L2: gametogenesis Flashcards
gamete are what type of cells?
haploid sex cells (oocytes & sperms)
gametogenesis is characterized in 4 stages
- The extraembryonic origin of the germ cells and their migration into the gonads.
- An increase in the number of germ cells by mitosis.
- A reduction in chromosomal number by meiosis.
- Structural and functional maturation of the eggs and spermatozoa
when are they ready for fertilization?
till puberty
What is the difference between mitosis and meiosis role in Gametogenesis?
- Mitosis > increase in the number of germ cells.
* Meiosis > reduction in chromosomal number.
When is the differences between male and female Gametogenesis start to exist?
The first phase of gametogenesis is identical in males and females,
whereas distinct differences exist between the male and female patterns in the last three phases.
Phase 1: Origin and Migration of Germ Cells: what happens?
- during development (proliferation) Primordial Germ > layers forming small disc > each layer of the disk develops to different systems of the body
- This procedure is very intricate anything can go wrong causing a defect
What could go wrong in phase 1?
Teratomas > when primordial germ cells stray and move to other places rather than the gonads
What is the chromosomal theory of inheritance?
It identifies chromosomes as the carriers of genetic material
who developed the chromosomal theory?
Sutton and Boveri in 1902.
How many genes do humans have?
35,000 genes on 46 chromosomes
what are autosome?
22 pairs of matching chromosomes
- 1 pair is of sex chromosome
each gamete contains how many chromosome?
Each gamete contains haploid number of 23 chromosomes
Why do we mean by linked genes?
Genes that are on the same chromosome
What is karyotyping
It’s a map of chromosomes
What can we define using karyotyping?
- if there is any defect
* the sex of the fetus
When does each chromosome replicate it’s DNA?
Before entering mitosis
How many chromosomes does each daughter cell receives?
the complete complement of 46 chromosomes- diploid progeny
how does The pattern of mitotic divisions differs from male and female germ cells
Female > high surge of mitosis five days after conception
Male > the surge happens after puberty
what happens in interphase
- uncoiled chromatin
- its the longest phase of mitotic division
what happens in prophase?
- condensation & duplication
- centrosome appears
- nucleus breaks down
what happens in metaphase?
- microtubules help align & pull chromosomes
- they align in the metaplate (in the middle) for equal segregation
what happens in anaphase?
- sister chromatids move away
what is the result in mitosis
results in 2 exact copies of the parent cell.
What is meiosis?
Cell division that takes place in germ cells to generate male and female gametes.
How many cell divisions are required to reduce the number of chromosomes to haploid (number
of 23)?
Requires two cell divisions.
Define homologous chromosomes
a pair of 2 chromosomes each pair with two chromatids
(Homologous chromosomes in meiosis one are haploid but the sister chromatids are
still together so we need another division to separate them)
When does each of the sister chromatids separate and why
each then separates into two daughter cells in meiosis two > resulting in a total of four
haploid daughter cells (each gamete then contains 23 chromosomes)
Why are the homologous chromosomes different than the parent cell?
Due to Crossover
What do you mean by crossover ?
- critical events in meiosis I
- interchange of chromatid segments between paired
homologous chromosomes.
What is the role of crossover in meiosis?
It enhances genetic variability
How is the genetic variability enhanced in meiosis?
- crossover
* random distribution of homologous chromosomes to the daughter cells
What are the results of ooginia ( one primary oocytes)?
- It gives rise to four daughters cells > each 22 plus 1 X chromosomes
- But only one develops into a mature gamete
o One mature gamete
o three polar bodies
How is spermatocyte similar to ooginia ?
In which it gives four daughters cells; however, all are mature gametes!
o two 22 plus 1 X
o two 22 plus 1 Y
What are the difference between ooginia and spermatogonia?
- Numerous oogonia undergo programmed degeneration called atresia until menopause
- Spermatogonia maintain the ability to divide throughout postnatal life
when does the number of germ cells peaks?
from 2nd to 5th month of pregnancy (due to mitosis)
For how long does ooginia undergo mitotic divisions?
Till the end of the third month
What happens in the end of the third month of pregnancy?
The ooginia stop undergoing mitotic division and arrange in clusters surrounded by a
layer of flat epithelium.
What is a primordial follicle?
It is Primary oocytes together with its surrounding flat epithelial cells.
when do primary oocytes starts prophase?
At birth, all primary oocytes have started prophase of meiosis I.
At birth, Do the primary oocytes completes meiosis one?
No they do not enter metaphase they enter diplotene stage until puberty.
how many oocytes are present by puberty
Most oocytes become atretic (die)during childhood- only 40,000 are present by puberty
and less than 500 will be ovulated
Why do the oocytes that reach maturity (ovulated late) late in life has a
higher probability to cause defects to the fetus?
Because the oocytes could have been dormant for 40 years or more before ovulation
• the longer the oocytes are arrested in meiosis the more
mutations
When does the primary oocyte complete
the first meiotic division?
At the time of ovulation
What results from the first meiotic
division?
The first polar body (non-functional cell) and a secondary oocyte (n).
The secondary oocyte arrest at?
In metaphase,2nd meiotic division
All Primordial follicle start meiosis 1?
Yes, all start meiosis one, however, not all of them complete it
All of oocyte will continue meiosis 1?
No, only for oocyte that need to be ovulated.
All secondary oocyte will complete meiosis
2?
No, only one which is fertilized by a sperm.
What results from the second meiotic
division (only when sperm is present)?
An ovum or a zygote + a second polar body
Which results from the fuse of the ovum
and sperm?
A zygote
What causes the completion of second
meiotic division?
Sperm penetration (fertilization).
What happens to the primordial follicles at puberty?
pool of growing follicles is established from primordial
follicles.
- Each month 15 to 20 follicles selected from this pool begin to mature
The selected follicles pass through three stages? what are the stages?
- Primary (preantral)
- Secondary (antral)
- Preovulatory (Graafian follicle)
What happen to the follicular cells surrounding the primary oocyte as 1*
oocyte begin to grow? and why ?
They change from flat to cuboidal > produce stratified epithelium of granulosa cells
- primordial follicle >
- growing follicle (secondary)à zona pellucida >
- vascular follicle (tertiary, Graafian follicle)àzona pellucida & Theca follicle
When does meiosis in males begin?
Begins after puberty.
in contrast to primary oocytes, not all
spermatogonia enter meiosis at the same time
First spermatogonium enters the meiotic phase as
primary spermatocytes (they spend several weeks dividing)
whats the result of primary spermatocytes?
This results in 2 secondary spermatocytes which enter the second meiotic division
Finally, four haploid spermatids are made
how long does the whole cycle of meiosis in males take?
This cycle takes 64 days
what happens in maturation of the sperm?
- formation of the acrosome
- Condensation of the nucleus.
- Formation of the neck, middle piece, and tail.
- Shedding of most of the cytoplasm as residual bodies
What is the importance of acrosome?
It covers half of the nuclear surface and contains enzymes to assist in fertilization
what’s the importance of maturation?
Important for mortality
what is nondisjunction
when Chromosomes fail to separate during meiosis.
what an example of nondisjunction?
One haploid gamete contains both members of chromosomal pair for a total of 24 chromosomes whereas the other haploid gamete has only 22
Define aneuploidy
General term for abnormal number of chromosomes in an embryo.
Why is Down syndrome known as trisomy 21?
Because there is an extra copy of chromosome 21
What are the symptoms of down syndrome?
- delayed growth
- distinct facial characteristics
- intellectual disability
What is the cause of down syndrome?
• Results from having 3 copies of chromosome 21
What increases the risk of having a child with down syndrome?
• The increase with the maternal age
Patients with down syndrome suffer from?
• Premature aging, thyroid dysfunction, infections
What are the symptoms of Trisomy 18?
- mental retardation
- conginetal heart defects
- low-set ears
- flexion of fingers and hands
What is the incidence rate of trisomy 18 ?
incidence rate is 1:5000
What is the estimated age for patients with trisomy 18?
• 2 months of age
What are the symptoms of trisomy 13?
- show cleft lip and palate
- eye defects
- Deafness
- mental retardation
What is the incidence rate of trisomy 13 ?
incidence rate is 1:20,000
What is the estimated age for patients with trisomy 13?
More than 90% die in their first month after birth
Ways to determine whether the patient has a
Chromosomal Abnormality
- Karyotyping
* Fluorescence in situ Hybridization (FISH)
Defects can occur also due:
- Family history
* relative marriage
