Lecture 6 Flashcards
Definition of haploid
Contains one set of chromosome
Definition of diploid
Contains two sets of chromosomes, one from each parent
Gametes:
Specialized reproductive cells; sperm or eggs generated through meiosis
Fertilization:
haploid gametes fuse to form a diploid cell (zygote)
Zygote:
Divides by mitosis to become a multicellular organism
Germline cells:
Gametes and their precursors
Somatic cells:
For the rest of the body and leave no progeny
Autosomes:
Chromosomes that are common to both sexes: one from each parent in each diploid nucleus
Sex chromosomes:
X and Y
How does meiosis create genetic diversity in two ways?
- random segregation of homologs during meiosis
- crossing over
Gametes are diploid or haploid?
Haploid
Meiosis:
Two cell divisions but one round of DNA synthesis to produce half the number of chromosomes
Chromosomes replicate in S phase
How are the two sister chromatids bound together?
By cohesin complexes
How long can it take for homologs to pair in prophase I?
days or weeks
bivalent:
4 chromatid structure
Chiasma
Crossing over that gives stable pairing
How are homologs joined?
Synaptonemal complex protein structure
What is the small region of homology between X and Y that allows them to pair?
pseudoautosomal region
Name the five phases of prophase
- Leptotene: homologs begin to condense/pair
- Zygotene: homologs pair and synaptonemal complexes form
- Pachytene: synapsis is complete; crossing over occurs
- Diplotene: Synaptonemal complex begins to break down; homologs begin to separate but remain attached at chiasmata
- Diakinesis: Reach maximum condensation; separation of homologs and transition stage into metaphase
What forms Synaptonemal complex?
Transverse filaments between homologs form synaptonemal complex which is important for crossing over
What binds sister chromatids to each other? What is the importance?
Cohesin complexes assemble on DNA during S phase and bind sister chromatids. Major components of axial core of each homolog. Plays important role in segregating homologs in meiosis I
What are distinguishing features of meiosis
- Kinetochores on the 2 sister chromatids attach to microtubules coming from same pole
- Homologs separate at anaphase I
- Arms of sister chromatids separate at AI, resolving chiasmata and allowing separation
- Sister chromatids separate in anaphase II when separase cleaves cohesin complexes
What stage is human female meiosis arrested at and for how long?
Arrested for years after diplotene. And Meiosis I is completed only at ovulation and Meiosis II after fertilization
Nondisjunction:
Homologs fail to separate properly
When are nondisjunction errors very common?
Durirng egg development and increase with advancing maternal age
What is the leading cause of spontaneous abortions and mental retardation in humans?
nondisjuction
Aneuploid:
Cells with abnormal chromosome number
Trisomy 21:
Down syndrome: most common single cause of mental retardation
What is the main origin of additional chromosome in Trisomy 21 individuals?
90% of trisomy 21 cases, the additional chromosome is maternal in origin and 70% of the time the nondisjunction occurs during MI
Euploid:
cells with normal number of chromosomes
Random distribution of maternal/paternal homologs can produce how many different gametes?
2^n
2^23=8.4x10^6
How is variation increased during meiosis?
crossing over
2-3 crossovers per human homolog
When does meiosis begin in male mammals?
Begins in the testes at puberty and proceeds without arrest
-24 days to complete meiosis
What is the error rate in aneuploidy in eggs vs sperm? Why do they differ?
20% of eggs are aneuploid vs only 3% of sperm
A cell cycle checkpoint is activated in sperm w meiotic errors, resulting in apoptosis
What percentage of all human conceptions are aneuploid?
25% mostly due to nondisjunction in oocytes at meiosis I
What gametes are more prone to new DNA mutations? Why?
Male gametes as a result of undergoing more mitotic cell divisions; advanced paternal age leads to increased risk of point mutations
Primordial germ cells (PGCs)
Found in all vertebrate embryos and give rise to gametes
During development, where to PGCs migrate to?
To developing gonad: ovaries/testes
What is the cell cycle of PCGs?
Proliferate through mitosis and then undergo meiosis to differentiate into mature haploid gametes
In some animals, what determines which cells become germ cells?
In some: the egg contains specific molecules that determine
Others, including mammals: rely on neighboring cell signaling to dictate which become PGCs
In mammals, the dividing egg is ________ for first few rounds of cell division; then differentiate into _______.
Totipotent;PGCs
What must be suppressed by gene repression in developing mammal organism?
Somatic cell fates
Translation of certain RNAs must be ________
Inhibited (nanos)
Cell signaling leads causes PGCs to do what?
Proliferate and migrate to developing gonad in genital ridge
PGCs are _________ to cell fate
Uncommited
What cells can be removed and cultured with singaling proteins to form cell lines for any cell type for the body except extraembryonic cells?
PGCs because they are pluripotent
What determines of gonad is ovary or testes in genital ridge?
Sex chromosomes that are not in the PCG
What determines sex of embryo?
Sperm; presence or absence of Y determines sex of individual
What is the sex of 45, X?
Female
What is the default pathway for sex?
Female, Y directs ridge to develop into testis
What is the sex determining region of Y?
SRY gene is necessary and sufficient for testis development in mammals
What can cause sex-reversal?
SRY
XX mouse + Sry = develops as a male but sterile
XY male with inactivating Sry mutation = develops as a female
Expression of Sry causes cells to differentiate into what?
Sertoli cells which are testis support cells
What does Sry encode?
Encodes DNA binding protein that regulates transcription of genes involved in Sertoli development
What gene is expressed in all vertebrates that activates Sertoli-specific genes, including anti Mullerian hormone?
Sox9s
What does ectopic expression of Sox9 in developing XX mouse gonads cause?
Causes them to develop as males (even without SRY)
What does Sry direct in somatic cells to do?
Differentiate into Sertoli cells instead of follicle cells
What type of hormone do Sertoli cells secrete?
Anti-Mullerian hormone
What does anti-Mullerian hormone do?
Suppresses female development and causes mullerian duct to regress
What do Leydig cells do?
Secrete testosterone and are responsible for secondary sexual characteristics
What happens in the absence of Sry gene?
Genital ridge becomes an ovary, PGC becomes an egg, and somatic cells differentiate into follicle cells (support cells) and theca cells ( estrogen producing cells)
An activated egg can give rise to what?
Completely new organism
What is egg activation usually triggered by?
fertilization which is fusion of sperm and egg
What is parthenogentic activation? What species does this occur in?
Activation in the absence of sperm. Can occur in some lizards and frogs but mammals cannot due to imprinting
Reproductive cloning:
Egg cytoplasm can be used to reprogram a somatic cell nucleus to direct development of a new individual
What do eggs contain for initial development of the embryo?
Stockpiles of nutrients
Yolk:
Nutritional reserve
Rich in lipids, proteins, and polysaccharides
Yolk granules:
discrete yolk-containing structures
Egg coat:
Glycoprotein-rich extracellular matrix
Protects egg from mechanical damage
Acts as species-specific barrier to sperm
What are the species-specific barriers to sperm?
Vitelline layer in non mammalian eggs
Zona pellucida in mammals
What blocks polyspermy?
Upon egg activation with one sperm, specialized secretory cells called corticle granules release contents, which enter egg coat and block
What are the stages of oogenesis?
- PGCs migrate into gonad
- Diploid oogonia undergo mitotic divisions
- Primary oocytes arrest at prophase I
- Resume progression at ovulation
- Cytoplasm divides asymmetrically creating a large secondary oocyte and polar body
- Secondary oocyte is arrested in Metaphase I
- Meiosis completed only after fertilization
How do eggs achieve a large size?
They have extra gene copies in the cell and growth occurs after DNA replication.
Some species produce many copies of some genes like rRNA
Imported yolk proteins through receptor mediated endocytosis
Accessory cells-nurse cells-make ribosomes, mRNA, proteins
Follicle cells connect to each other and oocyte (gap junctions)
Primordial follicle:
Oocyte surrounded by single layer of follicle cells
Developing follicles:
multiple layers of follicle cells (granulosa cells) surround growing oocyte, and some developing follicles go on to accuire fluid-filled cavity, antrum
Antrum:
Fluid filled cavity
What happens to antral follicles during puberty
FSH surge induces 10-12 antral follicles and one becomes dominant
What triggers ovulation? What happens to the primary oocyte during ovulation?
FSH + LH surge triggers ovulation and primary oocyte completes MI, becomes a secondary oocyte that arrests at metaphase II. Follicle enlarges and ruptures to release secondary oocyte
After primary oocyte completes MI, what stage does does it arrest at?
Metaphase II
What is a secondary oocyte surrounded by after ovulation?
granulosa cells
What is sperm optimized to do?
Carry DNA to egg
What organelles does the sperm cell contain in cytoplasm?
The cell is stripped down and has no cytoplasmic organelles
What are the two distinct regions of a sperm?
Head-contains nucleus
Tail-propulsion
What is the purpose of the midpiece in sperm?
It is packed with mitochondria to power flagellum for movement
Why is the midpiece packed with mitochondrion?
Dynein motor proteins require ATP hydrolysis to slide microtubules for movement
What does the acrosomal vesicle on sperm contain?
Hydrolytic enzymes that are released by exocytosis in acrosome reaction
What is the purpose of acrosomal vesicles in sperm?
Helps penetrated egg’s outer coat
When does meiosis begin for sperm?
Not until puberty
What are the steps in Spermatogenesis?
PGCs—->Enters gonad—->Spermatogonium—>proliferate by mitosis
Meiosis:
Primary spermatocyte
Prophase I: paired homologs cross over
Complete MI to produce two secondary spermatocytes with 22 duplicated autosomes and X or Y
Each secondary spermatocyte enters MII to produce 4 haploid spermatids
Differentiate into sperm which escape into lumen of seminiferous tubule
Pass into epididymis, where stored and undergo further maturation
Where does spermatogenesis occur?
In the seminiferous tubules
During spermatogenesis what do the cells stay in close contact with?
Sertoli cells
How many mitotic divisions does the Spermatogonium undergo before entering meiosis I?
Limited mitotic divisions
What is syncytium between developing sperm?
Progeny of spermatogonium dont complete cytokinesis and remain connected through cytoplasmic bridges. So each share a common cytoplasm
What do synchronous batches allow developing sperm to do?
Have access to diploid genome for their development
What is fertilization?
Fusion of egg and sperm
Capacitation:
Process required to acquire capacity to fertilize an egg
**(simpler terms)Changes that enables sperm to penetrate and fertilize an egg
How long does fertilization take?
5 to 6 hours
Where is fertilization completed?
Oviduct
What biochemical and functional changes does a sperm undergo during fertilization?
Greatly increases motility of flagellum
Makes sperm capable of undergoing acrosome reaction
What requirements are needed for fertilization that are all present in female genital tract? What are their purposes
Albumin-helps extract cholesterol from membrane, increasing ability to fuse with acrosome membrane
Ca2+ and HCO3-: activates adenyl cyclase in sperm to produce cAMP
-This helps to initiate capacitation-associated changes
How does capacitated sperm penetrate granulosa cells?
Uses hyaluronidase
What is the released egg surrounded by?
Granulosa cells and an ECM rich in hyaluronic acid
Once the sperm passes the granulosa cells, what does it bind to?
Binds to zona pellucida
What does the zona pellucida act as?
Species barrier
What does the zona pellucida induce?
Induces sperm to undergo acrosome reaction
What does the acrosome reaction help sperm to do?
Contents help sperm to tunnel through zona pellucida and alters sperm so it can bind and fuse with plasma membrane of egg
How does the sperm bind to egg plasma membrane? How
First by tip and then by side; microvilli on egg aide in the process
What certain membrane proteins are crucial to binding sperm and egg?
ZP1,2, and 3
ZP 2 and 3: form long filaments
ZP1: cross-links the filaments
What does fusion with the sperm induce?
Induces cortical reaction where cortical granules release contents and meiosis resumes
The events of egg activation are triggered by the increase of what ion?
Ca2+ in cytosol
Egg activation can be artificially triggered by injecting Ca2+ into egg
What causes increase of Ca2+ in cytosol of egg?
Fusion of sperm, or injection of sperm
Sperm head, or sperm extract into egg
When the fusion of sperm causes the cortical reaction to occur, what happens to the egg plasma membrane?
there are changes in the plasma membrane so other sperm cant fuse and prevents polyspermy
What does the cortical reaction do to the zona pellucida?
The release of enzymes cause the zona pellucida to change so sperm cant penetrate
inactivation of ZP3 so it no longer binds sperm or induces acrosome reaction
ZP2 is cleaved which makes ZP impenetrable
What happens after fertilization?
2 haploid nuclei (pronuclei) fuse in the zygote and forms a single diploid nucleus
Centrosome duplicates; and assemble mitotic spindle
What does the sperm contribute to the zygote?
Centrosomes and centrioles which are not present in eggs
What happens in polyspermy?
extra mitotic spindles are present and leads to faulty segregation of chromosomes and aneuploidy
What are the statistics of reduced fertility?
10% of couples have reduced fertility
No pregnancy after 12 months of unprotected sex
Defect can by in either partner 50/50
When was the first child born by IVF?
1978
What is not considered routine for couples with reduced fertility?
In vitro fertilization
How is In virto fertilization done?
Woman takes hormones to stimulate multiple oocytes to develop
Eggs are harvested and fertilized in culture dish with sperm
2-3 embryos are transferred to woman’s uterus
What is the risk percentage of multiple pregnancies with IVF?
> 30% of cases
What makes it possible for couples at risk for a genetic condition to conceive healthy children?
IVF combined with preimplantation genetic diagnosis (PGD)
What is ICSI?
Intracytoplasmic sperm injection that allows men with few or non-motile sperm to father children.
Egg is injected with a single sperm
What hurdles does ICSI allow sperm to bypass?
Capacitation, swimming to the egg, acrosome reaction, plasma membrane fusion
What is cloning?
Transferring nucleus of somatic cell to an unfertilized egg lacking a nucleus; cell is activated to create embryo
Compare reproductive vs therapeutic cloning
Reproductive cloning: clone embryo is placed in foster mother
Therapeutic cloning make ES cells genetically identical to donor (made in lab)-avoid immunological reaction associated with using organ donor
