topic 3.3: meiosis
Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. This process is required to produce egg and sperm cells for sexual reproduction.
This unit will take 2 school days
This unit will take 2 school days
Essential idea:
- Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.
Nature of science:
- Making careful observations—meiosis was discovered by microscope examination of dividing germ-line cells. (1.8)
- Discuss difficulties in microscopic examination of dividing cells.
- Describe the discovery of meiosis.
- Discuss difficulties in microscopic examination of dividing cells.
Understanding:
3.3.U1 One diploid nucleus divides by meiosis to produce four haploid nuclei. (Oxford Biology Course Companion page 160).
- 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 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. (Oxford Biology Course Companion page 161).
- Compare sexual and asexual life cycles.
- Explain why meiosis must occur as part of a sexual life cycle.
Meiosis is preceded by interphase, during which DNA is replicated (in the S phase) to produce two genetically identical copies. The two identical DNA molecules are identified as sister chromatids, and are held together by a single centromere. The sister chromatids are separated during meiosis II, following the separation of homologous chromosomes in meiosis I
3.3.U3 DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids. (Oxford Biology Course Companion page 162).
- 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.
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)
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)
images from http://www2.estrellamountain.edu/faculty/farabee/biobk/meiosis5.gif
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Meiosis I: metaphase I
- chromosomes continue to condense
- spindle microtubules attach to centromeres
- bivalents line up at equator
- separation of homologous pairs;
- chromosomes moved to poles by spindle
Meiosis I: telophase I
- chromosomes at poles
- nuclear envelope reappears
- spindle disappears
- chromosomes partially uncoil
Meiosis II: prophase II
- chromosomes condense again
- nuclear envelope disappears
- new spindle forms (at right angles to previous spindle)
- spindle microtubules attach to centromeres
- chromosomes move to the equator
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
3.3.U5 Orientation of pairs of homologous chromosomes prior to separation is random (Oxford Biology Course Companion page 162).
- Describe the attachment of spindle microtubules to chromosomes during meiosis I.
- Describe random orientation of chromosomes during meiosis I.
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. (Oxford Biology Course Companion page 163).
- Explain why meiosis I is a reductive division.
- State that cells are haploid at the end of meiosis I.
This is because the homologous chromosomes separate into different cells
image from IB Biology Help
3.3.U7 Crossing over and random orientation promotes genetic variation. (Oxford Biology Course Companion page 165).
- 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 involves the exchange of segments of DNA between homologous chromosomes during prophase I
Because it mixes differents sets of DNA basically.
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)
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
Because it mixes differents sets of DNA basically.
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)
image from IB Biology Help
3.3.U8 Fusion of gametes from different parents promotes genetic variation. Oxford Biology Course CompanionOutline 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
Application
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.
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.
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:
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. (Oxford Biology Course Companion page 163).
- 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
diagnosis of chromosomal abnormalities, such as Down syndrome, identifying extra chromosome 21 (trisomy 21
image from IB Biology Help
Skills
3.3.S1 Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells. (Oxford Biology Course Companion page 164).
- Outline the events of prophase, metaphase, anaphase and telophase in meiosis I and meiosis II.
- Draw diagrams of cells in prophase, metaphase, anaphase and telophase in meiosis I and meiosis II.
Meiosis consists of two divisions, both of which follow the same stages as mitosis (prophase, metaphase, anaphase, telophase). Meiosis is preceded by interphase, in which DNA is replicated to produce chromosomes consisting of two sister chromatids. A second growth phase called interkinesis may occur between meiosis I and II, however no DNA replication occurs in this stage.
Meiosis I
The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated
Meiosis II
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
Meiosis I
The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated
- P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
- M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
- A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
- T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
Meiosis II
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
- P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
- M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
- A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
- T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
Key Terms:
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meiosis
chromosome karyotype somatic cell non-sister chromatids bivalents reduction division gametes random orientation homologous chromosomes spindle microtubules reductive division |
diploid
crossing over chorionic villus gamete recombinant chromatid disintegrates amniocentesis chorionic villus sampling variation |
haploid
non-disjunction amniocentesis gene spindle fibres crossing over germ-line cell sister chromatid Meiosis I Interphase fertilization |
nuclei
Down's Syndrome pre-natal n prophase telophase haploid non-disjunction Meiosis II bivalent random orientation |
homologous
trisomy abnormalities 2n metaphase chromatid allele sexual asexual synapsis karyotype |
Powerpoint and Notes on Topic 3.3 by Chris Payne
Correct use of terminology is a key skill in Biology. It is essential to use key terms correctly when communicating your understanding, particularly in assessments. Use the quizlet flashcards or other tools such as learn, scatter, space race, speller and test to help you master the vocabulary.
Useful Links:
Learn.Genetics at Utah is an excellent resource for this topic, and all animations are easy enough to save.
Meiosis:
Good animation from Biostudio (the one used in the diagrams in the presentation)
Good animation from Learn.Genetics
Non-disjunction:
Trisomy from Learn.Genetics (their page on Down Syndrome is here)
Excellent animation of meiosis I and meiosis II from BioStudio
Karyotyping:
Animation and have-a-go from Learn.Genetics
Amniocentesis:
Simple animation from MedIndia
In the News:
Avoiding invasive tests for Down syndrome:
This news article outlines a new blood test for Down syndrome which does not require the invasive techniques of chorionic villus sampling or amniocentesis. See if you can work through the statistics posted there:
- What proportion of all pregnancies end up terminating due to invasive tests as a result of a ‘false-positive’ scan result?
- Compare the proportion of women needing amniocentesis or CVS in cases where the blood test is and is not used.
- How does the blood test work – what is being detected and how? Find out more
National Geographic Shutting Down the Extra Chromosome in Downs Syndrome
TOK
- In 1922 the number of chromosomes counted in a human cell was 48. This remained the established number for 30 years, even though a review of photographic evidence from the time clearly showed that there were 46. For what reasons do existing beliefs carry a certain inertia?
Video Clips:
Paul Andersen explains how the process of meiosis produces variable gametes. He starts with a brief discussion of haploid and diploid cells. He compares and contrasts spermatogenesis and oogenesis. He explains how each person is different due to independent assortment, crossing over and random fertilization.
In this video Paul Andersen explains the difference between diploid and haploid cells. He starts with a brief description of the central dogma and how genes code for proteins. He then uses the phenotype of red hair to explain that humans are diploid creatures. He discriminates between diploid somatic cells and haploid sex cells. He finishes with a brief description of different life cycles dominated by haploid and diploid individuals.
Hank gets down to the nitty gritty about meiosis, the special type of cell division that is necessary for sexual reproduction in eukaryotic organisms.
A brief stop-motion walkthrough of nondisjunction during Meiosis II.
