Meiosis Flashcards
Meiosis:
Production of haploid gametes
What are the sources of genetic variation?
Mutation
Sexual reproduction
—Meiosis: crossing over, independent assortment
Fertilization
Fertilization:
The fusion of haploid gametes
Genetic variation:
Consequences of meiosis
Meiosis interphase:
DNA synthesis and chromosome replication phase
Meiosis I:
Separation of homologous chromosome pairs, and reduction of the chromosome number by half
Meiosis II:
Separation of sister chromatids, also known as equational division
What are the major results of Meiosis?
Increase in cell numbers sometimes
Daughter cells are not genetically identical
Daughter cells are haploid
Meiosis results in the formation of gametes
What are the two types of gametogenesis?
Spermatogenesis and oogenesis
What are the products of gametogenesis?
Gametes: Egg and sperm
What is the pairing of homologous chromosomes?
synapsis
What are the major events in meiosis?
Synapsis and crossing over
What does meiosis require?
Two consecutive cell divisions BUT only one cycle of DNA replication
When does crossing over occur in meiosis?
During prophase I, segments of nonsister chromatids are exchanged
What produces variation without crossing-over?
independent assortment
When does reduction division occur?
The end of meiosis I
When does equational division occur?
At the end of meiosis II
Name the stages of prophase I
Leptotene Zygotene Pachytene Diplotene Diakinesis
What happens during leptotene?
Chromosomes are threadlike
Each chromosome consists of two chromatids
Chromosomes begin to condense and become visible
Describe the zygotene stage
Chromosomes continue to condense
Homologous chromosomes pair (synapsis)
Synaptonemal complex forms
Synapsis results in bivalents (or tetrads)
Describe the pachytene stage
Maximum coiling occurs and chromosomes become shorter and thicker
Tetrads become visible
Crossing-over begins
Describe the diplotene stage
Crossing-over continues
Chiasmata are well-defined
Describe the diakinesis stage
Crossing-over is complete
Terminalization occurs
Spindle apparatus is in place
Nuclear membrane is disrupted
Synapsis:
Close pairing of homologous chromosomes
Tetrad:
Closely associated four-sister chromatids of two homologous chromosomes
Crossing over:
Crossing over of chromosome segments from the sister chromatid of one chromosome to the sister chromatid of the other synapsed chromosome
What is the first mechanism of generating genetic variation in newly formed gametes?
Exchange of genetic information during crossing over
Describe metaphase I
Tetrads line up along equatorial plate
Centromeres do not divide
Describe anaphase I
Homologous chromosomes move to opposite poles
Each homologue consists of two chromatids because sister chromatids remain attached
Chromatids are not genetically identical because of crossing-over
Daughter cells will be haploid
Describe Telophase I
Cytokinesis occurs
Nuclear membranes reform
Spindle apparatus disassembles
Chromosomes may uncoil to varying degrees
Chromosome number is halved
Haploid daughter cells are genetically unique
What is the second mechanism of generating genetic variation in the newly formed gametes?
Random distribution of chromosomes into two newly divided cells
When does interkinesis occur and what happens during the stage?
Interkinesis happens after telophase I Nuclear membranes reform Spindle apparatus disassembles Chromosomes may uncoil to varying degrees Spindle usually breaks down
Describe prophase II
Chromosomes again condense
Nuclear membranes disappear
Spindle apparatus reforms in each cell
Each chromosome consists of two chromatids
Each daughter cell has one complete set of chromosomes (haploid)
Describe metaphase II
Chromosomes line up on equatorial plate
Kinetochores face opposite poles
Describe anaphase II
Centromeres of sister chromatids separate
Chromosomes move to opposite poles
Each chromosome consists of a single chromatid
Describe telophase II
Chromosomes uncoil
Spindle apparatus disintegrates
Cytokinesis is complete
Nuclear membranes reform
What is the end result of Meiosis
Four genetically unique haploid daughter cells
What holds chromatids together during early part of mitosis?
Cohesin
What does the break down of cohesin allow in mitosis?
Sister chromatids to separate during mitosis anaphase
In meiosis, what holds homologous chromosomes together at chiasmata through metaphase I?
Cohesin along chromosome arms
What keeps sister chromatids together during anaphase I and later stages in meiosis?
Meiosis-specific cohesin at centromere. They are the cohesins that remain after the breakdown of other cohesin when homologs separate during Anaphase I
What is the meiosis-specific cohesin at the centromere protected by?
shugoshin
What breaks down cohesin at the end of metaphase?
Separase
What is kept inactive during interphase and early mitosis?
Separase
What breaks down meiosis-specific cohesin at centromere? During what stage?
Separase during end of metaphase II
What keeps separase inactive during interphase and early mitosis?
Securin
What keeps sister kinetochores oriented towards the same pole during metaphase I?
Monopolins
In what process does the end result include daughter cells that are genetically identical?
Mitosis
In what process does the end result include the daughter cells being genetically unique?
Meiosis
In Spermatogenesis, what do primordial germ cells divide mitotically to produce?
Spermatogonia
PGCs are diploid or haploid?
Diploid
Describe spermatogonia
Diploid
Can undergo repeated rounds of mitosis to produce more spermatogonia
May initiate meiosis and enter prophase I to give rise to primary spermatocyte
Describe primary spermatocytes
Diploid
Undergo meiosis I to produce haploid secondary spermatocytes
Describe Secondary spermatocytes:
Haploid
Undergo meiosis II to produce haploid spermatids
Describe spermatids:
Haploid
Differentiate into haploid sperm
Describe PCGs in oogenesis
Diploid
Divide mitotically to produce diploid oogonia
Describe oogonia
Diploid
Can undergo repeated rounds of mitosis to produce more oogonia
Eventually initiate meiosis and enter into prophase I to gove rise to primary oocytes
Describe primary oocytes
All become frozen at diplotene stage of prophase I
Will not resume meiosis until beginning of puberty and then a number will resume meiosis during each menstrual cycle until they are all used up
When will some primary oocytes resume meiosis until metaphase II
During menstrual cycle
in most mammals, the egg is ovulated at what stage?
Metaphase II
When meiosis I has been completed in oogenesis what results?
Unequal cytokinesis
Large ovum and small polar body
When will meiosis resume after meiosis I?
Not until egg is fertilized
What will potentially result at the end of oogenesis?
One large egg and three small polar bodies
Describe secondary oocytes
Haploid
Result of unequal cytokinesis resulting in one secondary oocyte and one polar body
ootids:
haploid egg
Polar bodies:
three (?) haploid small “cells” for each ootid
