meiosis Flashcards
define homologs
define chromatids
- homologs: duplicated chromosomes derived from a common ancestor (chromosome from mother and father each have a homologous partner)
- chromatids: duplicated chromosomes, sister chromatids make up a chromosome (identical twins of a chromosome)
briefly describe each phase of meiosis
prophase 1: the chromosomes condense and the nuclear envelope breaks down. crossing over occurs.
metaphase 1: pairs of homologous chromosomes move to the equator of the cell
anaphase 1: homologous chromosomes move to the opposite poles of the cell
prophase 2: a new spindle forms around the chromosomes
metaphase 2: metaphase 2 chromosomes line up at the equator
anaphase 2: centromeres divide. chromatids move to opposite poles of the cells.
telophase 2 and cytokinesis: a nuclear envelope forms around each set of chromosomes. the cytoplasm divides
prophase 1:
1. what happens during prophase one?
- what does crossing over cause?
- chromosomes condense and the nuclear envelope breaks down
- crossing over occurs forming chiasmata, where there are special complexes which form at these complexes called the synaptial complex
what 2 things increase activity during the first and second divisions of meiosis?
- MPF (maturation promoting factor)
- a cyclin dependent kinase
controllers of meiosis:
1. what is used for regulation of activity?
- what what is key to the correct meiotic division (separation of chromatids)
- what holds chromosomes together?
- what are the two steps of breaking down these cohesion complexes?
- securing cyclin activation and degradation are also used for regulation of activity, just like in mitosis
- degradation of cohesion complexes is key to correct meiotic division
- condensin molecules hold and handcuff the chromosomes together, normally for the whole length of the chromosome, in meiosis the condensin molecule is called Rec 8
- in meiosis one, rec8 (cohesion is lost from arms) is destroyed at everywhere except the centromere
- in meiosis two, this complex is broken (cohesion is lost from centromere) and allows the chromosomes to separate properly and the cell becomes haploid
the difference between meiosis and mitosis:
1. what is the main function of mitosis?
2. what is the main function of mitosis?
- what are the products of meiosis? and how many rounds of replication are there? what is a bivalent sructure?
- what are the products of mitosis? and how many rounds of replication are there?
- mitosis is cell division for cell growth
- meiosis is cell division for gamete production
- meiosis results in the formation of haploid cells, includes one round of replication and two divisions (pairing of replicated maternal and paternal homologous chromosomes line up on the metaphase plate next to each other
- this is called a bivalent structure where replicated paternal/maternal chromosomes (made up of chromatids) are paired) - mitosis results in the formation of diploid cells and is one round of replication for one round of division (homologous chromosomes, one from the mother, one from the father, line up independently on metaphase plates)
what is the difference in meiosis in men and women (generation of gametes: spermatogenesis/oogenesis)
in spermatogenesis, the generation of gametes (sperm) is continuous and four gametes (sperm) is produced from on precurser
in oogenesis, the meiosis division 1 of the egg is completed at puberty and at menstruation, meiosis 2 starts and remains uncompleted and is only completed at fertilisation (pregnancy). one egg is produced from one precurser
what 2 features of meiosis contribute to genetic diversity?
recombination and independent assortment of homologs
recombination:
- recombination is important in genetic diversity, it causes crossover
- if genes causing a particular phenotype are very close together then they are likely to be inherited together, giving us strong clues on the organisation of chromosomes and the genetic transmission of disease
- forming this crossover structure and interlocking involves the Holliday junction
independent assortment:
independent assortment of homologs creates a chance for huge variation as there are 23 pairs of chromosomes which line up independently creating a vast number of combinations of chromosomes in the gametes
how is pairing and recombination of sex chromosomes in males different?
what does the resultant structure give?
- this pairing and recombination of sex chromosomes in males is different as they have one X chromosome and one Y chromosome which pair at the pseudo-autosomal region
- this chromosome structure then gives a way of tracing paternal ancestry because the rest of the chromosome does not undergo recombination
- what is a common meiotic error in meiosis 1?
- what specifically causes down syndrome?
- separation of the homologs resulting in a trisomic zygote (extra chromosome in the zygote)
- the most common disease arising from this is down syndrome in which an with three copies of chromosome 21 is formed - this is due to the non-disjunction in homologs in meiosis division 1 meaning that one of the precursers goes on to be an egg and does not inherit any copies of chromosome 1 and he other egg precurser goes on to acquire both copies of chromosome 21. if the latter egg is fertilised an individual with 3 copies of chromosome 21 is seen (down syndrome phenotype)
- what is a common meiotic error in meiosis 2?
- what diseases can this lead to?
- separation of sister chromatids, also resulting in a trisomic zygote (extra chromosome in zygote)
- this can lead to diseases such as:
- Angelman syndrome
- Prada-Willi syndrome
- Beckwith-Wiedemann syndrome
what is the trend with meiotic errors and maternal age?
why is this the case?
- meiotic errors increase with maternal age
- this is due to the differences in generation of gametes in men and women and centromeres are not held so closely together as maternal age increases and so cohesion defects increase
what is uniparental disomy? heterodysomy/isodysomy?
uniparental disomy occurs when a person receives two copies of a chromosome, or a part of a chromosome from one parent and no copy from the other
- heterodysomy is when 2 chromosomes are different members of a pair, still from one parent
- isodysomy is when both chromosomes are identical
what causes uniparental heterodysomy/isodysomy
the third chromosome does not get anchored to the spindle which can result in an anaphase lag in which it is separated into a micronucleus and then lost causing the individual to then end up with the right number of chromosomes however sometimes these copies have come from the same parent
