3.3. Meiosis

Asexual life cycle

Asexual life cycle

Offspring are genetically identical to the parent, having the exact same chromosome

Sexual life cycle

Offspring are genetical distinct from each other an from each of the parent. Chromosomes are different, which allows for genetic diversity

Fertilization

Fusion of gametes involved in eukaryotic sexual reproduction

Meiosis

Process in which the nucleus of an eukaryotic cell divides to produce genetically different cells

Why must meiosis occur?

Fertilization doubles the number of chromosome each time it occurs (n + n = 2n). Hence, the chromosome number of the sex cell must be halved to keep it from doubling every generation. Without it, the sexual life cycle of eukaryotes would not occur

When does meiosis occurs in animals?

During the creation of gametes

What occurs before meiosis?

• During interphase, all of the DNA in the nucleus of the parent cell is replicated

• Each chromosome now consists of two sister chromatids

• Sister chromatid are genetically identical because DNA replication is very accurate and the number of mistakes in the copying of DNA is extremely small

What is meiosis divided into?

Meiosis I and II

What are the phases of meiosis?

Prophase, metaphase, anaphase and telophase

Prophase I

1. Synapsis

2. Crossing over

Synapsis

Process in which homologous chromosomes pair up with each other

Name of combination formed after synapsis

Bivalent (pair of homologous chromosomes) or tetrad (group of four chromatids)

How is a bivalent also called?

Tetrad

Synaptonemal complex

A protein-based structure that forms between the homologous chromosomes in a bivalent

Crossing over

In a bivalent, two non-sister chromatids extend over each over at homologous sequences. Each sequence breaks apart and rejoins with the other non-sister chromatid at the region of the break

Chiasmata

X-shaped points that connect the non-sister chromatids at the site where crossing over has occurred

Where do crossovers occur?

At random position anywhere along the chromosome

How many crossovers occur?

At least one in each bivalent

What results from crossing over?

Since crossover occurs at the same base sequence on both chromatids, there is a mutual exchange of genes between the chromatids

Recombinant chromosomes

Chromosomes with a new combination of alleles resulting from crossing over

What characteristic of non-sister chromatids allow recombinant chromosomes to form?

Since non-sister chromatids are homologous but not genetically identical, some of the alleles of the exchanged genes are likely to be different

Metaphase I

1. Nuclear membrane breaks down

2. Spindle microtubules grow from the poles of the cell and attach to the bivalents, moving them to the equator of the cell

   1. Each of the homologous chromosome in a bivalent is attached to a different pole

3. Random and independent orientation

Orientation of bivalents

The direction in which the bivalent is facing

Random orientation

The pole faced by a chromosome in a bivalent is random. Thus, each chromosome has an equal chance of attaching to a pole, an eventually being pulled to it

Independent orientation

The orientation of one bivalent does not affect the orientation of other bivalents

Anaphase I

• Chiasma in bivalents slide to the end of the chromosomes, allowing them to separate

• Disjunction

Disjunction

Process in which the homologous chromosomes in each bivalent separate from each other, moving to opposite poles

Anaphase from mitosis vs Anaphase I

Unlike in mitosis, in which the centromere divides and the chromatids of a chromosome move to opposite poles, in anaphase I, the centromere does not divide and whole chromosomes move to the poles

Telophase I

• Chromosomes uncoil

• Cytokinesis (division of the cytoplasm) occurs

What is the product of meiosis I?

Two separate nuclei containing one of each type of chromosome (n) → two haploid cells

Why is meiosis called reduction division?

Because the chromosome number is halved

What happens immediately after meiosis I?

After meiosis I, daughter cells enter meiosis II without passing through interphase (i.e. no DNA replication).

Meiosis II

Similar to mitosis in that the replicated chromosome is separated into chromatids

How does meiosis II differ from mitosis?

Separated sister-chromatids are likely to be non-identical sister chromatids due to crossing over

What is the product of meiosis II?

Four haploid nuclei

Nuclei, chromosomes per nucleus and chromatids per chromosome AFTER interphase, meiosis I and meiosis II

Interphase I:

• 1 nuclei

• 46 chromosomes, each consisting of 2 chromatids

Meiosis I:

• 2 nuclei

• 23 chromosomes, each consisting of 2 chromatids

Meiosis II:

• 4 nuclei

• 23 chromosomes, each consisting of 1 chromatid

Alleles in gamete

• When the alleles are the same in both copies of the gene (one in each chromatid), each gamete produced by the parent will contain that allele

• When the alleles are different, each of the two alleles has an equal chance of being passed on in a gamete

Processes in meiosis that cause genetic variation

• Random orientation of bivalents

• Crossing over

Random orientation in genetic variation

• Combination of alleles that end up in each daughter cell depends on the orientation of the bivalents as they lined up

• The different combinations increases genetic variation between the gametes

# of possible chromosome combinations from random assortment

2^n

Crossing over in genetic variation

• Without crossing over, the combinations of alleles present in the parent cell would be forever linked together

• e.g. if one chromosome carries AB and another carries ab, only these combinations could occur in gametes. With crossing over, Ab and aB are also possible

# of possible chromosome combinations from crossing over

Since crossing over can occur anywhere in the non-sister chromatids, the number of possible combinations is infinite

Fertilization in genetic variation

• Allows alleles from to different individual to combine in one new individual

• Since any male gamete can fuse with any female gamete, the combination of alleles is unlikely to ever have existed before, promoting genetic variation

# of possible chromosome combinations after fertilization

(2n)^2

Non-disjunction

When chromosomes do not separate correctly: both chromosomes move to one pole and none to another pole

When does it occur?

Anaphase I or Anaphase II

What is the result of non-disjunction?

Gametes with either an extra copy of a chromosome or no copies of a particular chromosome

Chromosome abnormality

When abnormal gametes are fertilized, resulting in a zygote with the incorrect chromosome number

• Either 45 or 47 chromosomes

Non-disjunction in Down Syndrome

Occurs during anaphase I, when the 21st bivalent fails to separate

Why aren’t other trisomies in humans common?

Most other trisomies are so serious that the offspring does not survive

Examples of other trisomies

Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18) both have very low survival rates with few babies surviving past their first birthday

Klinefelter’s syndrome

When the individual has sex chromosomes XXY

• Does not impact life expectancy but may have a negative effect in fertility

Turners syndrome

When the individual has one sex chromosome

• Often reduced life expectancy and lack of sexual development during puberty

What is the relationship between parental age and non-disjunction?

Many studies show that as the age of the parents increases, the incidence of non-disjunction increases

• In particular, maternal age and Down syndrome