Meiosis, Germ Cells and Fertilization (Lec 6) Flashcards
Define diploid
contains two sets of chromosomes, one from each parent
Define haploid
contains one set of chromosomes
Define gamete
specialized reproductive cell; sperm or eggs generated through meiosis
Define fertilization
haploid gametes fuse to form a diploid cell (zygote)
Define zygote
divides by mitosis to become a multicellular organism
Define germline cells
gametes and their precursors
Define somatic cells
form the rest of the body and leave no progeny
Define autosomes
chromosomes common to both sexes: one form each parent in each diploid nucleus
Define sex chromosomes
X and Y
What are the two ways meiosis creates genetic diversity?
random segregation of homologs during meiosis, crossing over
In meiosis, are gametes haploid or diploid?
haploid
In regards to meiosis I, what phase of the cell cycle do chromosomes replicate?
S phase
Besides chromosome replication, what else occurs in meiosis I?
homologs pair, recombine and separate
What occurs in Meiosis II?
sister chromatids are divided to produce 4 haploid daughter cells
What occurs in prophase I in meiosis?
homologs begin to pair, 4 chromatid structure is called bivalent, stable pairing requires crossing over, homologs are joined by protein structure called synaptonemal complex, small region of homology between X and Y called pseudoautosomal region allows them to pair
Describe the 5 phrases 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
Diakinesis: reach maximum condensation; separation of homologs and transition stage into metaphase
What is the synaptonemal complex made up of?
transverse filaments between homologs, cohesion complexes that assemble on DNA during S phase and bind sister chromatids
What are the distinguishing features of meiosis?
kinetochores, homologs separate at anaphase I, arms of sister chromatids separate at anaphase I, sister chromatids separate in anaphase II
In females, when is meiosis I is completed?
only at ovulation
In females, when is meiosis II completed?
after fertilization
What is nondisjunction?
homologs fail to separate properly
What is aneuploidy?
cells with abnormal chromosome number
What is euploidy?
cells with normal number of chromosomes
When and where does meiosis begin for male mammals?
in testes at puberty
Compare the error rates in meiosis between males and females
20%% in eggs vs. 3% of sperm
In regards to gonadal development, what determines if gonad is ovary or testes?
sex chromosomes in genital ridge
What determines the sex of an embryo?
sperm; presence or absence of Y
What is the SRY gene necessary for?
testis development
Expression of SRY causes cells to differentiate into ____ cells
sertoli
What do sertoli cells serete?
anti-Mullerian hormone - suppresses female development
True or False?
Eggs are not specialized
false, highly specialized
What are the stages of oogenesis?
primordial germ cell enters gonad, diploid oogonia undergo mitotic cell divisions before entering meiosis, primary oocytes are formed and arrest in prophase I of meiosis I, after ovulation process resumes and cytoplasm divides forming small polar body and large secondary oocyte which arrests in metaphase II, meiosis is finished after fertilization
What are the stages of spermatogenesis?
meiosis does not begin until puberty; in prophase I of meiosis, paired homologs cross over; primary spermatocytes complete Meiosis I to produce two secondary spermatocytes with 22 duplicated autosomes and X or Y; each secondary spermatocyte enters meiosis II to produce 4 haploid spermatids; differentiate into sperm which escape into lumen of seminiferous tubule; pass into epididymis
Where does spermatogenesis occur?
in seminiferous tubules where close contact with sertoli cells is maintained
What are the requirements of fertilization?
albumin, Ca2+ and HCO3-
What are the stages of fertilization?
egg released, sperm penetrates granulosa cells, sperm binds to zone pellucida, zone pellucida induces sperm to undergo acrosome reaction, sperm binds egg plasma membrane, egg activation = meiosis resumes
What is the cortical reaction?
fusion of sperm causes change in egg plasma membrane so other sperm can’t fuse
What happens after fertilization?
2 haploid nuclei fuse in the zygote, sperm contribute centrosomes and centrioles, centrosome duplicates - assembles mitotic spindle
