Meiosis

What happens during sexual reproduction?

Reproductive cells called gametes unite to form a new individual in a process called fertilization

What are gametes?

reproductive cells and in animals are called sperm and eggs

What is meiosis?

The nuclear division that leads to a halving of chromosome number. The Gametes must contain half the chromosome number, and at fertilization, the full chromosome number is restored.

What are sex chromosomes?

determine the sex of the individual

females have XX chromosomes, and males have XY chromosomes

What are non-sex chromosomes called?

autosomes

What are homologous chromosomes/homologs?

pairs of matching chromosomes—one inherited from each parent—found in diploid cells that possess the same genes, size, and structure, but often different alleles

What is a gene?

a section of DNA that influences one or more hereditary traits

Different versions of specific genes are called alleles

Homologs may contain different alleles

What is ploidy?

The number of complete sets of chromosomes in a cell or organism (n,2n,3n, etc)

more than three or more versions of each type of chromosome are called polypoid (3n, 4n, etc)

Define Karyotype

identifies the number and types of chromosomes present in a species

humans are diploid (2n) - two homologs of each chromosome and two alleles of each gene, paternal chromosome that came from the father and maternal chromosome that came from the mother

other organisms haploid (n) - only one of each type of chromosome and one alleles of each gene

Define haploid number (n)

indicates the number of distinct types of chromosomes present

sex chromosomes count as a single type

humans have 23

What happens to the chromosomes before meiosis begins?

Each chromosome in the diploid (2n) parent cell is replicated. Each chromosome will have two identical sister chromatids.

Define sister chromatids

two identical copies of a single replicated chromosome that are joined together at a region called the centromere

What are the two divisions of meiosis?

Meiosis I: Homologous chromosomes separate

Meiosis II: Sister chromatids separate

How many cells result from meiosis?

Four haploid (n) daughter cells

What happens during fertilization?

two haploid gametes combine to form a diploid zygote

What restores the diploid chromosome number in organisms?

Fertilization

What structure move chromosomes during meiosis?

Microtubules of the spindle apparatus

Where do spindle microtubules attach to chromosomes?

Kinetochores at the centromere

What happens during interphase?

• each chromosome in the diploid (2n) parent cell is replicated and each chromosome has two identical sister chromatids

• The two attached sister chromatids are still considered a single replicated chromosome

Why is meiosis I called a reductional division?

it reduces chromosome number form diploid (2n) to haploid (n)

What separates during Meiosis I?

homologous chromosomes

What is the result of meiosis I?

two haploid cells with replicated chromosomes

Describe stages of meiosis I: Prophase 1

• occupies 90% of meiosis

• chromosomes condense

• synapsis homologous chromosomes pair gene-by-gene

• bivalent a pair of homologous chromosomes

• crossing over, non-sister chromatids exchange DNA segments

• Each bivalent usually has one or more chiasmata, X-shaped regions where crossing over occurred

• Produces chromosomes with a combination of maternal and paternal alleles

What is the genetic result of crossing over?

chromosomes with both maternal and paternal alleles

Describe stages of meiosis I: Prophase I, Metaphase I

What happens in metaphase I? How do homologs align in metaphase I?

homologous chromosome pairs line up at the metaphase plate

randomly (independent assortment)

Describe stages of meiosis I: Prophase I, Metaphase I, Anaphase I

homologous chromosomes separates

Describe stages of meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I?

homologs move to opposite poles

cytokinesis divides the cell

How is meiosis II similar to mitosis?

sister chromatids separate

Does DNA replication occur before meiosis II?

No

What is the result of meiosis II?

Four haploid daughter cells with unreplicated chromosomes

What two mechanisms create genetic variation in meiosis?

crossing over

random assortment of homologous chromosomes

What is the final result of meiosis?

1 diploid cell —> 4 haploid cells

What are the four steps of homolog pairing and crossing over?

prophase I

1. chromosomes condense; sister chromatids remain joined by cohesins

2. homologous chromosomes pair by synapsis via the synaptonemal complex

3. DNA breaks occur and crossing over happens between non-sister chromatids

4. the synaptonemal complex breaks down, homologs remain attached at chiasmata

Why doesn’t crossing over occur in mitosis?

homologous chromosomes do not pair, so crossing over cannot occur

What is the main difference between mitosis and meiosis

Mitosis: conserves chromosomes number (2n —> 2n), produce identical cells

Meiosis: reduces chromosomes number (2n —> 1n), produces genetically diverse cells

What is the ploidy change in meiosis?

Diploid (2n) —> Haploid (1n)

What three events occur only in Meiosis I?

1. synapsis and crossing over (prophase I)

2. homologous pairs align at metaphase plate (metaphase I)

3. homologous chromosomes separate (anaphase I)

Why does meiosis produce genetically different gametes?

Due to independent assortment and crossing over

What type of reproduction produces genetically identical offspring?

Asexual reproduction (clones)

What type of reproduction produces genetic diversity?

sexual reproduction

How many chromosome combinations are possible from independent assortment?

2^n, where n=haploid number

How many combinations are possible in humans from independent assortment?

2²³ = 8.4 million combinations

What does crossing over produce?

New allele combinations on the same chromosome

What is genetic recombination?

new allele combinations produced by crossing over and independent assortment

Why is crossing over important?

It increases genetic variability beyond independent assortment

What is random fertilization?

any sperm can fuse with any egg

Why is each zygote genetically unique?

Due to independent assortment + crossing over + random fertilization

What is nondisjunction?

failure of homologous chromosomes or sister chromatids to separate properly

What gametes result from nondisjunction?

n+1 (extra chromosome)

n-1 (missing chromosome)

What is trisomy? Monosomy? Aneuploidy?

trisomy - a condition where a zygote has three copies of a chromosome

monosomy - a condition where a zygote lacks one chromosome

aneuploidy - cells with abnormal numbers of chromosomes

What causes down syndrome?

trisomy 21 (extra copy of chromosome 21)

What increases the risk of nondisjunction?

maternal age

Why does maternal age increase nondisjunction risk?

oocytes pause in prophase I for many years, increasing error likelihood

When do primary oocytes begin meiosis?

during female embryonic development

When do primary oocytes pause in meiosis?

prophase I

When is meiosis completed in females?

at ovulation and fertilization

Why is asexual reproduction more efficient?

it can produce twice as many offspring in the same time

What are two hypotheses explaining why sexual production perists?

1. purifying selection hypothesis

2. changing environment hypothesis

Which statement correctly describtes non-sister chromatids?

a) they are only formed during meiosis

b) they are held together by centromere

c) they are identical

d) they carry the same gene loci

e) they are always separated during mitosis

d) they carry the same gene loci

Which of the following best represents the four gametes that would be produced if nondisjunction occurred during meiotic division II?

a) two of the gametes will be n+1 wand two will be n-1

b) two of the gametes will be haploid and two will be diploid

c) one of the gametes produced

d) all of the gametes will be diploid

c) one of the gametes produced