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Meiosis
3/11/16, 10:12 PM
Meiosis
Haploid = single set of chromosomes
Diploid = double set of chromosomes (one from each parent)
Homologous chromosomes: Pairs of chromosomes having the same genes. Diploid cells have
homologous chromosomes (one from each parent). See fig on page 170.
Sister chromatids: a chromosome and its identical copy (formed by DNA duplication), still held
together by centromeres.
Meiosis requires two cell divisions and results in four daughter cells. Replication of the DNA takes
place before mitosis begins.
This replication is followed by two consecutive cell divisions: meiosis I and meiosis II.
There is no replication of DNA during the interkinesis between meiosis I and meiosis II.
During Meiosis I the homologs undergo synapsis (resulting in a bivalent) and crossing over occurs
between nonsister chromatids
During Meiosis II, the daughter chromosomes separate in a manner similar to that in mitosis.
The result is four daughter cells that are NOT genetically identical to the parent cell.
They are haploid and due to crossing over their chromosomes carry a different combination of
genes.
Meiosis I
Prophase I- nuclear envelope disappears, synapsis occurs.
the homologs undergo synapsis (resulting in a bivalent) and crossing over occurs between nonsister
chromatids. (see fig below from
http://www.members.aol.com/BearFlag45/Biology1A/LectureNotes/lec14.html)
·
Cross-overs help to hold homologous chromosomes together
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At the cross-over the 2 chromatids can exchange tips (recombination)
o The exchange is catalyzed by enzymes
o Promotes genetic diversity
o Homologous chromosomes can have more than 1 cross-over
The cross-over process is unbelievably precise
Average human chromosome has about 100 million DNA base pairs
In cross-overs neither chromosome can gain or lose a single base pair
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Meiosis
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Metaphase I- Bivalents (pairs) independently align at the metaphase plate
Anaphase I- Homologous chromosomes separate, and sister chromatids remain attached at the
centromere and move toward the same pole while their homologs move towards the opposite pole.
Telophase I- Each pole now has a haploid set of chromosomes that are each still composed of two
sister chromatids attached at the centromere.
Usually cytokinesis occurs simultaneously with telophase I, forming two haploid daughter cells.
Cleavage furrows form in animal cells and cell plates in plant cells.
In some species, nuclear membranes and nucleoli reappear and the cell enters a time of
interkinesis before meiosis II. In other species, the daughter cells immediately prepare for
meiosis II.
Regardless of whether a cell enters interkinesis, no DNA replication occurs before meiosis II.
Meiosis II
Prophase II- If the cell entered interkinesis, the nuclear envelope and nucleoli disperse. Spindle
apparatus forms and chromosomes move toward the metaphase II plate.
Metaphase II- Chromosomes align singly on the metaphase plate, and kinetochores of sister
chromatids point toward opposite poles
Anaphase II- Centromeres of sister chromatids separate. Sister chromatids of each pair (now
individual chromosomes) move towards opposite poles of the cell.
Telophase II- Nuclei form at opposite poles of the cell.
Cytokinesis- cytoplasm is split forming four haploid daughter cells.
Comparing Mitosis and Meiosis:
Things common to both:
· Both start with DNA replication during interphase
· Use of the mitotic spindle to separate chromosomes
· Division with a set of stages
Things unique to Mitosis Produces 2 identical cells
· Does not promote genetic variability
· Used for growth and repair (reproduction in single celled organism)
· A single cell division
o Separates sister chromatids
o Centromeres divide at anaphase
Things unique to Meiosis
· Produces 4 haploid cells with half the original number of chromosomes
· Promotes genetic diversity
· Used for sexual reproduction (produces sperm and egg cells)
· No duplication of DNA between the 1st and 2nd divisions
· Homologous chromosomes come together (synapsis) in 1st division
· Crossing over between homologous chromosomes is common.
· Two cell divisions
o First division separates homologous chromosomes
o Second division separates sister chromatids
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Meiosis
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o Centromeres divide at 2nd anaphase, but not at 1st.
Comparison of Meiosis I and Mitosis
Meiosis I
Prophase
Metaphase
Anaphase
Mitosis
Synapsis occurs to form bivalents (tetrads),
Chiasmata appear as evidence that crossing
over has occurred.
Homologous pairs (tetrads) align on the
metaphase plate.
Meiosis I separates pairs of chromosomes.
Centromeres do not divide and sister
chromatids stay together. Sister chromatids of
each chromosome move to the same pole of
the cell. Only the homologs separate.
Neither synapsis nor crossing
over occurs
Individual chromosomes align on
the metaphase plate.
Centromeres divide and sister
chromatids move to opposite
poles of the cell.
LAB
Crossing over and mapping gene distance to the centromere:
Sordaria fimicola is an ascomycete fungus that can be used to demonstrate the results of crossing over during
meiosis. Sordaria is a haploid organism for most of its life cycle. It becomes diploid only when the fusion of
mycelia (filament like groups of cells) of two different strains results in the fusion of the two different types
of haploid nuclei to form a diploid nucleus. The diploid nucleus must then undergo meiosis to resume its
haploid state.
Meiosis, followed by mitosis, in Sordaria results in the formation of eight haploid ascospores contained
within a sac called an ascus (plural asci). Many asci are contained within a fruiting body called a perithecium.
When ascospores are mature the ascus ruptures releasing ascospores. Each ascospore can develop into a new
haploid fungus. The life cycle of Sordaria fimicola is shown below.
Using Figure 3.15: Meiosis with Crossing Over, you will determine the number of hybrids in MI (from
Meiosis I) also noted as 4:4 pattern, the number of hybrids that are 2:2:2:2 or 2:4:2 patterns (from Meiosis II).
You will indicate the total numbers found in the images given to your group in the data table below.
Data table 1: Research Team Data for crossing over in Sordaria
Number of 4:4
Number of 2:2:2:2
Total Asci
% Asci showing
Gene to
patterns
and 2:4:2 patterns
crossover
Centromere
Distance (Map
Units)
Data Table 2: Class Data for crossing over in Sordaria
Number of 4:4
Number of 2:2:2:2
Total Asci
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% Asci showing
Gene to
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Meiosis
patterns
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and 2:4:2 patterns
crossover
Centromere
Distance (Map
Units)
The frequency of crossing over appears to be governed largely by the distance between genes, or in this case,
between the gene for spore coat color and the centromere. The probability of a crossover occurring between
two particular genes on the same chromosome (linked genes) increases as the distance between those genes
becomes larger. The frequency of crossover, therefore appears to be directly proportional to the distance
between genes.
A map unit is an arbitrary unit of measure used to describe the relative distances between linked
genes. The number of map units between two genes or between a gene and the centromere is equal to the
percentage of recombinants. Customary units cannot be used because we cannot directly visualize genes with
the light microscope. However, due to the relationship between distance and crossover frequency, we may use
the map unit.
Analysis of Results:
1. Using the data in Table 2, determine the distance between the gene for spore color and the centromere.
Calculate the percent of crossovers by dividing the number of crossover asci (2:2:2:2 or 2:4:2 patterns) by the
total number of asci X 100. To calculate the map distance, divide the percent of crossover by 2. The
percentage of crossover asci is divided by two because only half the spores in each ascus are the result of a
crossover event. Record your results in Table 2.
2. Draw a pair of chromosomes in MI and MII and show how you would get a 2:4:2 arrangement of
ascospores by crossing over. (Hint use fig 3:15).
3. Perform a chi-square analysis on the data to determine the usefulness of the data.
Applying your knowledge
1. What advantage does the process of crossing over bring to reproduction?
2. Can you think of any way in which new gene combinations resulting from crossovers might be
disadvantageous?
3. In the photomicrograph of dividing root cells below, identify interphase and the following phases of
mitosis: prophase, metaphase, anaphase, telophase, and cytokinesis.
4. Describe the activity of chromosomes on each stage of meiosis I.
5. Describe the activity of chromosomes in each stage of meiosis II.
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