Meiosis
• Process that occurs to form haploid cells from diploid cells
• Forms gametes– sperm and egg
Homologous Chromosomes
• A pair of chromosomes containing the same sets of genes – One chromosome from
each parent– Don’t necessarily contain
identical genetic material– Ex. You get one
chromosome #4 from mom and one chromosome #4 from dad
Homologous Chromosomes vs. Sister Chromatids
• Homologous chromosomes contain the same information, but aren’t identical to one another
• Sister chromatids are identical copies of the same chromosome.
Meiosis Overview
• Chromosomes are replicated to form sister chromatids before meiosis
• 2 stages– Meiosis I homologous pairs are separated – Meiosis II sister chromatids are separated
• Results in 4 haploid daughter cells– In humans, this means there are 23
chromosomes in each haploid cell
Metaphase I: homologous pairs line up in the middle of the cell
Anaphase I: homologous pairs pulled apart
Unique to Meiosis
• Synapsis and Crossing Over in Prophase I– Synapsis = when
homologous chromosomes line up next to one another form a tetrad
– Crossing over = the non-sister chromatids in the pair exchange genetic material
Crossing Over and Genetic Diversity
• Crossing Over leads to more genetic diversity in a species
• Why?• By exchanging some
genetic material, the haploid cells formed through meiosis are no longer identical– more variety
Unique to Meiosis
• Homologous Chromosomes line up next to each other (form tetrads)
• Homologous chromosomes, NOT sister chromatids, separate from each other during Anaphase I
Sexual vs. Asexual Reproduction
Asexual Reproduction
Sexual Reproduction
Who does it? Bacteria, Yeast Hydra, Fungi
Animals, Plants
Methods (a few examples)
Budding, Binary Fission
Pollination, Fertilization
Advantages Very quick, produces clones
More genetic diversity
Disadvantages Less genetic diversity– only through mutations
Uses more energy, offspring may not be as fit
Recombination
• Another name for Crossing Over
• Occurs in Prophase I when homologous chromosomes pair up
• Results in genetic variation
• In humans, 1-3 crossover events per chromosome
Recombination Frequency
• Likelihood of crossing over between 2 genes is directly related to the distance between them– If 2 genes are close to one another on a
chromosome there is less of a chance they’ll be separated by crossing over
– If 2 genes are far from one another on a chromosome there is a greater chance they’ll be separated by crossing over
Recombination Frequency• When 2 genes are separated by crossing
over we say they have recombined.
• High recombination frequency = likely to be separated by crossing over
• Low recombination frequency = not likely to be separated by crossing over
Recombination Frequency
• If 2 genes have a high recombination frequency, are they close together or far apart?– Far apart– greater % chance of being separated
Linkage Maps
• A map (diagram of relative positions of genes) can be made by considering recombination frequencies
Sample Problem
• What is the order of genes given the following recombination frequencies?
Genes Recombination Frequency
A & B 40%
A & C 10%
A & D 15%
B & D 25%
B & C 30%
C & D 5%
Answer
• Step 1: Start by determining which genes are closest together– C & D are closest 5% recombination freq.
C--- (5) --- D
• Now, determine the position of one more gene relative to these, let’s say gene A– A is closer to C (10%) compared to D (15%)
A ----- (10)------ C --- (5) --- D
Answer (continued)
• Now, put in the final gene– B is farthest from A, then C, then D according
to the numbers
A ----- (10) ------- C --- (5) --- D ---- (25) ---- B
Nondisjunction • Meiotic spindle works
incorrectly.
• Chromosomes fail to separate properly.
• May occur during meiosis I or II– Meiosis I: homologous
chromosomes don’t separate
– Meiosis II: sister chromatids don’t separate
Polyploidy vs. Aneuploidy
• Polyploidy = having too many chromosomes ( more than 2 copies/sets of a chromosome)
• Aneuploidy = having an abnormal number of chromosomes (either too few or too many)
Trisomic vs. Monosomic
• Trisomic = having 3 copies of a chromosome
• Monosomic = only having 1 copy of a chromosome
Meiosis Problems
• Extra or missing parts of chromosomes can also occur due to errors in crossing over
Why is this a problem?
• Why might an increase of DNA result in physical problems? – Proteins overproduced, not enough regulators
to regulate all genes, etc.
• Why might a decrease of DNA result in physical problems? – Missing genes or portions of genes, regulatory
proteins not made, etc