Lecture 6: Meiosis, Germ Cells and Fertilization Flashcards
- understand how sexual reproduction and meiosis generate genetic diversity - review the development of germ cells and sex determination in mammals - understand the process of fertilization
Diploid
contains two sets of chromosomes, one from each parent
Haploid
contains one set of chromosomes
gametes
specialized reproductive cells; sperm or eggs generated through meiosis
fertilization
haploid gametes fuse to form a diploid cell
zygote
divides by mitosis to become a multicellular organism
germline cells
gametes and their precursors
somatic cells
form the rest of the body and leave no progeny
In what two ways does meiosis create genetic diversity?
random segregation of homologs during meiosis
crossing over
Are gametes haploid or diploid?
haploid
What phase is chromosomes replicated?
S phase
What are sister chromatids bound by
cohesion complexes
What happens in meiosis I?
homologs pair, recombine and separate
True or false? There is no further DNA replication in meiosis II
true
What happens in meiosis II?
sister chromatids are divided to produce 4 daughter haploid cells
What happens in prophase I?
homologs begin to pair
What is a bivalent?
4 chromatid structure
Where does crossing over occur in the bivalent?
the chiasma
What are the five phases of prophase?
leptotene zygotene pachytene diplotene diakinesis
What occurs in leptotene?
homologs begin to condense/pair
What occurs in zygotene?
homologs pair and synaptonemal complexes form
What occurs in pachytene?
synapsis is complete; crossing over occurs
What occurs in diplotene?
synaptonemal complex begins to break down; homologs begin to separate but remain attached at chiasmata
What occurs in diakinesis?
reach maximum condensation, separation of homologs and transitions stage into metaphase
What forms the synaptonemal complex?
transverse filaments between homologs
When do homologs separate?
anaphase I
When do sister chromatids separate?
anaphase II
When is the human female meiosis arrested?
after diplotene (meiosis I)
What is nondisjunction?
homologs fail to separate properly
What is aneuploid?
cells with abnormal chromosome number
What is euploid?
cells with normal number of chromosomes
How many gametes can be produce?
2^n
primordial germ cells are found in all vertebrae _____ and give rise to ______
embryos; gametes
In mammals the dividing egg is _____ for few rounds of cell division, then differentiate into _______
totipotent; primordial germ cells
True or false? Primordial germ cells are uncommitted to cell fate
True
Does the sperm or the egg determine the sex of the embryo?
sperm
What gene is necessary and sufficient for testis development in mammals?
SRY gene
What is the yolk of an egg for?
nutritional reserve, rich in lipids, protein and polysaccharides
What is the egg coat?
glycoprotein rich extra cellular matrix that protects from mechanical damage
How do eggs achieve large size?
extra gene copies in the cell
When does meiosis begin for sperm?
puberty
Where does spermatogenesis occur?
seminiferous tubules
What is capacitation?
process required for a sperm to acquire capacity to fertilize an egg
Requirements for fertilization
albumin: extracts cholesterol from membrane, increasing ability to fuse with acrosomal membrane
Ca2+ and HCO3-: activate adenyl cyclase in sperm to produce cAMP
What must capacitated sperm penetrate?
granulosa cells
What does the sperm bind to? what does this do?
sperm binds to zona pellucida to act as species barrier
What does the zone pellucid do?
induces sperm to undergo acrosome reaction
What membrane proteins are crucial to binding?
ZP 1, 2, 3
ZP 2 and 3 form long filaments
ZP 1 cross links the filaments
Fusion with sperm induces what?
cortical reaction, meiosis resumes
What does the cortical reaction do?
releases enzymes that change zone pellucid so sperm can’t penetrate, inactivation of ZP3 so it no longer binds sperm or induces acrosome reaction
