Gametogenesis Flashcards
(56 cards)
1
Q
What is meant by a somatic cell?
A
A cell that contains 46 chromosomes arranged as 23 pairs
- there are 22 pairs of matching chromosomes (autosomes) and 1 pair of sex chromosomes
- these are known as diploid cells
2
Q
What is a haploid cell?
A
A gamete (sperm or oocyte) is a haploid cell as it contains only 23 chromosomes
This is half the number of chromosomes as a somatic cell
3
Q
What type of cell are the only cells in the body not to be a somatic cell?
A
Gametes are the only cells in the body that are not somatic cells
4
Q
When does development begin?
A
Development begins following fertilisation
- this occurs when the male gamete (sperm) unites with the female gamete (oocyte) to form the zygote
5
Q
What actually is gametogenesis?
When does it occur in relation to fertilisation?
A
it is the conversion of primordial germ cells into male and female gametes
- it occurs in preparation for fertilisation
- it involves meiosis to reduce the number of chromosomes allowing diploid precursor cells to become haploid gametes
6
Q
What are the “4 M’s” of gametogenesis?
A
- Migration - of PGCs to the gametes
- Mitosis - an increase in the number of PGCs
- Meiosis (I & II) - to reduce the chromosomal numbers
- Maturation - of gametes
7
Q
What cells give rise to gametes?
Where are these formed from and where do they migrate to?
A
Gametes are derived from primordial germ cells (PGCs)
- they are formed from the epiblast layer of the bilaminar disc during week 2
- they move through the primitive streak during gastrulation to migrate to the wall of the yolk sac
- they remain in the yolk sac for a number of weeks
8
Q
Where do the PGCs migrate to after residing in the yolk sac?
What process do they undergo whilst migrating?
A
- during week 4 they migrate from the yolk sac towards the developing gonads
- they pass through the gut tube and across the dorsal mesentery to reach the primitive gonads by the end of week 5
- as they migrate towards the primitive gonads from the yolk sac, they undergo mitosis to form genetically identical cells
9
Q
What is a teratoma?
How can they form during migration of PGCs?
A
- a teratoma is a benign (usually) tumour that contains tissue from all 3 germ cell layers
- it often contains skin, hair, cartilage, brain tissue and/or teeth
- they are formed when PGCs are misdirected to extragonadal sites
- this describes areas outside of the genital ridge
- PGCs are pluripotent and have the ability to develop into many different cell types
10
Q
What are the 2 most common types of foetal teratoma?
Will those affected still have genitals?
A
- sacrococcygeal is the most common, followed by oropharyngeal
- if someone has a teratoma, the gonads will still develop if there is migration of some PGCs to the genital ridge
- if there are no PGCs reaching the genital ridge then the gonads will not develop
11
Q
What is mitosis?
What type of cells does it produce?
A
- the process by which one cell divides to produce 2 genetically identical daughter cells
- each cell receives the complete complement of 46 chromosomes
12
Q
What are the 4 stages of the cell cycle?
What proportion of this cycle is taken up by mitosis?
A
G1 phase
- growth phase in preparation for DNA replication
S phase
- replication of DNA to form 2 sister chromatids attached at the centromere
G2 phase
- growth phase in preparation for mitosis
M phase
- mitosis occurs to produce 2 genetically identical daughter cells, each containing 46 chromosomes
- this is a relatively short component of the cell cycle as most of it is spent in interphase
13
Q
What mnemonic can be used to remember the 4 stages of mitosis?
A
PMAT
- P - prophase
- M - metaphase
- A - anaphase
- T - telophase
14
Q
What occurs during prophase?
What does each chromosome consist of at this stage?
A
- the chromosomes condense and become visible
- the nuclear membrane disintegrates
- each chromosome consists of 2 sister chromatids joined at the centromere
- chromosomes continue to condense, shorten and thicken but chromatids only become distinguishable at prometaphase
15
Q
What happens during metaphase?
A
- the chromosomes line up along the equatorial plane of the cell
- each chromosome is attached by its centromere to the centriole via microtubles, forming the mitotic spindle
16
Q
What happens during anaphase?
A
- the centromere of each chromosome divides
- the sister chromatids are pulled towards opposite ends of the cell by the spindle (microtubles)
17
Q
What happens during telophase?
A
- the chromosomes uncoil and lengthen
- a nuclear membrane forms around the chromosomes at opposite poles of the cell
- the cytoplasm divides (cytokinesis) to form 2 genetically identical daughter cells
18
Q
What are the main differences in mitotic ability in males and females?
A
- Mitosis of PGCs continue once they reach the genital ridge
In females:
- there is rapid mitosis of oogonia during months 2-5 of foetal development
- this peaks at around 7 million oogonia
- the majority of oogonia then undergo atresia (degenerate and break down)
- atresia of oogonia continues throughout life until the menopause
In males:
- spermatogonia maintain the ability to divide throughout life
- there is no rapid mitosis during development
19
Q
What are the 3 key differences between meiosis and mitosis?
In what way are they similar?
A
- meiosis reduces the number of chromosomes to produce haploid cells (23 single chromosomes)
- meiosis involves 2 successive divisions - meiosis I & II
- meiosis produces daughter cells that are genetically unique
- they are similar in the way that they are both preceeded by the S phase to replicate the DNA of each chromosome prior to division
20
Q
What is different about the way the chromosomes line up during prophase I?
A
- S phase occurs so that each of the 46 chromosomes is replicated into sister chromatids
-
homologous chromosomes align themselves in pairs in a process called synapsis
- replicated maternal and paternal homologues line up with each other
- Following synapsis, crosslinking occurs which allows maternal and paternal chromosomes to swap some genetic material
- the point where the crosslinking occurs is called a chiasma
21
Q
A
22
Q
How is genetic variability produced during meiosis I?
How many daughter cells are produced by meiosis?
A
- crosslinking redistributes genetic material between maternal and paternal homologues
- there is random distribution of homologous chromosomes to the daughter cells as the way the pairs align on the equator during metaphase I is random
- meiosis produces 4 genetically unique haploid daughter cells
23
Q
What is meant by oogenesis?
A
the process by which oogonia differentiate into mature oocytes
24
Q
What happens to PGCs once they reach the gonad of a genetic female?
A
- PGCs differentiate into oogonia, which then undergo a number of mitotic divisions
- by the end of month 3 they are arranged in clusters surrounded by a layer of flat epithelial cells (follicular cells)
- these are derived from the surface epithelium covering the ovary
25
How are primary oocytes formed?
* the majority of oogonia continue to divide by mitosis
* some oogonia reside in the **_prophase stage of meiosis I_** - these are **primary oocytes**
* they undergo **rapid mitosis until the 5th month** of development where the number of germ cells in the ovary reaches 7 million
* following this, the oogonia and primary oocytes begin to undergo **_atresia_**
26
Following atresia of oogonia and primary oocytes, which structures remain?
* all surviving primary oocytes have entered **prophase I of meiosis**
* they are individually surrounded by a layer of flat follicular epithelial cells to form a **_primordial follicle_**
27
Near the time of birth, what happens to the primary oocytes that remain?
* all primary oocytes have started **prophase I of meiosis**
* they **_do not_ proceed into metaphase**, but enter the **_diplotene stage_**
* this is a **resting stage of prophase** that the oocytes will remain in until puberty
28
How many primary oocytes remain at puberty?
What happens during a menstrual cycle?
* around **40,000** primary oocytes remain at puberty
* with each cycle, a number of follicles begin to develop but usually only one will reach full maturity
* *the others will degenerate and become atretic*
* this **secondary follicle** induces a **_surge in LH_**, which causes the primary oocyte to **_complete meiosis I_**
* there is formation of 2 daughter cells of unequal size - a **_secondary oocyte_** and the **first polar body** (receiving less cytoplasm)
29
What happens to the secondary oocyte after its formation?
* the cell enters meiosis II but **_arrests in metaphase II_** around **3 hours prior** to ovulation
***meiosis II is only completed if the _oocyte is fertilised_***
* the cell degenerates 24 hours after ovulation if fertilisation does not occur
30
What is meant by spermatogenesis?
When does this occur?
* this is the process by which spermatogonia are transformed into spermatozoa
* unlike oogenesis (which begins prior to birth), this begins at puberty
* the sperm cells complete meiosis II immediately
31
What are the 2 different types of chromosomal abnormalities?
chromosomal abnormalities can be **_numerical_** or **_structural_**
* numerical abnormalities are **_aneuploidies_** in which the cell contains an **abnormal number of chromosomes**
* structural abnormalities can include **_deletions_** and **_fragile sites_**
in a deletion, small parts of the chromosome are accidentally lost
a fragile site is a region of the chromosome that is susceptible to damage and often DNA loss occurs here
32
What are the 3 mechanisms by which Down's syndrome can occur?
What type of chromosomal abnormality is this?
* it is **_trisomy 21_** - a form of aneuploidy as each cell contains an extra chromosome so is not euploid
* 95% cases are due to **_non-disjunction_** occuring during **M1 or 2**
* 4% of cases are due to an **_unbalanced translocation_**
* 1% of cases are due to **_mosaicism_** (mitotic non-disjunction)
33
What are the features of Down's syndrome?
1. hypotonia
2. small nose with flat nasal bridge
3. small mouth with protruding tongue
4. eyes that slant upwards and outwards
5. flat occiput
6. sandal gap (large space between first and second toe)
7. broad hands with short fingers
8. single transverse palmar crease
9. below average weight and length at birth
* 1 in 10 will also have additional difficulties such as ASD or ADHD
34
Why do people with Down's syndrome often have a reduction in life expectancy?
Due to the **associated complications**, such as:
1. glue ear
2. 50% also have congenital heart defect
3. bowel problems (constipation, diarrhoea, coeliac disease, reflux)
4. visual impairments
5. hypothyroidism
6. increased risk of infections
7. increased risk of developing dementia at a younger age
35
What is meant by non-disjunction?
this is the **failure of homologous chromosomes to separate** during meiosis I
or of **sister chromatids to separate** in meiosis II or mitosis
this results in daughter cells with an **abnormal number of chromosomes** - **_ANEUPLOIDY_**
36
What is the result of non-disjunction occurring during meiosis I?
* a **pair of homologous chromosomes fails to separate**, leading to a dissimilar number of chromosomes in the second set of cells
* this unequal sharing of genetic materal means that **_all 4 daughter cells_** have an **abnormal chromosome number**
* 2 daughter cells will have an **_extra chromosome (24 or n+1)_** and 2 will be **_missing a chromosome (22 or n-1)_**
* this will result in either a **_trisomy_** (if a gamete with 24 chromosomes is fertilised) or a **_monosomy_** (if a gamete with 22 chromosomes is fertilised)
37
What is meant by mosaicism?
mosaicism is **non-disjunction** that occurs **_during mitosis_**
* this results in some daughter cells having an **abnormal chromosome number** (aneuploid) and some being **normal**
* the aneuploid cells continue to replicate and make up a **subpopulation of cells**
38
How can Down's syndrome occur due to mosaicism?
Why might someone have a varied expression of this condition?
* Down's syndrome due to mosaicism can occur:
1. in mitosis of a **normal zygote**
2. in mitosis of a **trisomy 21 zygote**
* there is **varied expression** of the symptoms of DS as there are **_2 different cell populations_** within the **same zygote**
* *as well as abnormal trisomy 21 cells, there are also normal cells*
39
How can mosaicism in a normal zygote lead to trisomy 21?
* the sperm and oocyte both contain **1 copy** of ch21
* fertilisation occurs to produce a **normal zygote** containing **2 copies** of ch21
* mitosis occurs to produce genetically identical cells
* **non-disjunction** occurs, leading to **1 cell containing 1 copy** of ch21 and the **other containing 3 copies**
* the cell containing 1 copy of ch21 does not survive
* the cell containing 3 copies **continues to divide** to produce a **_subset cell population_**
40
How can Down's syndrome result from mosaicism occurring in a trisomy 21 zygote?
* one of the gametes is **abnormal** and contains **2 copies of ch21**
* fertilisation occurs to produce a zygote containing **3 copies of ch21**
* non-disjunction occurs to produce a daughter cell with **4 copies of ch21** and the other with **2 copies of ch21**
* the cell with 4 copies of ch21 does not survive
* the cell with 2 copies of ch21 continues to divide and make up a **normal diploid cell population**
* there is also a **trisomy 21 cell population** from normal mitosis following fertilisation with an abnormal gamete
41
What is meant by a chromosomal translocation?
What is the difference between an unbalanced and a balanced translocation?
* translocations occur when a **chromosome breaks** and a **_piece of one chromosome attaches onto another_**
**_Balanced translocation:_**
* this occurs when there is **_no loss of genetic material_**
* breakage and reunion occurs between 2 chromosomes but as no critical genetic material is lost, the individuals are **often asymptomatic**
**_Unbalanced translocation:_**
* this involves **_loss or duplication of genetic material_**, resulting in an **altered phenotype**
42
How can Down's syndrome result from a translocation?
it can result from a translocation occurring in a **_14-21 carrier_**
43
What is meant by a 14-21 carrier?
* in an individual, the **_long arm of ch21_** breaks off and **_joins onto ch14_**
* this individual is **asymptomatic** as the translocation is balanced
* there is **_no loss of any genetic material_**
44
How can Down's sydrome result from a 14-21 carrier?
* if a parent has a **balanced 14-21 translocation**, they can pass this on to their offspring
* this results in **extra material from ch21**
* there is trisomy 21 as there is an **_extra copy of ch21 attached onto the ch14_** that has been inherited from the aysymptomatic parent
45
What is Turner syndrome?
What is significant about this condition?
There is **absence of the second X chromosome**, resulting in a **_45,X karyotype_**
It is the only monosomy that is compatible with life
46
By which 3 mechanisms can Turner's syndrome occur?
* non-disjunction of the male gamete
* structural abnormalities of the X chromosome
* mosaicism
* 98% of foetuses with this condition are spontaneously aborted
47
What are the key features of Turner's syndrome?
* female gender and appearance
* gonadal dysgenesis (failure of the ovaries to form)
* short stature
* webbed neck
* lymphedema of extremities
* broad chest with widely spaced nipples
48
What is meant by Klinefelter syndrome?
What symptoms does it tend to produce?
there is an **_extra copy of the X chromosome in males_**, producing an **47,XXY karyotype**
* individuals tend to be **asymptomatic** but may have some speech / learning difficulties and dystonia of the face
* clinical features are usually detected by amniocentesis and include:
* testicular atrophy
* sterility
* hyalinisation of the seminiferous tubules
* gynaecomastia
49
Why is there an increased incidence of chromosomal abnormalities with increased maternal age?
* from puberty, **10-30 primary oocytes complete meiosis I** each menstrual cycle
* some primary oocytes can be **_arrested in prophase I for 40 years_**
* primary oocytes are **_vulnerable to damage during M1 arrest_**
***The longer an oocyte is arrested in development, the greater chance of chromosomal abnormalities***
50
Why can problems with fertilisation also become more common with increasing maternal age?
* as more oocytes undergo atresia, the **follicle pool reduces over time**
* in older women, there is a **_smaller pool of remaining viable follicles_**
51
Why does the risk of Down's syndrome increase with maternal age?
* the **_rate of non-disjunction increases with age_** and this is responsible for 80% cases of DS
* this is related to the **length of time** that the primary oocyte is **arrested in the M1 stage** of development
52
Why does only one follicle become mature during each menstrual cycle?
What happens to the others?
* **10-30 primary follicles** begin the maturation process each cycle
* while meiosis resumes, the **follicular cells** surrounding the oocyte also u**ndergo maturation**
* only one follicle will mature to a **_Graafian follicle_** which can erupt from the ovary
* the other follicles undergo **atresia** to form **polar bodies**
53
How long does spermiogenesis take?
What 4 main changes have to occur during this process?
* the process takes around **74 days**
* several changes occur as the spermatid matures into a spermatozoom:
1. condensation of the nucleus
2. formation of the acrosome
3. formation of the neck, middle piece and tail
4. shedding of the cytoplasm
54
Where are developing sperm ejected into?
At what point do they become motile?
* developing sperm are ejected into the semiferous tubercles, which push them along towards the epidydimis
* they do not become motile until maturation is completed within the epididymis
55
How frequently are abnormal sperm formed?
What are the 4 most common abnormalities?
* unlike oocytes, abnormal sperm occur quite frequently (10%)
* the most common abnormalities are:
1. duplicated head or tail
2. absent tail
3. giant or dwarf sperm
4. immotility
56
How can the presence of abnormal sperm affect fertility?
* the effect of sperm morphology on fertility is not known
* **_sperm concentration_** and **_motility_** have a correlation with fertility
* **_subfertility_** can occur when there is a **small portion of abnormal sperm**, leading to increased difficulty with conception
