Meiosis, Germ Cells and Fertilization (Lecture 6) Flashcards
Contains two sets of chromosomes, one from each parent
Diploid
Contains one set of chromosomes
Haploid
Specialized reproductive cells; sperm or eggs generated through meiosis
Gametes
Haploid gametes fuse to form a diploid cell (zygote)
Fertilization
Divides by mitosis to become a multicellular organism
Zygote
Gametes and their precursors
Germline cells
Form the rest of the body and leave no progeny
Somatic cells
Chromosomes common to both sexes: one from each parent in each diploid nucleus
Autosomes
Meiosis creates genetic diversity in two ways
- Random segregation of homologs during meiosis
- Crossing over
Chromosomes replicate in ____ phase
S
The two sister chromatids are bound by ____ complexes
cohesin
Homologs pair recombine and separate in
meiosis I
Is there DNA replication in Meiosis II
No
In Meiosis II is there further DNA replication
Yes
in meiosis II sister chromatids are divided to produce ___ haploid daughter cells
4
Homologs begin to pair in ______, which can last days/weeks
Prophase I
a 4 chromatid structure is called a ____-
bivalent
Stable chromosome pairing requires
crossing over (chiasma)
Homologs are joined by protein structure called _______
synaptonemal complex
Small region of homology between X and Y called _______ allows them to pair
pseudautosomal region
What are the 5 phases of prophase
- Leptotene
- Zygotene
- Pachytene
- Diplotene
- Diakinesis
What phase of prophase do homologs begin to condense/pair
Leptotene
What phase of prophase do homologs begin to pair and synaptonemal complexes form
Zygotene
What phase of prophase is synapsis complete; crossing over occurs;
Pachytene
What phase of prophase is synaptonemal complex begins to break down; homologs begin to separate but remain attached at chiasmata
Diplotene
Reach maximum condensation; separation of homologs and transition stage into metaphase
Diakinesis
_______ filaments between homologs form synaptonemal complex
transverse
Cohesin complexes assemble on DNA during ____ phase and bind sister chromotids
S
What are the major components of axial core of each homolog and play an important role in segregating homologs in meiosis I
Cohesin complexes
arms of sister chromatids separate at ________, resolving chiasmata and allowing homologs to separate
Anaphase I
Sister chromatids separate in anaphase II when _____ cleaves chosen complexes
Separase
Homologs separate at _____
anaphase I
In meiosis Kinetochores on the 2 sister chromatids attach to microtubules coming form ___ pole
the same
The human female meiosis is arrested for years after _____ stage of prophase
Diplotene
Homologs fail to separate properly
Nondisjunction
nondisjunction errors are very common during egg development and increase with advancing _____ age
maternal
What is the leading cause of spontaneous abortions and mental retardation in humans
Nondisjunction
Cells with abnormal chromosome number
Aneuploid
example of Aneuploidy that is the most common single cause of mental retardation
Trisomy 21/Down syndrome
___% of trisomy 21 individuals, the additional chromosome is maternal in origin
90
Of the 90% of trisomy 21 individuals in which the additional chromosome is maternal in origin ___% occur during meiosis I and __% in meiosis II
70%, 30%
Cells with normal number of chromosomes
Euploid
Random distribution of maternal/paternal homologs can produce ___ different gametes
2^n, human example: 2^23=8.4x10^6
The variation produced in random distribution of maternal/paternal homologs can be further increased by
Crossing-over (2-3 crossovers per human homolog)
________ are found in all vertebrate embryos and give rise to gametes
Primordial germ cells (PGCs)
Primordial germ cells (PGCs) migrate to _____
developing gonad: ovaries/testes
Primordial germ cells proliferate through _____ and then undergo _____ to differentiate into mature haploid gametes
mitosis, meiosis
Primordial germ cells proliferate through _____ and then undergo _____ to differentiate into mature haploid gametes
mitosis, meiosis
Primordial germ cells proliferate through _____ and then undergo _____ to differentiate into mature haploid gametes
mitosis, meiosis
In some animals, egg contains specific molecules that determine which cells become germ cells while others, including mammals, rely on ___________ signaling to dictate which become PGCs
Neighboring cell signaling
In some animals, egg contains specific molecules that determine which cells become germ cells while others, including mammals, rely on ___________ signaling to dictate which become PGCs
Neighboring cell signaling
In mammals, dividing egg is _____ for first few rounds of cell division; then differentiate into PGCs
totipotent
When cells differentiate into PGCs somatic cell fates must be ____ by _____ and translation of certain _____ must be inhibited
suppressed by gene repression, RNAs
When cells differentiate into PGCs somatic cell fates must be ____ by _____ and translation of certain _____ must be inhibited
suppressed by gene repression, RNAs
Cell signaling leads to proliferation and migration of PGCs to developing gonad forming the ____ ridge
Genital
PGC can be removed and cultured with signaling proteins to form cell lines
any cell type for the body except extra embryonic cells (pluripotent)
_____ in genital ridge determine if gonad is ovary or testes
sex chromosomes (note not the sex chromosomes in the PCG itself)
____ directs genital ridge to develop into testis. The default pathway is _____
Y, default pathway is female
Sex determining region of Y
SRY
____ gene is necessary and sufficient for testis development in mammals
Sry
____ gene is necessary and sufficient for testis development in mammals
Sry
____ gene is necessary and sufficient for testis development in mammals
Sry
The SRY gene can cause sex reversal such as
- XX mouse + SRY - develops as a male (sterile)
- XY male with inactivating SRY mutation -develops as a sterile female
expression of SRY causes cells to differentiate into ____ cells, which are the support cells of the Testis
Sertoli
SRY encodes _________ that regulates transcription of genes involved in Sertoli development
DNA binding protein
What is the gene that is transcribed for Sertoli development
Sox9 (which is expressed in all vertebrates)
Sox9 activates ____-specific genes, including _____ hormone
Sertoli-specific genes, including anti-Mullerian hormone
ectopic expression of Sox9 in developing XX mouse gonads causes them to
develop as males (even without SRY)
SRY in somatic cells direct differentiation into _____ instead of ____
Sertoli cells instead of follicle cells
Sertoli cells secrete anti-Mullerian hormone which function include
- Suppresses female development
- Causes Mullerian duct to regress
Sertoli cells induces ____ cell differentiation in other somatic cells
Leydig
What are the functions of of Leydig cells
Secrete testosterone (responsible for secondary sexual characteristics)
What happens in absence of the SRY gene
- Genital ridge becomes an ovary
- PGC becomes an egg
- Somatic cells differentiate into: follicle cells (support cells), theca cells (estrogen-producing cells)
An activated egg can give rise to
completely new organism
Egg activation is usually triggered by _____
fertilization
Activation of egg in absence of sperm is called
Parthenogentic (this can occur in some lizards and frogs)
Mammals cannot do Parthenogentic activation of egg due to
Imprinting
Egg _____ can be used to reprogram a somatic cell nucleus to direct development of a new individual.
Cytoplasm
Eggs are _____ specialized
highly
Eggs contain stockpiles of _____ for initial development of the embryo
nutrients
____ is the nutritional reserve of an egg
yolk
Yolk is rich is ____, ____ and ______
lipids, proteins, and polysaccharides
________ are discrete yolk-containing structures
Yolk granules
Egg coat is a ________ - rich extracellular matrix
Glycoprotein
What is the function of the egg coat
- protects from mechanical damage
- acts as species-specific barrier to sperm
What is the egg coat called in non-mammalian eggs
Vitelline layer
What is the egg coat called in mammals
Zona pellucida
Upon egg activation, specialized secretory cells called _____ release contents, which enter egg coat and block polyspermy
cortical granules
Diploid oogonia undergo ______ before entering meiosis
Mitotic cell divisions
Primary oocytes are formed during
gestation of the human embryo
Primary oocytes arrest at ______ of meiosis I
prophase I
Secondary oocyte is arrested at
metaphase II
eggs have ____ gene copies in the cell
extra
egg growth occurs after _________
DNA replication
eggs import yolk proteins through
receptor-mediated endocytosis
eggs import yolk proteins through
receptor-mediated endocytosis
