cell cycle progression until repairs are made or, if the damage is excessive until the cell is instructed to die by apoptosis
Genes can regulate checkpoints - like TP53
Medications can affect checkpoints
How long does interphase typically last vs mitosis?
Interphase - takes a total of 16 to 24 hours, whereas mitosis lasts only 1 to 2 hours
Describe steps of DNA replication
Double helix unwinds
Both strands serve as template for new DNA molecule
DNA polymerase adds new nucleotides to the new strand
DNA polymerase synthesizes from 5’ to 3’
Leading strand synthesized continuously
Lagging strand is synthesized in short segments which are
joined by ligase (Okazaki fragments)
Multiple origins of replication. ~ 10,000 in mammals
Heterochromatin replicates later than euchromatin
Correct maintenance of the ends of chromosomes requires a
special enzyme called telomerase, which ensures that the
very ends of each chromosome are replicated
Describe prophase
Gradual condensation
of the chromosomes, formation
of the mitotic spindle, and
formation of a pair of
centrosomes, from which
microtubules radiate and
eventually take up positions at
the poles of the cell.
Describe prometaphase.
Nuclear membrane dissolves.
Chromosomes attach to microtubules of the mitotic spindle.
Describe metaphase
Chromosomes are
maximally condensed and line up
at the equatorial plane of the cell
Describe anaphase
Chromosomes separate
at the centromere, and the sister
chromatids of each chromosome
now become independent
daughter chromosomes, which
move to opposite poles of the cell
Describe telophase
Chromosomes begin to
de-condense and a nuclear
membrane begins to re-form
around each of the two daughter
nuclei, which resume their
interphase appearance.
To complete the process of cell
division, the cytoplasm cleaves by a
process known as cytokinesis.
What type of mosaicism does mitotic nondisjunction lead to?
leads to chromosomal mosaicism in
tissues
Describe the process of meiosis.
Diploid cells give rise to haploid gametes
One round of DNA replication followed by two
rounds of chromosome segregation and cell
division - meiosis I and meiosis II
Sequence of events in male and female
meiosis is the same; however, the timing of
gametogenesis is very different
What is the result at the end of meiosis I?
2 sister chromatids in different cells
What is the result at the end of meiosis II?
4 haploid games
Describe genetic recombination.
Genetic recombination (also called meiotic crossing over) occurs in prophase I between non-sister
chromatids of each pair of homologous
chromosomes, thus ensuring that none of the
gametes produced by meiosis will be identical to
another
What are the stages of meiosis I?
prophase 1, metaphase 1, anaphase 1,
telophase/cytokinesis
Which chromosomes have crossover?
Each chromosome arm with exception of acrocentric p arms usually has at least one crossover
Recombination pattern and potential issue.
Recombination is suppressed near centromeres and increases greatly near telomeres in both males and females (but more so in males).
Because recombination involves the physical intertwining of the two homologues, it is also critical for ensuring proper chromosome segregation . Failure to recombine properly can lead to nondisjunction in meiosis I
Describe Meiosis II
- Meiotic interphase has no S phase (i.e., no DNA synthesis and duplication of the genome) between the first and second meiotic divisions (unlike mitotic interphase)
- In meiosis II, the sister-chromatids of each of the 23 chromosomes separate, and one chromatid of each chromosome passes to each daughter cell
- Due to crossing over in meiosis I, the chromosomes of the resulting gametes are not identical
Non-disjunction in meiosis pictograph
meiotic nondisjunction, particularly in oogenesis, is the most common mutational
mechanism responsible for chromosomally abnormal fetuses (aneuploidy)
Correlates to maternal age
When are primary oocytes developed?
by approximately
the third month of fetal development
What stage do oocytes stay arrested in at from the time of fetal development?
prophase of meiosis I
Approximately how many oocytes eventually mature out of the millions present at birth and are ovulated as part of a woman’s menstrual cycle?
400
What happens to the oocyte just before ovulation and then what is formed?
Just before ovulation, the oocyte rapidly completes meiosis I, dividing in such a way that one cell becomes the secondary oocyte (an egg or ovum), containing most of the cytoplasm with its organelles; the other cell becomes the first polar body
(Polar bodies remove one-half of the diploid chromosome set produced by meiotic division in the egg)
When does meiosis II begin and halt?
Meiosis II begins promptly after the secondary oocyte (egg) and first polar body are formed and proceeds to the metaphase stage during ovulation, where it halts again, only to be
completed if fertilization occurs
When does meiosis II complete?
Fertilization is followed by the completion of meiosis II,
with the formation of the second polar body (Polar bodies remove one-half of the diploid chromosome set produced by meiotic division in the egg)
Oogenesis and spermatogenesis table
