Lecture 14: Mitosis and Meiosis Flashcards
What are the six phases of mitosis?
- Interphase
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
What is interphase?
Everything that is not mitosis(G1, S, G2)
What occurs during interphase?
DNA is spread out, Centrosomes are duplicated(G2)
What happens during prophase?
-Chromosomes condense(chromosome refers to two chromatids)
-Spindle forms
-Kinteochores
-Centrosomes move to opposite ends of the nucleus
What are centrosomes?
Microtubule organizing centres
What happens during prometaphase?
-Nuclear envelope breaks(surrounds chromosomes originally)
-Microtubules probe the cytoplasm and begin attaching to kinetochores
-Microtubules form
What happens during metaphase?
-Centromeres of chromosomes become aligned in a plane in the cell’s equator due to tension caused microtubules bound to the kinetochores
-Sister chromatids bound to kinetochore microtubule on opposite spindles
What happens after metaphase? (hint: not anaphase)
Cell has a checkpoint to ensure that all of the chromosomes are correctly lined up
What happens during anaphase?
-Centromere that was holding the sister chromatids together separates
-The microtubules shorten pulling the sister chromatids apart
-Spindle Elongates
How are the sister chromatids pulled apart during anaphase?
Each sister chromatid is attached to a microtubule, that applies tension to the sister chromatid and forces the sister chromatids apart
What happens during telophase?
Daughter chromosomes are brought to the poles of the cell and decondense,
-nuclear envelope reforms
-microtubules dissolve
-Spindle breakdown
What do the microtubules bind to ?
Kinetochores of chromosomes
-Microtubules can also bind to each other
What is a mitotic spindle?
The mitotic spindle is the macromolecular machine that segregates chromosomes to two daughter cells during mitosis
True or False: Sister chromatids must remain paired until all chromatids are attached to a kinetochore microtubule
True
Explain the probing of microtubules
Microtubules grow and shrink from the centrosome, probing the cytoplasm. Microtubules do not become stable until they reach a kinetochore.
Can microtubules from the same spindle connect to the same kinetochore of the same chromosome?
No
When are the centrosomes of the sister chromatids allowed to dissolve?
Once all sister chromatids have been captured by microtubules then their centromeres are allowed to dissolve
Is cytokinesis the same in plants and animals?
No
How is cytokinesis done in animals?
-Via purse string mechanism
-A ring of actin and myosin forms in the middle of the cell and then it begins to contract until it pinches off the cell(actin-myosin contraction)
How is cytokinesis done in plants?
-Vesicles with cell wall material segregate in between the two cells and form a new wall between the two nuclei
Does cytokinesis need to occur? I not in what cells does it not occur in?
-No its does not need to occur
- In muscle cells, cells will undergo mitosis but not cytokinesis these cells have more than usual number of chromosomes
What is sex?
Mixing of the genetic material of two organisms
What must happen before sex?
Before sexual reproduction you must reduce the number of homologs by half
Are homologous chromosomes haploid or diploid?
Haploid(n)
Are somatic cells haploid or diploid?
Diploid(2n)
Are gametes haploid or diploid?
Haploid(n)
What is meiosis?
Process by which haploid gametes are created
What are the six phases of meiosis I?
- Early Prophase
- Mid Prophase
- Late Prophase
- Metaphase
- Anaphase
- Telophase
What are the six phases of meiosis II?
- Interphase
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
What happens during early prophase?
-Centrosome duplicated and going to the poles
-Chromosomes begin to condense
How do the chromosomes know how to pair?
-By using their DNA sequence and comparing it to the other chromosomes
What happens during mid prophase?
-The chromosomes now pair as homologs(tetrad) and align(synapsis)
How do X and Y chromosomes find a pair?
-X and Y sex chromosomes pair up as homologs
What happens during late prophase?
-Lined up chromosomes start crossing over
What is crossing over?
Switching DNA between sister chromatids
What happens during metaphase?
-Homologous chromosomes line up down the equator/metaphase plate of the cell
-They are then captured by microtubules attaching to the kinetochores
What happens during anaphase?
-Two homologous chromosomes are pulled to opposite sides of the cell and two cells are created
What happens during telophase?
Nucleus reforms, chromosomes are brought to the poles of the cell, they decondense, nuclear envelope reforms, microtubules dissolve and cell gets divided(telophase is optional)
What is meiosis II?
Exactly the same as mitosis
What is the end product of meiosis?
Four cells(gametes) that each contain 23 chromosomes
How long does mitosis roughly take?
20 mins
How long does meiosis last?
Depends, in females meiosis lasts roughly 40 years(eggs remain in prophase 1)
What is nondisjunction?
Abnormal distribution of chromosomes in daughter cells
How do unstable kinetochore microtubules become stable?
By binding to a kinetochore
Define syncytial and when is it seen ?
Cells with multiple nuclei. Occurs in cells that undergo mitosis but not cytokinesis
Is mitosis asexual reproduction or sexual ?
Asexual, since there is no mixing of genes
How many homologs of each chromosome are in haploid and diploid cells?
Haploid: One of each homolog chromosome
Diploid: Two of each homolog chromosome
What is synapsis?
Synapsis: pairing of homologous chromosomes
What is it called when homologous chromosomes are in the middle of crossing over?
They form chiasmata
How do the homologous chromosome pairs stay together at the metaphase plate during metaphase I?
The chiasmata keeps them together
What is Down syndrome caused by?
A problem in meiosis
What is an example of a non-disjunction event?
Down syndrome
- During meiosis instead of separating the 21 chromosome homologs both of them end up in one gamete
How many copies of chromosome 21 will a person with down syndrome have?
3 copies
True or False: Frequency of Down syndrome increase in older women?
True
True or False : some organisms spend the majority of their lives with haploid cells?
True
Why is it ok for some species to be haploid?
It is all about ratio. Since all of their chromosomes have one homolog everything is balanced. However when someone with Down syndrome only has one chromosome with three homologs it is unbalanced and causes a problem.
Are multiples of n okay, meaning 4 homologs per chromosome or 5 or 6?
Yes
Why does’t meiosis work for odd numbered ploidy (ie. 3n, 5n,7n, etc. )
During meiosis I, homologs are supposed to pair up however due to the odd number one will not have a pair and therefore the progeny will end up with multiple trisomies
Can odd numbered ploidy do mitosis?
Yes, because it is just a clone of the cell
Does higher ploidy mean bigger species?
Yes
What does syncytial mean?
Cells that undergo mitosis but not cytokinesis
