Lec 06- Meiosis, Germ Cells, and Fertilization Flashcards
(185 cards)
1
Q
Contains 2 sets of chromosomes, one from each parent
A
diploid
2
Q
Contains 1 set of chromosomes
A
haploid
3
Q
Specialized reproductive cells; sperm or eggs generated through meiosis
A
gametes
4
Q
Process by which haploid gametes fuse to form a diploid cell (zygote)
A
fertilization
5
Q
How does a zygote divide to become a multicellular organism?
A
divides by mitosis
6
Q
What are germline cells?
A
gametes and their precursors
7
Q
What cells form the rest of the body and leave no progeny?
A
somatic cells
8
Q
What are autosomes?
A
- chromosomes common to both sexes
- one from each parent in each diploid nucleus
9
Q
What are the 2 sex chromosomes?
A
X and Y
10
Q
What are 2 ways that meiosis creates genetic diversity?
A
- random segregation of homologs during meiosis
- crossing over
11
Q
Gametes are ______
A
haploid
12
Q
Gametes arise from meiosis which involved _______ cell divisions and ______ round of DNA synthesis to produce __________ the number of chromosomes
A
- 2 divisions
- 1 round of DNA synthesis
- half number of chromosomes
13
Q
When do the chromosomes replicate during meiosis I?
A
S-phase
14
Q
What binds the 2 sister chromatids in meiosis I?
A
cohesion complexes
15
Q
What happens to homologs during meiosis I?
A
They pair, recombine, and separate in meiosis I
16
Q
Is there DNA replication in meiosis II?
A
NO
17
Q
What happens to sister chromatids in meiosis II?
A
sister chromatids = divided (Anaphase II)
18
Q
What is the result of meiosis II?
A
4 haploid daughter cells
19
Q
What happens to the homologs in prophase I of meiosis?
A
homologs begin to pair
lasts days/weeks
20
Q
What is a bivalent?
A
the structure of 4 chromatids in prophase I of meiosis
21
Q
What is required for stable pairing of homologs in prophase I of meiosis?
A
crossing over at the chiasma
22
Q
What protein structure joins homologs in prophase I of meiosis?
A
synaptonemal complex
23
Q
What is the pseudoautosomal region?
A
- small region of homology between X and Y
- allows X and Y to pair
- prophase I of meiosis
24
Q
What are the 5 phases of prophase of meiosis?
A
- Leptotene
- Zygotene
- Pachytene
- Diplotene
- Diakinesis
25
What happens in the Leptotene phase of prophase?
homologs begin to condense/pair
26
What happens in the Zygotene phase of prophase?
- homologs pair
| - synaptonemal complexes form
27
What happens in the Pachytene phase of prophase?
- synapsis is complete
| - crossing over occurs
28
What happens in the Diplotene phase of prophase?
- synaptonemal complex begins to break down
| - homologs begin to separate but remain attached at chiasmata
29
What happens in the Diakinesis phase of prophase?
- reach max condensation
- separation of homologs
- transition stage into metaphase
30
What is the synaptonemal complex made of?
transverse filaments between homologs
31
What is the synaptonemal complex important for?
crossing over
32
When do cohesion complexes assemble on the DNA?
during S-phase
33
What do the cohesion complexes bind to when they assemble on DNA?
bind sister chromatids
34
What is the function of cohesion complexes?
- major component of axial core of each homolog
| - play important role in segregating homologs in meiosis I
35
Where do kinetochores attach?
on the 2 sister chromatids to the microtubules coming from the same pole
36
What phase of meiosis do homologs separate at?
anaphase I
37
When do the arms of sister chromatids separate in meiosis?
anaphase I
38
What can occur when the arms of sister chromatids separate in meiosis?
- chiasmata resolved
| - allows homologs to separate
39
When do sister chromatids separate in anaphase II of meiosis?
when separate cleaves cohesion complexes
40
What happens when human female meiosis is arrested for years after diplotene?
- Meiosis I = completed only at ovulation
| - Meiosis II = completed after fertilization
41
What is nondisjunction?
when homologs fail to separate properly
42
When are nondisjunction errors common?
during egg development
43
What happens to nondisjunction errors with age?
increase errors with age
44
What is the leading cause of spontaneous abortions/mental retardation in humans?
nondisjunction
45
What are aneuploid errors?
cells with abnormal chromosome number
46
What is the most common single cause of mental retardation?
Trisomy 21
47
In 90% of trisomy 21 individuals, the additional chromosome is from what origin?
maternal
48
What % of trisomy 21 occurs in meiosis I and meiosis II?
70% in meiosis I
30% in meiosis II
49
What does euploid mean?
cells with normal number of chromosomes
50
How man different gametes can random distribute of maternal/paternal homologs produce?
2^n
2^23 = 8.4x10^6
51
What increases genetic variation?
crossing over
52
How many crossovers occur per human homolog?
2-3
53
When do female mammals being meiosis?
in fetal ovary
54
When is meiosis I completed for female mammals?
at ovulation (until about 40 years later)
55
When is meiosis II completed for female mammals?
only at fertilization
56
When do male mammals begin meiosis?
at puberty (in the testes)
57
When is meiosis completed for male mammals?
24 days (proceeds without arrest)
58
What % of eggs are aneuploid?
20%
59
What % of sperm is aneuploid?
3-4%
60
What leads to apoptosis of abnormal sperm?
a cell cycle checkpoint that is activated in sperm with meiotic errors
61
What % of all human conceptions are aneuploid?
25%
62
What is human aneuploid due mostly to?
nondisjunction in oocytes at meiosis I
63
Which gametes are more prone to new DNA mutations?
male gametes
64
Why are male gametes more prone to new DNA mutations?
they undergo more mitotic cell divisions
65
What does advanced paternal age lead to?
increased risk of point mutations in male gametes
66
Where are primordial germ cells (PGCs) found? What do they give rise to?
- found in all vertebrate embryos
| - give rise to gametes
67
Where do the PGCs migrate to?
the developing gonads (ovaries/testes)
68
What do PGCs proliferate through?
mitosis then meiosis to differentiate into haploid gametes (sperm/egg)
69
What is different in some animal eggs compared to mammals?
some animal eggs contain specific molecules that determine which cells become germ cells
70
What do mammals rely on?
rely on neighboring cell signals to dictate which cells become PGCs
71
What is unique about the mammal dividing egg?
- it is totipotent for the first few rounds of cell division
| - then it differentiates into PGCs
72
What suppresses somatic cell fates in mammals?
gene repression
Translation of certain RNAs must be inhibited (Nanos)
73
How is the genital ridge formed?
cell signaling leads to proliferation and migration of PGCs to the developing gonad
74
Are PGCs committed to their cell fate?
NO, they are pluripotent
75
What does pluripotent mean?
cells can be removed and cultured with signaling proteins to form cell lines for any type of cell for the body except extraembryonic cells
76
What determines if the gonad is ovary or testes?
the sex chromosomes in the genital ridge
| NOT the sex chromosomes in the PGC
77
Are X patients with 45 chromosomes male or female?
female
78
What does the Y chromosome do?
directs genital ridge to develop into testis
79
What is the default sex determination pathway?
female
80
What is SRY?
sex determining region of Y
81
What is the function of the SRY gene?
necessary and sufficient for testis development in mammals
82
What can SRY gene cause?
sex-reversal
83
What will the XX mouse + SRY develop as?
male (sterile)
84
What will the XY male with inactivating SRY mutation develop as?
female
85
What does the expression of SRY cause?
cells to differentiate into Sertoli cells
86
What are sertoli cells?
testis support cells
87
What does SRY encode?
Sox9
88
What is Sox9?
- DNA binding protein that regulates transcription of genes involved in Sertoli development
- expressed in all vertebrates
89
What does Sox9 activate?
-activates Sertoli-specific genes (including anti-Mullertian hormone)
90
What results from ectopic expression of Sox9 in developing XX mouse gonads?
causes them to develop as males (even without SRY)
91
What does SRY do in somatic cells?
- directs differentiation into Sertoli cells instead of follicle cells
- induce Leydig cell differentiation
92
What do Sertoli cells secrete?
anti-Mullerian hormone
93
What does anti-Mullerian hormone do?
- suppresses female development
| - causes Mullerian duct to regress
94
What does the induction of Leydig cell differentiation do?
Secretes testosterone
95
What is the function of testosterone secreted from Leydig cells?
responsible for secondary sexual characteristics
96
What happens to gonadal development in the absence of SRY?
- genital ridge becomes an ovary
- PGC becomes an egg
- Somatic cells differentiate differently
97
What do the somatic cells differentiate into when there is no SRY?
- Follicle cells (support cells)
| - Theca cells (estrogen-producing cells)
98
What triggers egg activation?
fertilization (fusion of sperm and egg)
99
What is parthenogentic activation?
activation in absence of sperm
100
In what animals does parthenogentic activation occur?
some lizards and frogs
101
Why can't mammals do parthenogentic activation?
due to imprinting
102
What can the egg cytoplasm be used for?
can be used to reprogram a somatic cell nucleus to direct development of a new individual
REPRODUCTIVE CLONING
103
What is the nutritional reserve for eggs?
yolk sac
104
What is the yolk sac made of?
- lipids
- proteins
- polysaccharides
105
What are yolk granules?
discrete yolk-containing structures
106
What is the egg coat?
a glycoprotein-rich extracellular matrix
107
What does the egg coat do?
- protects from mechanical damage
| - acts as species-specific barrier to sperm
108
What is the egg coat called in mammals vs. non-mammalian eggs?
mammals = Zona pellucida
non-mammals = Vitelline layer
109
What happens upon egg activation?
- corticle granules release their contents
- contents enter egg coat
- block polyspermy
110
What are cortical granules?
specialized secretory cells
111
What are the stages of oogenesis?
1) Primordial germ cells migrate into gonad
2) Diploid oogonia undergo mitotic cell divisions then enter meiosis
3) Primary oocytes arrest at prophase I of meiosis I
4) Progression resumes at ovulation
5) Cytoplasm divides asymmetrically
6) Secondary oocyte arrested at metaphase II
7) Meiosis completed after fertilization
112
When are primary oocytes formed?
during gestation of human embryo
113
What results from the cytoplasm's asymmetrical division?
- small polar body
| - large secondary oocyte
114
How do eggs achieve large size?
-have extra gene copies in the cell
115
When does egg growth occur?
after DNA replication
116
Some species produce many copies of some genes like ______
rRNA
117
How are yolk proteins imported?
through receptor-mediated endocytosis
118
What do accessory cells (like nurse cells) do?
- make ribosomes, mRNA, and proteins
| - transport into oocyte (insect)
119
What do follicle cells connect to?
connect to each other and oocyte
120
How do follicle cells communicate?
though gap junctions
121
What is the primordial follicle?
oocyte surrounded by single layer of follicle cells
122
What are developing follicles?
multiple layers of follicle cells (granulosa cells) surrounding growing oocyte
123
What do some developing follicles acquire?
antrum (fluid-filled cavity)
124
What happens at puberty?
FSH surge induces antral follicles (one becomes dominant)
125
What triggers ovulation?
FSH + LH in the middle of menstrual cycle
126
What triggers primary oocyte to complete meiosis I?
FSH + LH in the middle of menstrual cycle
127
When is the secondary ooctye released (still surrounded by granulosa cells)?
when the follicle enlarges and ruptures
128
When does the secondary oocyte complete meiosis II?
only when fertilized
129
Why are sperm smaller than the egg?
optimized to carry DNA to the egg
130
Do sperm have cytoplasmic organelles?
NO
They have stripped down cells
131
What are the 2 distinct regions of sperm?
- Head (nucleus)
| - Tail (propulsion)
132
What is the mid piece of sperm packed with?
mitochondria
to power flagellum for movement
133
What do the dyenin motor proteins require in order to slide microtubules for sperm movement?
ATP hydrolysis
134
What enzymes does the acrosomal vesicles contain?
hydrolytic enzymes
135
What releases the hydrolytic enzyme?
released by exocytosis in acrosome reaction
136
What does hydrolytic enzyme do?
helps penetrate egg's outer coat
137
What results when primary spermatocytes complete meiosis I?
produce 2 secondary spermatocytes with 22 duplicated autosomes and X or Y
138
What does each secondary spermatocyte produce after going through meiosis II?
4 haploid spermatids
139
What do the 4 haploid spermatids differentiate into?
sperm
140
What do sperm escape into?
the lumen of seminiferous tubule
141
What happens in the epididymis?
sperm pass through it and are stored and undergo further maturation
142
Where does spermatogenesis occur?
in seminiferous tubules
143
What do the seminiferous tubules maintain close contact with?
sertoli cells
144
Spermatogonia undergo ______ mitotic divisions before entering meiosis I
limited
145
What happens to progeny of spermatogonium since they don't complete cytokinesis?
they remain connected through synctium
146
What are synctium?
cytoplasmic bridges that connect progeny of spermatogonium
147
The progeny of spermatogonium each share a common ________
cytoplasm
148
Sperm are produced in _________ batches
synchronous
149
What do sperm have access to for their development?
diploid genome
150
Order or sperm production
- spermatogonium (mitosis) (46)
- spermatogonia
- primary spermatocytes (meiosis I)
- secondary spermatocytes (meiosis II)
- spermatids (23)
- differentiating spermatids
- mature spermatozoa + residual bodies
151
What is capacitation?
process required to acquire capacity to fertilize an egg
152
How long does fertilization take in humans?
5-6 hours
153
Where is fertilization completed?
in oviduct
154
Why do sperm undergo many biochemical and functional changes during fertilization?
- increases motility of flagellum
| - makes sperm capable of undergoing acrosome reaction
155
What are the requirements for fertilization?
- Albumin
- Calcium
- HCO3
156
Where are all the requirements for fertilization located?
in female genital tract
157
What is the function of albumin for fertilization?
- helps extract cholesterol from membrane
| - increases membrane ability to fuse with acrosome membrane
158
What is the function of calcium and HCO3 for fertilization?
activate adenyl cyclase in sperm to produce cAMP
-helps initiate capacitation-associated changes
159
What is the released egg surrounded by?
- granulosa cells
| - hyaluronic acid rich ECM
160
How do the capacitated sperm penetrate granulosa cells?
hyaluronidase
161
What happens when the sperm binds to the zone pellucida?
zona pellucida acts as a species barrier
162
What is the function of the zona pellucida?
-induces sperm to undergo acrosome reaction
163
What does the acrosome reaction do?
alters sperm so it can bind and fuse with plasma membrane of egg
164
What do the contents of the acrosome reaction do to help the sperm?
help sperm tunnel through zona pellucida
165
How does the sperm bind to the egg plasma membrane?
```
1st = tip
2nd = side
```
166
What aids in the process of the sperm binding to the egg?
microvilli on the egg
167
What membrane proteins are crucial to the egg and sperm binding?
ZP1
ZP2
ZP3
168
Which membrane proteins form long filaments?
ZP2
| ZP3
169
Which membrane protein cross-links the filaments?
ZP1
170
Steps of fertilization
1) binding of sperm to zona pellucida
2) acrosome reaction
3) penetration through zona pellucida
4) fusion of plasma membranes
5) sperm contents enter egg cytoplasm
171
What does fusion of an egg and a sperm (egg activation) induce?
- cortical reaction (cortical granules release contents)
| - meiosis to resume
172
How are the cortical reaction and meiosis resumption triggered?
increased calcium in the cytosol
173
What can egg activation be artificially triggered by?
- injecting calcium into the egg
| - fused sperm or injection of sperm, a sperm head, or sperm extract into the egg
174
Fusion of sperm causes change in egg plasma membrane so other sperm can't fuse. What is this called?
inhibition of polyspermy
175
What happens to ZP3 when cortical reaction occurs?
ZP3 = inactivated
cannot bind sperm or induce acrosome reaction
176
What happens to ZP2 when cortical reaction occurs?
ZP2 = cleaved
makes zona pellucida impenetrable
177
What happens to the 2 haploid nuclei (pronuclei) after fertilization?
fuse in the zygote
forms a single diploid nucleus
178
What do the sperm contribute after fertilization that are not present in eggs?
- centrosomes
| - centrioles
179
What happens to the centrosome after fertilization?
centrosome = duplicates
assemble mitotic spindle
180
What happens in polyspermy?
- extra mitotic spindles are present
- leads to faulty segregation of chromosomes
- aneuploidy
181
What % of couples have reduced fertility?
10%
182
When was the first child born by IVF?
1978
183
What is ICSI?
intracytoplamic sperm injection
-allows men with few/non-motile sperm to father child
184
What does ICSI bypass?
- capacitation
- swimming to the egg
- acrosome reaction
- plasma membrane fusion
185
Clones have been made by transferring ________ to an unfertilized egg lacking a nucleus
nucleus of somatic cell
