L02 Meiosis Flashcards
Contrast Mitosis and Meiosis
- produces two daughter cells with the full diploid compliment that the parent cell had while meiosis produces haploid gametes
- Meiosis has two discrete steps in order to reduce the amount of genetic material by half while mitosis is a single cycle
Why does meiosis produce gametes with a haploid set of chromosomes?
Ensures that during sexual reproduction, haploid gametes combine to reconstitute the diploid content found in parents
What is the purpose of crossing over?
Ensures genetic variation
State the events of prophase 1 of Meiosis 1
Homologous chromosomes pair up (synapses) to form a bivalent which condenses and shortens to form a tetrad.
Chromatin intertwine to form a chiasmas, crossing over occurs, centromeres attach to the spindle fiber
State the events of Metaphase 1 in Meiosis 1
- Chromosomes at maximum shortness
- Chiasmas hold non-sister chromosomes together
- binding of spindle fibers to centromeres moves chromatids to metaphase plate
State the occurrences of Anaphase 1
- Cohesin is degraded between sister chromatids
- Genetic material at crossovers can now separate.
- Homologous chromosomes separate to and move toward each pole
State the events of Telophase 1
- Nuclear membranes form
- Cytokinesis
- Two haploid cells result for Prophase 2
- Each cell has one of the homologous chromosomes which still has duplicated DNA (the sister chromatid)
Early Prophase 1 of meiosis where homologous chromosomes pair up is
Is called the synapse
A synapse pair is initially called a Bivalent
The Bivalent will condense, shorten and the Bivalent is then called a tetrad
How does prophase 1 ensure Genetic Variation?
- homologous chromosomes pair up(synapsis)
- synapsis is initially called Bivalent
- Bivalent condenses to tetrad
- As prophase 1 continues, Chiasmas are formed in one or more areas - here, non sister chromatids exchange information
- At the end of prophase 1 the tetrad pair of chromosomes separate but are still intertwined by Chiasmas
- Nuclear envelope breaks down and centromeres attach to spindle.
State the events of prophase 2
- The second Meiotic division begins at Prophase 2 with one each homologous chromosomes which were separated at Telophase 1
- Spindle fibers from the centrioles are attached to the centromeric region of each sister chromatid
State the events of metaphase 2
- The centromeres are positioned at the equatorial plate
- The centromeres of each sister chromatid are attached to a spindle fiber which were protected by Shugoshin complex until here at metaphase 2 where Shugoshin is degraded
What are the events of Anaphase 2?
- Shugoshin is completely degraded and sister chromatids are separated to opposite poles of the cell(Shugoshin is degraded by APC)
- Each haploid daughter cell from this meiosis 2 has one member of an original pair of homologous chromosomes
State the events of Telophase 2
- One sister chromatid from an original homologouspair is present at each pole
- Each chromosome is now a monad
- Nuclear membrane forms
Describe the cytokinesis of Telophase 2
- Each gamete achieves the haploid State
- Each gamete has a unique mixture of maternal and paternal genetic information originally from grandparents
- Meiosis significantly increases genetic variation in each generation
In sex chromosomes in meiosis……… does not occur
Crossing over
Which regions in sex chromosomes form the synapse in Prophase 1?
The short homologous psuedo-autosomal regions
These are only present at one end of each sex chromosome
In mitosis, sister chromatids are separated during ___________, while in meiosis, sisters are separated in ____________________.
Anaphase
Anaphase 2
How and where are male gametes produced?
By spermatogenesis in the Testes
How and where are female gametes produced ?
Female gametes are produced by oogenesis in the ovary
Describe spermatogenesis
Spermatogonia undergo growth and maturation to form primary spermatocytes
Primary spermatocyte undergoes meiosis 1
- this produces two secondary spermatocytes
Spermatocytes undergo meiosis 2
- Produces a total of four haploid spermatids
Which male animals would spermatogenesis occur continuously/ periodically in?
- Animals that reproduce sperm year-round would produce sperm continuously
- Animals that have breeding periods only produce sperm during breeding time
Describe oogenesis
- Formation of ova in the ovaries
- Oogonium undergoes growth and maturation to create a primary oocyte
- the primary oocyte undergoes meiosis 1 and in Telophase 1, the dyads at one pole get very little cytoplasm
- This yields one secondary oocyte and the first polar body
- Secondary oocyte undergoes Meiosis 2 and genetic material is equally distributed with very little cytoplasm in one of the daughter cell
- This yields one ootid which will mature to the ovum and a secondary polar body
How do spermatogenesis and oogenesis differ in their continuous events?
Unlike spermatogenesis, the two meiotic events of oogenesis may not be continuous
Explain the arrest/continuation of oogenesis
- In some animals such as humans, the first division of all oocyte’s begins in the embryonic ovary but arrests in prophase 1
- Years later, meiosis resumes in each oocyte just prior to ovulation
- The second division is completed only after fertillization
Crossing over…
Ensures genetic variation
Which stage generates dyads?
Anaphase 1
Which phase does synopsis occur?
Prophase 1
Describe sex chromosomes during meiosis
- sex chromosomes X and Y have short homologous pseudo-autosomal regions
- these regions form synapsis in Prophase 1
- Only present at one end of each sex chromosome
- crossing over does not occur
Describe development of gametes
The development of gametes occurs between spermatogenesis and oogenesis
Discuss the genetic content “n” and chromosome number “c” of each type of gamete
Primary gametocytes- 2n, 2c becomes 2n, 4c(after DNA is replicated relight before meiosis 1)
Secondary gametocytes -n, 2c
Potential gametes- n, c
