mitosis and meiosis- lecture 7 Flashcards
each cell nucleus contains about how many feet of dna
6 feet
dna is wrapped around special proteins called
histones
nucleosome
combined loop of dna and histone potein
chromatin
nucleosomes packed into a thread
chromosome
a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.
short arm of chromosome
p arm
long arm of chromosome
q arm
middle of chromosome
centromere
tip of chromosome
telomere
heterochromatin vs euchromatin
gene poor repeat rich vs gene rich
sister chromatid
A sister chromatid refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be ‘one-half’ of the duplicated chromosome
centromere function and is home to
constricted region of the chromosome where the kinetochores form
home to kinetochore, where the spindle microtubules attach in mitosis and meiosis
dna replication
why do dna molecules (chromosomes) get shorter over time
unidirectional feature of dna polymerase
how do cells stop chromosomes from getting shorter
telomeres (like tips of shoelace) use enzyme telomerase to prevent division by division erosion of tips of chromosome
how to create karyotype
ease of viewing chromosomes facilitated by colchine, which disrupts spindle formation in mitosis and creates freeze frame mitotic moment that we can stain
how are chromosomes stained for a karyotype
stained using glemsa which stains preferentially a-t regions of the genome
chromosomes get categorized in what ways
where they keep their centromeres and size
n number
haploid chromosome complement (number of chromosomes)
diploid
two copies of each chromosome, one from each parent
homologous chromosomes vs non homologous chromosomes
Homologous chromosomes consist of alleles of the same gene type found in the same loci unlike non-homologous chromosomes, which constitute alleles of varying gene types
different ways of visualizing human chromosomes
you can id different chromosomes based on banding pattern and size
how do we lay out karyotypes
by size; chromosome 1 is the largest and 22 is the smallest
how many chromosomes do we have
Normally, each cell in the human body has 23 pairs of chromosomes (46 total chromosomes). Half come from the mother; the other half come from the father. Two of the chromosomes (the X and the Y chromosome) determine your sex as male or female when you are born.
trisomy 21
down syndrome- extra chromosome 21
aneuploidy
having extra or missing chromosomes
dosage imbalance
Genetic imbalance is to describe situation when the genome of a cell or organism has more copies of some genes than other genes due to chromosomal rearrangements or aneuploidy. Changes in gene dosage, the number of times a given gene is present in the cell nucleus, can create a genetic imbalance because youre pumping out too much of a protein when the amount of each protein in a cell is crucial
why is down syndrome more common than autosomal trisomies
chromosome 21 is a small chromosome so its impact on dosage imbalance is minor
chromosomal rearrangements
deletion
duplication
inversion
translocation
cell cycle
A cell cycle is a series of events that takes place in a cell as it grows and divides. A cell spends most of its time in what is called interphase, and during this time it grows, replicates its chromosomes, and prepares for cell division. The cell then leaves interphase, undergoes mitosis, and completes its division
stages of cell cycle
The cell cycle is a four-stage process in which the cell:
1) increases in size (gap 1, or G1, stage)
2) copies its DNA (synthesis, or S, stage)
3) prepares to divide (gap 2, or G2, stage)
4) divides (mitosis, or M, stage)
checkpoints of cell cycle
There are many checkpoints in the cell cycle, but the three major ones are:
the G1/S checkpoint, also known as the Start or restriction checkpoint or Major Checkpoint
the G2/M checkpoint
and the metaphase-to-anaphase transition, also known as the spindle checkpoint.
why are cellular checkpoints important
need them to regulate cell growth; without them you can get cancer which is uncontrolled cell growth
what happens when dna gets tangled
topoisomerase untangles a pair of dna molecules by cleaving one dna duplex, passing the other duplex through the gap and then repairing the dna
mitosis
each daughter cell has an identical complement of chromosomal material
5 stages of mitosis
1) prophase
2) prometaphase
3) metaphase
4) anaphase
5) telophase: cytokineseis
prophase
Chromosomes, consisting of two sister chromatids, condense. The sister chromatids are held together along their length via the action of cohesin proteins. The cohesin proteins attach to the chromosomes during S-phase and persist until the chromatids are separated. As the chromosomes condense, if they become entangled or crossed, the enzyme topoisomerase cleaves one chromosome and passes the other through to resolve the tangle, then re-seals the break.
prometaphase
microtubules polymerize at the centrioles (in the centrosome) which start to form the poles of a bipolar spindle. ends of the microtubules try to attach to protein structures at the centromeres called kinetochores
metaphase
The microtubules move the chromosomes to the central plane of the cells by pulling on the attached chromatids until they reach the
center of the cell. This central plane is also known as the metaphase plate or the equator, and at metaphase the chromosomes are all aligned
there. When the chromosomes are aligned in the center, there is approximately equal tension “pulling” on each chromosome from the two
poles of the cell. This tension sends a signal to the cell that all of the chromosomes are attached to both poles, and it is safe to proceed through mitosis.
anaphase
Contraction of the microtubules creates tension that stimulates the enzyme separase to cleave the cohesin proteins connecting the sister
chromatids. The sister chromatids then separate and are pulled to opposite poles of the cell. Upon separation, each chromatid becomes a chromosome and there cease to be any sister chromatids in the cell.
telophase
The chromosomes arrive at the nuclear pole and the nuclear envelope reforms forming two distinct nuclei, each containing a full
complement of chromosomes. The chromosomes decondense back into
diffuse chromatin.
Cytokinesis often (but not always) occurs after telophase
centrosome vs centriole
tubulin
Tubulin is the protein that polymerizes into long chains or filaments that form microtubules, hollow fibers which serve as a skeletal system for living cells. Microtubules have the ability to shift through various formations which is what enables a cell to undergo mitosis or to regulate intracellular transport.
kinetochore
The kinetochore plays an essential role in facilitating chromosome segregation during cell division. This massive protein complex assembles onto the centromere of chromosomes and enables their attachment to spindle microtubules during mitosis
what happens in both prophase and metaphase
the cohesin proteins embrace both sister chromatids, holding them together. once the polewards tension is established in metaphase, and the two sister chromatids are correctly connected to opposing poles, opposed forces activate separase, which cleaves cohesin releasing the embrace and allowing the chromosomes to move toward the poles
mitosis vs meiosis
mitosis ensures that the genetic complement of the daughter cells is identical to that of the parent cell
meiosis shuffles chromosomes to maximize the genetic diversity of gamete. here the goal is to take diploid cells and form haploid ones that contain half the number of chromosomes.
Therefore, they are genetically distinct from the diploid parent cell
major phases of meiosis
interphase, prophase I, metaphase I, anaphase I, telophase I, cytokinesis, interphase II, metaphase II, anaphase II, and telophase II
Meiosis I
The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated
prophase 1 meiosis 1
During prophase I the homologous chromosomes pair up, gene by gene, along their entire lengths, forming the synaptonemal complex. While paired, non-identical sister chromatids (one from each homolog) exchange genetic information in the process of crossing over at chiasmata (singular: chiasma). Crossing over (or recombination) is an essential feature of meiosis I. Cohesins are deposited at the sites where recombination occurs and help to stabilize the homologous pairs as they
are moved to the metaphase plate. Crossing over is one mechanism by which genetic diversity is generated during meiosis, as it produces novel combinations of alleles
metaphase 1 meiosis 1
The homologous pairs align at the metaphase plate during metaphase I. The order in which the chromosome align is completely random. This has important consequences, since as the chromosomes segregate during anaphase I, each daughter cell receives a unique combination of maternal and paternal chromosomes. This independent assortment of chromosomes is the second mechanism by which genetic
diversity is created during meiosis I.
telophase 1 meiosis 1
Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
meiosis 2
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
prophase 2 meiosis 2
Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
metaphase 2 meiosis 2
Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
anaphase 2 meiosis 2
Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
telophase 2 meiosis 2
Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
The final outcome of meiosis is the production of
four haploid daughter cells
These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)
meiosis 1 explain
role of cohesin
meiosis creates genetic diversity in two ways:
1) recombination between parental chromosomes creating new permutations of alleles at different loci
2) by randomizing the associations of chromosomes
nondisjunction
the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei.
why is maternal age associated with nondisjunction
how many chromosomes do we have
In humans, there are 23 unique chromosomes, numbered 1-22 and X / Y, with chromosome 1 being the longest and chromosome 22 being
the shortest. X and Y are known as the sex chromosomes, and the non-sex chromosomes (Chr1-22) are known as autosomes. In eukaryotic cells chromosomes are found in the nucleus, bounded by the nuclear envelope. Although each chromosome contain distinct genes, each
chromosome has the same general structure
Centromeres and telomeres are comprised of densely packed DNA, which is known as
heterochromatin. This tightly compacted
DNA is inaccessible to the transcriptional machinery, and as such centromeres and telomeres do not contain actively transcribed genes. In contrast, the chromosomal regions that encode actively transcribed genes are comprised of relaxed, open DNA known as euchromatin.
homologous chromosomes
Homologous chromosomes possess the same genes, in the same order, though they may possess different alleles or versions of those genes. Normal human karyotypes contain 46 chromosomes
trisomy
an extra chromosome
which phase of meiosis is most like mitosis
meiosis 2
difference between meiosis 1 and mitosis
in meiosis 1, the sister chromatids remain connected to each other but in mitosis they split
what is the role of cohesin and shugosin in meiosis
how does meiosis create genetic diversity 1
how does meiosis create genetic diversity 2
homologous chromosomes
Two chromosomes in a pair – normally one inherited from the mother and one from the father. For example, the two copies of Chromosome 1 in a cell would be referred to as homologous chromosome
Homologous chromosomes
possess the same genes, in the same order, though they may possess
different alleles or versions of those genes
when is the dna in a germline cell duplicated
before meiosis begins- in s phase
how can you tell which phase of meiosis nondisjunction occurred?
The consequences of meiotic non-disjunction differ depending on whether chromosomes or chromatids fail to separate during the first or
second division. Nondisjunction during meiosis I results from a failure of homologous chromosomes to separate. Consequently, both homologs of a pair segregate into a single daughter cell and the other daughter cell
does not receive a copy of this chromosome. These aberrant daughter cells then proceed through meiosis II, producing two gametes that lack this chromosome and two that contain two copies (instead of only one). Thus, non-disjunction in meiosis I results in all four gametes being aneuploid. When these abnormal gametes combine with normal
gametes, they produce aneuploid offspring that are trisomic (from the gamete that contained both homologs) or monosomic (from the gamete that was lacking the chromosome).
In contrast, non-disjunction in meiosis II results in two normal and two aneuploid gametes. Here, the cell proceeds normally through meiosis I, and the homologous chromosomes segregate evenly into the two daughter cells. Non-disjunction in meiosis II results in a failure of sister chromatids to separate in one of the cells during anaphase II. One daughter cell therefore receives both sister chromatids, and the other does not receive a copy of this chromosome. Non-disjunction typically
only occurs in one of the two products of meiosis II, while the other cell proceeds normally through meiosis, producing two normal haploid gametes. Therefore, non-disjunction in meiosis II produces two aneuploid
and two haploid gametes. When the abnormal gametes combine with normal gametes, they produce aneuploid offspring that are trisomic (from the gamete that contained both copies of the chromosome) or monosomic (from the gamete that was lacking the chromosome).
Amniocentesis
Amniocentesis is a test you may be offered during pregnancy to check if your baby has a genetic or chromosomal condition, such as Down’s syndrome, Edwards’ syndrome or Patau’s syndrome. It involves removing and testing a small sample of cells from amniotic fluid, the fluid that surrounds the baby in the womb (uterus).
how many chromosomes does each cell have
Normally, each cell in the human body has 23 pairs of chromosomes (46 total chromosomes). Half come from the mother; the other half come from the father. Two of the chromosomes (the X and the Y chromosome) determine your sex as male or female when you are born.
Robertsonian translocation
a special case in which two chromosomes are combined into one with relatively little loss of
genetic material
G1/S checkpoint
Checkpoints are
vital for quality control and help prevent the production of aberrant,
aneuploid cells; cells without sufficient materials will not proceed past
specific checkpoints. At the G1/S checkpoint, the cell evaluates whether
there are sufficient enzymes and components for DNA replication
stages of cell cycle
g1
s
g2
those are interphase, then there is:
m
gene dosage
This is because of the importance of maintaining a balance of gene dosage from all chromosomes. The symptoms of Down syndrome, such as
intellectual disability and facial abnormalities, are due to an imbalance in the amounts of gene products produced from chromosome 21. Wild type cells have two copies of each chromosome and as such the cell is considered ‘balanced,’ meaning that the correct amount of mRNA and
protein is produced from each gene. However, in the case of trisomy 21, these cells have an extra copy of chr21 and thus there is increased
expression from all the genes present on this chromosome. Since many cellular processes are highly sensitive to the relative amounts of gene
products, this increased expression disrupts many essential cellular functions.
Chromosome 21 has the fewest number of protein-coding genes of all the chromosomes and this explains why trisomy 21 is relatively
common, but trisomy 1 is never observed. During meiosis, there will be non-disjunction of chromosomes other than 21 – errors such as
nondisjunction occur more or less randomly, so all chromosomes are affected with approximately equal frequency. However, the additional
copy of these larger chromosomes, which contain more genes, upsets the normal equilibrium in cells so much that the resulting embryos are inviable.
gene dosage
This is because of the importance of maintaining a balance of gene dosage from all chromosomes. The symptoms of Down syndrome, such as
intellectual disability and facial abnormalities, are due to an imbalance in the amounts of gene products produced from chromosome 21. Wild type cells have two copies of each chromosome and as such the cell is considered ‘balanced,’ meaning that the correct amount of mRNA and
protein is produced from each gene. However, in the case of trisomy 21, these cells have an extra copy of chr21 and thus there is increased
expression from all the genes present on this chromosome. Since many cellular processes are highly sensitive to the relative amounts of gene
products, this increased expression disrupts many essential cellular functions.
Chromosome 21 has the fewest number of protein-coding genes of all the chromosomes and this explains why trisomy 21 is relatively
common, but trisomy 1 is never observed. During meiosis, there will be non-disjunction of chromosomes other than 21 – errors such as
nondisjunction occur more or less randomly, so all chromosomes are affected with approximately equal frequency. However, the additional
copy of these larger chromosomes, which contain more genes, upsets the normal equilibrium in cells so much that the resulting embryos are inviable.
the number of sets of chromosomes in a cell, or in the cells of an organism.
Ploidy
