Meiosis (Lec 2) Flashcards
What two key aspects of genetic variation occur in meiosis?
independent assortment, crossing over
Define meiosis
production of haploid gametes
Define fertilization
fusion of haploid gametes
What happens in Interphase of meiosis?
DNA synthesis and chromosome replication phase
What happens in meiosis I?
separation of homologous chromosome pairs, and reduction of the chromosome number by half
What happens in meiosis II?
separation of sister chromatids, also known as equational division
What are the major results of meiosis?
increase in cell numbers, daughter cells are not genetically identical, daughter cells are haploid, formation of gametes
What is the pairing of homologous chromosomes called?
synapsis
What is the first stage of prophase I and what happens in it?
Leptotene; chromosomes begin to condense
What is the second stage of prophase I and what happens in it?
Zygotene; Synaptonemal complex forms and homologous chromosomes pair (synapsis)
note: synapsis results in bivalents (tetrads)
What is the third stage of prophase I and what happens in it?
Pachytene; crossing over
What is the fourth stage of prophase I and what happens in it?
Diplotene; crossing over continues, chiasmata are well defined at this point
What is the fifth and final stage of prophase I and what happens in it?
Diakinesis; crossing over is incomplete, spindle apparatus is in place, and nuclear membrane is disrupted
Do centromeres divide?
no
What happens in metaphase I?
tetrads line up along equatorial plate
What happens in anaphase I?
homologous chromosomes move to opposite poles, each homologue consists of two chromatids because sister chromatids remain attached, daughter cells will be haploid
What happens in telophase I?
cytokinesis occurs, nuclear membrane reform, chromosome number is halved, haploid daughter cells are genetically unique
What happens in interkinesis?
spindle breaks down, chromosomes may uncoil
What happens in prophase II?
chromosomes condense again, spindle apparatus reforms, each chromosome consists of two chromatids, each daughter cell has one complete set of chromosomes
What happens in metaphase II?
chromosomes line up on equatorial plate, kinetochores face opposite poles
What happens in anaphase II?
centromeres of sister chromatids separate, chromosomes move to opposite poles, each chromosome consists of a single chromatid
What happens in telophase II?
chromosomes uncoil, cytokinesis completes, end result is four genetically unique haploid daughter cells
What is cohesin?
enzyme that holds chromatids together during early part of mitosis, holds homologous chromosomes together at chiasmata through metaphase I in meiosis, meiosis-specific cohesin at centromere keeps sister chromatids together during anaphase;- protected by shugoshin
What does separase do?
kept inactive during interphase and early mitosis, breaks down meiosis-specific cohesin at centromere during end of metaphase II
What does securin do?
keeps separase inactive during interphase and early mitosis
What does monopolins do?
keeps sister kinetochores oriented toward same pole during metaphase I
In spermatogenesis, are primordial germ cells haploid or diploid?
diploid
Primordial germ cells divide mitotically to produce diploid ___
spermatogonia
Spermatogonia enter prophase I and give rise to ____ _____
primary spermatocytes
Primary spermatocytes undergo meiosis I to produce haploid ___ _____
secondary spermatocytes
Secondary spermatocytes undergo meiosis II to produce haploid _____
spermatids
Spermatids differentiate into ___
sperm
Primordial germ cells involved in Oogenesis divide mitotically to produce diploid ___
oogonia
Oogonia initiate meiosis and enter into prophase I to give rise to ___ ____
primary oocytes
Primary oocytes become frozen at what stage of prophase I
diplotene
When does meiosis resume after being frozen in prophase I?
ovulation
Meiosis stops again at what point?
metaphase II; resumes once egg is fertilized
summarize oogeneis
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
