2.4 - Meiosis Flashcards
What is Meiosis?
- It is the process by which gametes are made in the reproductive organs.
- It involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei
What are the two divisions involved?
- Meiosis I
- Meiosis II
I (PMAT) x 2
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
TELOPHASE I
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
First Stage of Meiosis (INTERPHASE)
- During interphase, DNA is replicated (in the S phase) to produce 2 genetically identical copies
- ## The 2 identical DNA molecules are called ‘sister chromatids’ and are held together by single centromeres.
PROPHASE I
- Chromosomes condense and get shorter/fatter
- then arrange themselves side by side into homologous pairs which form bivalents
- centrioles move to opposite ends of the cell forming the spindle fibres
- nuclear envelope dissolves
- crossing over of genetic information occurs in this stage
What is crossing over?
- It is the process by which non-sister chromatids of the same homologous pair exchange alleles
Explain the process of crossing over.
- During prophase I, homologous chromosomes pair up and are in very close proximity to each other
- The non-sister chromatids can cross over and get twisted around each other
- the crossing points are ‘chiasmata’
- the twisting places stress on the DNA molecules
- This results in a section of chromatid from one chromosome being broken and rejoined with the chromatid from the other chromosome
- This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes.
METAPHASE I
- Bilavents line up along the equator of the cell
- spindle fibres attach to the centromeres
- orientation of each homologous pair is random and independent of any other homologous pair
- This is called independent assortment
What is independent assortment?
- the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I
- This means when they are pulled towards the equator of the spindle:
1. each pair can be arranged with the maternal on the left and paternal on the right or vice versa.
2. The orientation of one homologous pair is unaffected by the orientation of any other pair - when the homologous chromosomes are separated and pulled to different poles, the combination of alleles that end up in each daughter cell depends on how the homologous pair lines up
TELOPHASE I
- chromosomes assemble at each pole and decondense
- cell undergoes cytokinesis to form two haploid daughter cells
PROPHASE II
- start of second division
- chromosomes pair up and re-condense
- nuclear envelop breaks down again and spindle fibres reform
- centrioles move to opposite poles of the cells
METAPHASE II
- chromosomes are lined on the equator of the spindle fibres
ANAPHASE II
- Centromere divides
- Pairs of sister chromatids are separated
- Each new daughter cell inherits one chromatid from each chromosome
- spindle fibres contract
TELOPHASE II
- chromatids uncoil and decondense
- spindle fibres break down
- nuclear envelopes reform
- cell undergoes cytokinesis
CYTOKINESIS
- cytoplasm and surface membrane forms
- this creates four genetically different haploid daughter cells (these are gametes)
What are the three main ways that genetic variation occurs in meiosis?
- Crossing Over:
It is the process by which non-sister chromatids of the identical homologous pair exchange alleles. This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes. - Independent Assortment:
the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I. This means that each pair can be arranged with the maternal on the left and paternal on the right or vice versa and the orientation of one homologous pair is unaffected by the orientation of any other pair. So when they separate into either pole the combination of alleles depends on how the homologous pair line up. - Random Fusion of gametes
During fertilisation, any male gamete can fuse with any female gamete to form a zygote.
This random fusion of gametes at fertilization creates genetic variation between zygotes as each
will have a unique combination of alleles.
What are chromosome mutations?
refer to structural change in the chromosome that can affect the arrangement of genes
Give an example of a chromosome mutation.
Translocation - occurs when a segment of one chromosome breaks off and attaches to another chromosome
What are the effects of translocation?
This process can disrupt the normal arrangement of genes, which could potentially lead to genetic disorders or cancer the the rearranged genes affect cell growth or function
What is non-disjunction?
A type of chromosomal error that occurs when chromosomes fail to separate properly during meiosis. Resulting in gametes with an abnormal number of chromosomes
When such gametes are involved in fertilisation, they produce a zygote with either extra or missing chromosomes, leading to either polysomy (extra chromosome) or monosomy (missing chromosome)
Polysomy
- occurs when an individual has one or more extra copies of chromosome
- this happens if non-disjunction occurs in either meiosis I or II.
- if it happens in meiosis I, homologous chromosomes fail to separate, resulting in two gametes with an extra chromosome
- if it happens in meiosis II, sister chromatids fail to separate, leading to one gamete with an extra chromosome (one missing and two normal)
- this produced 47 chromosomes (polysomy) leading to a genetic disorder like Down syndrome
Example of polysomy
Down Syndrome (trisomy 21)
- condition happens when there is an extra copy of chromosome 21, giving the individual three copies instead of the usual two
- results in 47 chromosomes than the typical 46
- down syndrome is associated with physical characteristics like distinct facial features and development delays
- increases various health issues like congenital heart defects and certain autoimmune conditions
Monosomy
- occurs when an individual is missing a chromosome
- happens when non-disjunction occurs during either meiosis I or II
- if it happens in meiosis I, homologous chromosomes fail to separate and two gametes have a missing chromosome
- if it happens in meiosis II, sister chromatids fail to separate and one gamete is missing a chromosome. (one extra, two normal)
- produces 45 chromosomes leading to genetic disorders like Turner syndrome.
Example of monosomy
Turner Syndrome (monosomy x)
- when an individual has only one X chromosome (45 chromosomes in total, missing the second sex chromosome)
- affects females and can result in short stature, infertility, and other developmental issues
- affected individuals often experience heart and kidney abnormalities, and may have certain learning difficulties though intelligence is generally unaffected.
