Lecture 14 mitosis and meiosis Flashcards
What is chromatin?
A dense material made of double stranded DNA molecules and proteins
What are the two double stranded DNA molecules formed after DNA replicates during S phase called?
Sister chromatids
How are sister chromatids held together?
By a protein complex called cohesin along most of their length
When is chohesin removed?
Mitosis, most is removed, except at the centromere
What is the centromere?
The point at which sister chromatids are held together
After DNA replication, how is DNA further compacted?
Condensins coat the DNA molecule
What is the diameter of the nucleus?
5 micrometers
How long is the typical DNA in a human cell, end to end?
2m
What proteins are contained within chromosomes?
Histones
How many classes of histones are there?
5
What charge do histones have?
Positive at cellular pH levels
Why are histones positively charged?
High content of basic amino acids lysine and arginine
What do positive charges on histones do?
Attract negative phosphate groups on DNA
What interactions result in nucleosomes?
DNA- Histone interactions
Histone-histone interactions
What are nucleosomes like?
Bead like
How many histone molecules are their in each nucleosome?
Eight, 2 each of 4 of the histone classes
How are histones arranged within a nucleosome?
United to arrange a core or spool
How many base pairs are there in a nucleosome?
146
How many turns of base pairs are wrapped around each histone spool?
1.65 turns
What is the final component of a nucleosome?
Histone H1
What does H1 do?
Found on the outside of the DNA
clamps it to the histone core
What runs between nucleosomes?
A variable amount of non-nucleosomal linker DNA
How is DNA arranged during interphase?
A single DNA molecule running between chromosomes
What is DNA exposed to during interphase?
Nuclear environment
Including proteins involved in replication and regulation of expression
What happens to chromatin during mitosis and meiosis?
It becomes more tightly coiled and condensed
Up until what point does further coiling of chromatin take place?
Until chromatids begin to move apart
What is mitosis?
A single nucleus gives rise to two nuclei that are genetically identical to each other and to the parent nucleus
What are the subdivisions of mitosis, in order?
Prophase Prometaphase Metaphase Anaphase Telophase
What is the centrosome?
An organelle in the cytoplasm near the nucleus
What happens to the centrosome during S phase?
It doubles, forming a pair of centrosomes
What does a centrosome consist of?
A pair of centrioles- each one a hollow tube lined with nine micro tubules. The tubes are at right angles to each other.
When do the two centrosomes separate from each other?
At the G2 to M transition
Where do the centrosomes go after they have separated?
To opposite ends of the nuclear envelope
What does orientation of the centrosomes correlate with?
The plane at which the cell will divide
What is the consequence of determining the plane at which the cell divides?
It determines the spatial relationship of the two new cells to the parent cell- important for making body tissue
What surrounds the centrioles?
A high concentration of tubulin dimers
What do tubulin dimers do?
Initiate the formation of microtubules which orchestrate chromosomal movement
Plants lack centrosomes. How is chromosomal movement controlled?
A distinct microtubule organizing center at either end of the cell plays the same role
What does formation of microtubules lead to?
The formation of the spindle structures that segregate chromosomes
When do chromatids become visible?
prophase
When does the spindle form?
Prophase
What is visible under a light microscope during interphase?
Nuclear envelope
Nucleoli
Tangle oc chromatin
What happens during prophase to cohesin?
It is removed (not removed at centromere)- chromatids become visible
What structure develops late in prophase?
Specialized three-layered structures called kinetochores
Where do kinetochores develop?
In the centromere region, one on each chromatid
What are kinetochores important for?
Chromosome movement
What do the two centromeres act as?
A mitotic center, or pole, towards which the chromosomes move
How does the spindle form?
Microtubules form between each pole and the chromosomes
What is the purpose of the spindles?
A structure to which chromosomes attach and as a frame work for keeping the poles apart
What is the behavior of the spindles when they first form?
Initially unstable- they constantly form and fall apart
When do spindles become more stable?
When they contact microtubules from the other half spindle
What is the structure of the spindle?
Made up of 2 half spindles- each microtubule runs from one pole to the middle of the spindle, where it overlaps with the microtubules from the other half spindle
What are the two types of microtubule in the spindle?
Polar microtubules
Kinetochore microtubules
What are polar microtubules?
Microtubules that form the framework of the spindle
What is the structure of polar microtubules?
Abundant tubulin around centrioles
How are polar microtubules made up?
Tubulin dimers aggregate to form long fibers that extend to the middle region of the cell
What are kinetochore microtubules?
Microtubules that attach to the kinetochores on the chromosomes
When do kinetochore microtubules form?
Later than the polar microtubules
How do sister chromatids become attached to the kinetochore microtubules?
Sister chromatids in each chromosome pair become attached by their kinetochore to kinetochore microtubules in opposite halves of the spindle
What does the attachment of kinetochores to kinetochore microtubules ensure?
That one chromatid of the pair will move to one pole and the other chromatid to the opposite pole
What marks prometaphase?
The disappearance of the nuclear envelope and the nucleoli
What happens to the material that composes the nuclear envelope and nucleoli when it disappears during prometaphase?
It remains in the cytoplasm to be reassembled when daughter nuclei form
What begins to happen in prometaphase but is countered?
The movement of chromosomes towards the poles
What counters the movement of chromosomes towards the poles during prometaphase?
- Repulsive force from the poles pushes the chromosomes towards the middle region (equatorial plate) of the cell
- The two chromatids are still held at the centromere by cohesin
What is the middle region of the cell called?
The equatorial plate, or metaphase plate
What does the movement and countermovement of chromosomes during prometaphase do?
Makes the chromosomes appear to move aimlessly back and forth between poles and the middle as they approach the equatorial plate
What marks the start of metaphase?
When the centromeres arrive at the equatorial plate
When is the best time to see the sizes and shapes of the chromosomes and why?
Metaphase
Because they are maximally condensed
What happens by the end of metaphase?
All of the chromatid pairs separate simultaneously.
What marks the beginning of anaphase?
Separation of the chromatids
What happens during anaphase?
The sister chromatids move to opposite ends of the spindle
What does each chromatid contain during anaphase?
One double stranded DNA molecule called the daughter chromosome
Why does separation of the chromatids occur during anaphase?
One subunit of cohesin holding sister chromatids together is hydrolyzed
What type of molecule hydrolyzes the subunit of cohesin during anaphase?
A protease called separase
How is separase present in a cell?
It is present but inactive until anaphase, bound to an inhibitory subunit called securin
What happens to securin once all of the chromatids are connected to the spindle?
Securin in hydrolyzed
What effect does the hydrolysis of securin during anaphase have?
Separase catalyzes the cohesin breakdown
What is the process of hydrolyzing securin in order for separase to break down cohesin called?
The spindle checkpoint
What does the spindle checkpoint sense?
Whether there are any unattached kinetochores- if so, securin breakdown is blocked.
What two things move chromosomes along?
- Molecular motors
- Shortening microtubules
What act as molecular motors?
Proteins at the kinetochore
What are the names of the proteins at the kinetochore that act as molecular motors?
Cytoplasmic dynein
How do cytoplasmic dynein’s work?
They hydrolyze ATP to ADP
This releases energy to move chromosomes along microtubules towards the poles
What % of the force of motion do molecular motors account for?
75%
What % of force does the shortening of microtubules towards the poles account for?
25%
What happens to the poles during anaphase?
They are pushed further apart, doubling the distance between them
Why does distance between poles increase during anaphase?
Overlapping polar microtubules extending from opposite ends contain motor proteins that cause them to slide past each other
What happens to make the cell enter telophase?
The chromosomes stop moving at the end of anaphase
What is the situation at the start of telophase?
Two sets of identical daughter chromosomes are at opposite ends of the spindle
What happens to the spindle in telophase?
It starts to break down
What happens to the chromosomes in telophase?
They start to uncoil and become diffuse tangle of chromatin
What happens to the nuclear envelope and nucleoli in telophase?
They coalesce and re-form their respective structures
What does mitosis only refer to? (Hint: what separates it from cytokinesis?)
The division of the nucleus
Division of cytoplasm is cytokinesis
What effects how cytokinesis occurs?
The type of organism- for example, plants and animals are very different in their cytokinesis
How do animal cells divide during cytokinesis?
By furrowing of the plasma membrane as if an invisible thread was cinching the cytoplasm between poles
What is the invisible thread between poles that separates the cells during cytokinesis?
Microfilaments of actin and myosin in a contractile ring beneath the plasma membrane
How does actin and myosin form a contractile ring?
They interact like they do during muscle contraction to produce contraction
What do the microfilaments used to form the contractile ring assemble from?
Actin monomers that are present in the interphase cytoskeleton
What controls the assembly of microfilaments during cytokinesis?
Calcium ions released from storage sites in the center of the cell
Why do plant cells divide differently to animal cells?
Because of the presence of the cell wall
What happens after the spindle breaks down in mitosis?
Membranous vesicles derived from the golgi apparatus appear at the equatorial plate.
What happens after membranous vesicles appear at the equatorial plate of a plant cell during cytokinesis?
They are propelled along microtubules by motor protein kinesin and fuse to form new plasma membrane
What happens to the contents of the vesicles used to form a new plasma membrane in plant cells during cytokinesis?
Contribute vesicle contents to the cell plate
What is the cell plate?
The beginning of a new cell wall
What is not distributed precisely during mitosis?
Organelles such as ribosomes, mitochondria and chloroplasts
Why are organelles not precisely distributed?
Because it doesn’t matter as long as some of each is present
Why is unequal distribution of cytoplasmic components important?
It can have functional significance for the two new cells
What is the product of meiosis?
4 daughter cells that cannot undergo further duplications
What type of nuclear division is involved in reproduction?
Mitosis and meiosis, however, they have different reproductive roles.
What are the off spring produced in asexual reproduction called?
Clones- if the offspring are genetically identical
What can cause variation among asexually produces offspring?
Mutations
What is an advantage to asexual reproduction?
It is rapid and effective
What is an advantage to sexual reproduction?
It results in genetic diversity
How does meiosis produce some variation?
Gametes are different to each other and from their parent
What are cells not specialized for reproduction called?
Somatic cells
What are a set of chromosomes found in somatic cells called?
Homologous chromosomes
What are the characteristics of chromosomes found in homologous pairs?
They are similar size and appearance (except some sex chromosomes)
What genetic information do homologs have?
Similar but not usually identical genetic information
What type of cell are gametes?
Haploid
What is it called when two haploids fuse? What is the name of this process?
Zygote
Fertilization
What are the three hall marks of all sexual life cycles?
- Two parents, each provides chromosomes in the form of gametes produced by meiosis
- Each gamete is haploid
- Two gametes fuse to form a single cell (zygote) (diploid)
What are the three types of sexual life cycle that can be observed after zygote formation?
Haplontic
Alternation of generations
Diplontic
What organisms are haplontic?
Most protists, many fungi
What is a haplontic organism?
One where the zygote is the only diploid cell in the life cycle
What happens to the zygote in a haplontic organism?
It undergoes meiosis to produce haploid cells (spores)
What do spores of haplontic organisms do?
Form a new organism (single or multicellular) by mitosis
How do mature, haploid organisms produce gametes?
By mitosis
What organisms have alternation of generation sexual life cycles?
Plants, some protists
What does meiosis give rise to in alternation of generations?
Not gametes
Haploid spores
What happens to the haploid spores formed during alternation of generations?
Divide by mitosis for form an alternate, haploid life stage called gametophyte
What does the gametophyte produce?
Gametes, by mitosis
What happens to the gametes produced by haploid gametophytes?
Fuse to form a zygote
What happens to the zygote produced during alternation of generations?
It divides by mitosis to become the diploid sporophyte
What organisms are diplontic?
Animals, some plants
What are the only haploid cells in the life cycle of diplontic organisms?
The gametes
How are diplontic organisms zygotes produced?
Meiosis
What is able to be observed during metaphase of mitosis and hence counted and characterized?
Individual chromosomes
What is made to be able to observe an entire set of chromosomes?
A photomicrograph
What happens to the photomicrograph?
Images are arranged to reveal number, shape and sizes of the chromosomes
What do the number, shape and sizes of the chromosomes constitute?
The karyotype
What can the chromosomes of a karyotype be divided into?
22 pairs of autosomes
1 pair of sex chromosomes
How many nuclear divisions are there during meiosis?
2
How many times is the DNA replicated during meiosis?
once
What are the names of the two mitotic divisions?
Meiosis 1 and meiosis 2
What is a unique feature of meiosis 1 that separates it from mitosis?
Homologous chromosomes come together to pair along their entire lengths
What happens during metaphase 1 of meiosis?
The homologous chromosomes separate
When does crossing over occur?
Late stage prophase 1- prometaphase
What does crossing over result in?
exchange of genetic material
How is prophase 1 in meiosis 1 different?
It is long,
Chromosomes change markedly
How do chromosomes change during prophase 1 in meiosis 1?
The homologous chromosomes adhere along their length in a process called synapsis
When does the pairing process of synapsis start and end?
Starts during prophase 1
ends at the end of metaphase 1
Where does the joining of homologous chromosomes begin?
At the telomeres
How is joining of homologous chromosomes mediated?
Recognition of homologous DNA sequences on homologous chromosomes
What else helps to mediate joining of homologous chromosomes during meiosis 1?
A special group of proteins form a scaffold called synaptonemal complex
What is the structure of the synaptonemal complex?
Runs lengthwise along the homologous chromosomes ti join them together.
What are the four chromatids of each pair of homologous chromosomes called?
tetrad, or bivalent
How many chromatids are there in prophase 1 of meiosis 1 in humans?
96
During meiosis 1, when homologous chromosomes seem to repel each other, how are they held together?
By physical attachments mediated by cohesins
different to cohesins holding sister chromatids
What are regions of homologous chromosomes held together shaped like and called?
X-shaped
Chiasmata
What does a chiasma reflect?
An exchange of genetic material between non-sister chromatids- called crossing over
When do chromosomes start exchanging genetic material during meiosis 1?
Shortly after synapsis begins
not visible until repelling begins
How long does it take for prophase I to occur in the human male testes? What about for the entire meiotic cycle?
1 week, 1 month for entire meiotic cycle
In meiosis I, what attaches to the same half spindle?
The kinetochores of both chromatids, so the entire chromosome moves to the pole
How are homolgous chromosomes sorted?
The member attached is random, so sorting to each pole is random
What do the daughter nuclei at the end of meiosis I contain?
One set of chromosomes consisting of two chromatids
What happens in some organisms following anaphase I of meiosis I?
Telophase I
What follows telphase I of meiosis I?
Interphase called interkinesis
What happens during interkinesis?
Chromatin is partially uncoiled,
No replication of genetic material
What is the purpose of the second meiotic division?
To separate the chromatids
What are the three major difference between mitosis and meiosis II?
- DNA replicates before mitosis
- Chromatids making up chromosome are identical in mitosis
- Number of chromosomes on equatorial plate is 1/2 for meiosis II then mitosis
What is the result of meiosis II?
4 haploid nuclei
Why are the products of meiosis I genetically diverse?
- Synapsis during prophase I creates recombinant chromatids (crossing over)
- Independent assortment
What is independent assortment?
Homologous pairs line up randomly at metaphase I, paternal chromatids and maternal chromatids assort independently of each other
What is it called when homologous chromosomes fail to separate during meiosis I, or sister chromatids fail to separate during meiosis II?
Nondisjunction
How else may a meiotic error occur?
when chromosomes fail to remain together
What is the result of a meiotic error?
Aneuploid cells –> aneuploidy
What is aneuploidy?
A condition in which one or more chromosomes are either lacking or present in excess
What can cause aneuploidy?
Lack of cohesins
Why does a lack of cohesins cause aneuploidy?
Without them, homologs line up randomly at metaphase I, 50% chance both will go to the same pole.
What is a zygote with three copies of a particular chromosome called?
Trisomic
What causes Down syndrome?
Trisomy of chromosome 21
What are some symptoms of Down syndrome?
Impaired intelligence
Characteristic abnormalities of the hands, tongue, eyelids
Increased susceptibility to cardiac abnormalities and leukemia
What is a zygote with only one copy of a chromosome called?
Monosomic for that chromosome
Name another type of abnormal chromosomal event that can occur?
Translocation
What is translocation?
A piece of chromosome breaks off and becomes attached to another chromosome
How common are trisomies and monosomies in human zygotes?
10-30% of all conceptions
What happens to most embryo’s that develop from aneuploidy zygotes?
Do not survive to birth, or die before age 1
How many pregnancies are spontaneously terminated (miscarried) during the first 2 months, largely due to trisomies/monosomies?
1/5th
What occurs when organisms have complete extra sets of chromosomes?
Polyploidy
Why can polyploid cells still under go mitosis?
Because chromosomes behave independently of each other
Why can’t polyploid cells undergo normal meiosis?
Homologous chromosomes cannot synapse to begin division
Where is polyploidy common?
Modern agriculture:
-modern bread, wheat plants are hexaploids
