MeiosisMeiosis

AP BiologyUnit 3

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

No DNA is copied between Meiosis I

and II

Metaphase I: homologous pairs line up in the middle of the cell

Anaphase I: homologous pairs pulled apart

Metaphase II: Chromosomes line up single file

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

Example of Asexual Reproduction-- Budding

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

Karyotypes

• Karyotypes are used to determine if there are chromosomal abnormalities– Only tells you if there are the right number of

chromosomes– Does NOT tell you if specific genes are

mutated