Lec 2 Mitosis and Meiosis Flashcards
What does mitosis produce?
two identical diploid daughter cells
What does meiosis produce?
Gametes - sperm and egg cells
they are gentically diverse haploid cells
they have half the number of chromosones
What is a somatic cell?
any non reproductive cell
In what cases do we need mitotic growth?
some tissues, such as the epidermis, mucosae and bone marrow
Describe the structure of a chromosome before mitosis
has a short p arm and a long q arm
a centromere
telomeres on each end
made up of a pair of sister chromatids
What are the possible positions of a centomere
metacentric
submetacentric
acrocentric
telocentric
What are the phases of mitosis
prophase, prometaphase, metaphase, anaphase, telophase
Followed by cytokenisis
What are the phases of meiosis
prophase 1, metaphase 1, anaphase 1, telophase 1, cytokenisis
prophase 2, metaphase 2, anaphase 2, telophase 2, cytokenisis
Describe the process of prophase in detail
Prophase - nuclear envelope breaks down
chromosones begin to condense
spindle fibres appear
centrioles begin to move to opposite ends of the cell
kinetocores recognise and bind to the centromeres
Describe prometaphase in mitosis
chromosones begin to line up on the metaphase plate,
pulled by the kinetocore spindle fibres
the centrioles are now on opposite ends of the cell
Describe Metaphase and anaphase
Metaphase - chromosomes are now lined up on the metaphase plate
spindles begin to pull the chromosomes apart
Anaphase - 2 daughter chromsomes are pulled to opposite ends of the cell, as centromeres divide
Describe telophase and cytokenisis
Telophase- nucleus/nucleolus begin to reform
cleavage furrow forms as cleavage begins
they are now 2 identical copies
Cytokenisis- The cytoplasm divides
what is a homologous chromosome pair?
they have the same DNA, and same genes, they may have different alleles
sister chromatids have?
the same DNA and the same alleles
Noo sister chromatids have?
The same DNA and potentially different alleles
what is an allele
a variant of the same gene
what does meiosis produce?
4 non identical haploid cells
what is formed during prophase 1? why?
a tetrad(bivalent form) this allows crossover (at chiasma) to happen between non sister chromatids
the maternal and paternal DNA is swapped
this is known as recombination
what happens during prophase 1?
same as in mitosis - go over this aswell
except tthe homologus pair locate each other to form a tetrad
they can do this as they have the same DNA
This allows recombination (crossing over) to happen
what happens during metaphase 1 and anaphase 1?
metaphase 1 - the Homogulous pair lines up on the metaphase plate
Anaphase 1 - the tetrad separates in two. they are pulled to opposite end of the cell randomly
describe telophase 1
same as in mitosis - explain
there are now 23 chromosomes in each cell.
2 daughter cells are produced
what happens in meiosis 2?
both cells undergo prophase 2
then metaphase 2 where the chromosones line up randomly
sister chromatids are separated in anaphase 2
refomrs in telophase 2
this produces 4 non indentical haploid cells
what is the purpose of meiosis ?
to maintain a constant number of chromosomes over generations
produces genetic diversity via the random chromosone assortment
and via the crossing over of non sister chromatids
Describe Spermatogenisis
Check notes in book - same process as meiosis
can undergo mitosis to maintain sperm count
process takes about 60 days
prodiuces spermatids which mature to become sperm
Describe Oogenisis
Check Using notes (write this up later)
Takes 12-50 years
Bonus!
tip: crossing over is essential from meiosis to work
it will fail other wise and we dont know why
Humans are bad at meiosis, what are the consequences of this?
30 in 100 fail,
this causes a third of all identified miscarriages
the leading cause of mental retardation
causes infertility
What Error causes downs syndrome?
a non dis junction, where the chromatids fail to separate in anaphase. this causes aneuploidy (an abnormal chromosome number)
How do we visualise and log chromosomes ?
We use a metaphase spread, this requires actively dividing cells, the spreads may look different depending on where the chromosomes were harvested from, and how long the process takes
Karyotyping
what is a 47,xx, +21 ?
a female with trisomy ( Downs Syndrome), having one extra chromosome
Bouns : monosomy often terminates the cell line
non disjunctions are also important in mitosis (particularly the first 50 miotic divisons)
what can a NDJ in mitosis cause?
a NDJ in the first post zygotic division will mean all cells are eg. 47,xy, +21, and hence all cells will have trisomy
this gives a non mosaic karyotype.
A NDJ after this will produce 47,+21/46, N
a mosaic karyotype
What is Mosaicsm ?
the presence of two or more cell lines in an individual
can be body wide or tissue specific, this is dependent on when the NDJ occurs
Explain Non Disjunctions is meiosis.
a NDJ in meiosis can happen in either stage 1 or 2. This causes different things.
In meiosis 1 a ndj will lead to one N+1 cell and one N-1 cell, which then divide to give two of each.
These are then fertilised by the sperm.
the potential products can be two Trisomy cells and two monosmy cells
If the NDJ happens in meiosis 2 then there are two normal cells, one with trisomy and one with monosomy.
So can get a healthy fetus.
What do you know about the nucleolus?
a group of chromosomes gather, that are related to RNA coding and transcription and ribosome production.
it vanishes during M phase as the chromosomes condense into there individuals
What is Stage G0?
when cells exit the cell cycle
they can do this permanently like neurons, or temporarily before a signal makes them re join
how to chromosomes find each other in pro-phase 1 of meiosis?
Autosomes find each other as they have the same DNA sequence, allowing them to identify each other. Same Sequence Identity
Sex Chromosones although different, must have a similar sequnce to find each other. The PseudoAutosomalRegions are identical and hence the same process
What are intersex people, how are they caused?
the SRY gene codes for initiation of male traits.
If crossing over occurs on sex chromosomes, and the SRY gene is swapped its possible to be a XY with no SRY gene and hence develop as a female
Or it is possible to be XX and have the SRY gene, so develop as a male who is XX
just before/ after division what are the numbers you need to know for chromosones?
Before
46 chromosomes (23 Pairs), 92 Chromatids, 92 DNA molecules, 1 chromosome = 2 DNA Molecules
After
46 Chromosomes (23 Pairs), 46 Chromatids, 46 DNA molecules, 1 chromosome = 2 DNA Molecules
