3.3 Meiosis
Essential idea: Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.
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3.3.U1 One diploid nucleus divides by meiosis to produce four haploid nuclei.
3.3.U1 One diploid nucleus divides by meiosis to produce four haploid nuclei.
Be able to:
Compare divisions of meiosis I and meiosis II.
Meiosis is the process by which sex cells (gametes) are made in the reproductive organs. It involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei
The process of meiosis consists of two cellular divisions:
The first meiotic division separates pairs of homologous chromosomes to halve the chromosome number (diploid → haploid)
The second meiotic division separates sister chromatids (created by the replication of DNA during interphase)
3.3.U2 The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
3.3.U2 The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
Be able to:
State that DNA is replicated in interphase before meiosis.
Given a diploid number (for example 2n=4), outline the movement and structure of DNA through the stages of meiosis.
DNA is replicated in interphase, which is not a part of meiosis, but must precede it
replication = formation of sister chromatids
during S phase of interphase, DNA replication produces chromosomes made of two chromatids each, joined at the centromere
3.3.U4 The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
3.3.U4 The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
Be able to:
Describe the different stages of meiosis
Explain how crossing over occurs
Meiosis –animation of meiosis
Meiosis I: prophase I
Meiosis I: prophase I
chromosomes condense
nucleolus disappears
spindle forms
synapsis of bivalents: pairing of homologous chromosomes
crossing over occurs between non-sister chromatids
nuclear membrane disappears
At the start of prophase I, the chromosomes have already duplicated. During prophase I, they coil and become shorter and thicker and visible under the light microscope.
The duplicated homologous chromosomes pair, and crossing-over (the physical exchange of chromosome parts) occurs. Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous chromosome pair is visible as a bivalent (tetrad), a tight grouping of two chromosomes, each consisting of two sister chromatids. The sites of crossing-over are seen as crisscrossed nonsister chromatids and are called chiasmata (singular: chiasma)
Meiosis I: metaphase I
Meiosis I: metaphase I
chromosomes continue to condense
spindle microtubules attach to centromeres
bivalents line up at equator
Meiosis I: anaphase I
Meiosis I: anaphase I
separation of homologous pairs;
chromosomes moved to poles by spindle
Meiosis I: telophase I
Meiosis I: telophase I
chromosomes at poles
nuclear envelope reappears
spindle disappears
chromosomes partially uncoil
Meiosis II: prophase II
Meiosis II: prophase II
chromosomes condense again
nuclear envelope disappears
new spindle forms (at right angles to previous spindle)
Meiosis II: metaphase II
Meiosis II: metaphase II
spindle microtubules attach to centromeres
chromosomes move to the equator
Meiosis II: anaphase II
Meiosis II: anaphase II
sister chromatids separate
spindle microtubules move chromatids to opposite poles
Meiosis II: telophase II
chromosomes (=chromatids) arrive at poles
spindle disappears
nuclear membrane reappears
nucleolus reappears
chromosomes decondense into chromatin
**Cytokinesis not part of meiosis cell division
**Cytokinesis not part of meiosis cell division
3.3.U5 Orientation of pairs of homologous chromosomes prior to separation is random
3.3.U5 Orientation of pairs of homologous chromosomes prior to separation is random
B able to:
Describe the attachment of spindle microtubules to chromosomes during meiosis I.
Describe random orientation of chromosomes during meiosis I.
Independent assortment:
Independent assortment:
Variety produced by recombination of maternal and paternal chromosomes
For each pair of homologous chromosomes, maternal and paternal chromosomes assort to daughter cells randomly
Possible arrangements of chromosomes in haploid daughter cells = (2)nth, where n = number of homologous pairs
In humans, n = 23, and possible arrangements = (2)23 = about 8 million
Mendel’s law of independent assortment applies only to traits carried on different chromosomes, i.e.unlinked genes
Independent assortment occurs as a result of the alignment of homologs during metaphase I, determining which maternal and paternal chromosomes assort to each daughter cell
Each pair of alleles separates independently of every other pair of unlinked alleles
3.3.U6 Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
3.3.U6 Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
Be able to:
Explain why meiosis I is a reductive division.
State that cells are haploid at the end of meiosis I.
Cells are haploid at the end of meiosis I because the homologous chromosomes separate into different cells
3.3.U7 Crossing over and random orientation promotes genetic variation.
3.3.U7 Crossing over and random orientation promotes genetic variation.
Be able to:
Explain how meiosis leads to genetic variation in gametes.
State that the number of chromosome combinations possible due to random orientation is 2n.
The advantage of meiotic division and sexual reproduction is that it promotes genetic variation in offspring
The three main sources of genetic variation arising from sexual reproduction are:
Crossing over (in prophase I)
Random assortment of chromosomes (in metaphase I)
Random fusion of gametes from different parents
Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I mixing different sets of DNA.
The possible combinations for random orientation in human are quite large, in fact, there are 2^23 possible combinations or 107,3741,824 possible orientations. (2 to the power of haploid number of chromosomes)
3.3.U8 Fusion of gametes from different parents promotes genetic variation
3.3.U8 Fusion of gametes from different parents promotes genetic variation
Be able to:
Outline the role of fertilization as a source of genetic variation.
Both the father and the mother have sex cells produced through meiosis and so have a variety of unique gametes with unique genomes. And when the gametes fuse the egg and the sperm that will be fertilized is random. Out of the many eggs that woman have and the millions sperm males produce the two that will fertilize is completely random. And this also leads to genetic variation
3.3.A1 Non-disjunction can cause Down syndrome and other chromosome abnormalities.
3.3.A1 Non-disjunction can cause Down syndrome and other chromosome abnormalities.
Meiosis is the process of creating haploid gametes from a diploid cell. If everything goes smoothing during meiosis, chromosomes will be separated and distributed evenly to produce four haploid gametes. However, sometimes chromosomes do not separate properly. This is called nondisjunction and results in gametes with either too many or too few chromosomes. In humans, nondisjunction becomes more common the older one gets.
If nondisjunction occurs during anaphase I of meiosis I, this means that at least one pair of homologous chromosomes did not separate. The end result is two cells that have an extra copy of one chromosome and two cells that are missing that chromosome. In humans, n + 1 designates a cell with 23 chromosomes plus an extra copy of one for a total of 24 chromosomes. n - 1 designates a cell missing a chromosome for a total of only 22 chromosomes in humans.
If one of these abnormal gametes undergoes fertilization, then a baby with an abnormal number of chromosomes in its cells could be born. Trisomy is the condition of having 3 copies of one chromosome type. It is designated as 2n + 1 because the cell has the normal two sets of each 23 types of chromosomes plus an extra copy of one chromosome. Monosomy is the condition of having only 1 copy of a chromosome and is designated as 2n - 1.
3.3.A2 Studies showing age of parents influences chances of nondisjunction.
3.3.A2 Studies showing age of parents influences chances of nondisjunction.
Studies show that the chances of non-disjunction increase as the age of the parents increase
There is a particularly strong correlation between maternal age and the occurrence of non-disjunction events
This may be due to developing oocytes being arrested in prophase I until ovulation as part of the process of oogenesis
Other studies also suggest that:
The risk of chromosomal abnormalities in offspring increase significantly after a maternal age of 30
There is a higher incidence of chromosomal errors in offspring as a result of non-disjunction in meiosis I
Mean maternal age is increasing, leading to an increase in the number of Down syndrome offspring
3.3.A3 Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.
3.3.A3 Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.
Be able to:
Describe the two procedures for obtaining fetal cells for production of a karyotype.
Amniocentesis or chorionic villi sampling allows prenatal karyotyping, by isolation of amniotic fluid or chorion, containing fetal cells in mitosis, which are stained and paired
diagnosis of chromosomal abnormalities, such as Down syndrome, identifying extra chromosome 21 (trisomy 21)
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