BIO102 Ch13

Making Models 13.1 Tips on Drawing Chromosomes (II)

Drawing models of cells helps understand ploidy.
Example:
- 2n cell where n = 2 has unreplicated chromosomes: 2n = 4
- 3n cell where n = 1 has unreplicated chromosomes: 3n = 3

Chapter 13 Opening Roadmap

Meiosis promotes genetic diversity and allows for the benefits of sex.
Examining:
- How meiosis occurs (13.1), comparing it to mitosis (Ch. 12).
- How meiosis promotes genetic variation (13.2) through independent assortment, crossing over, and fertilization.
- What happens when things go wrong (13.3).

Meiosis: Nuclear Division

Meiosis is a nuclear division in which the number of chromosomes is reduced by half.
Meiosis produces haploid nuclei from a diploid nucleus.

Chromosomes

Chromosomes come in distinct sizes and shapes.
Every organism has a characteristic number and type of chromosomes.
Sex Chromosomes:
- Present in different numbers in the two sexes.
Autosomes:
- All chromosomes other than the sex chromosomes.
- Present in the same copy numbers in the two sexes.

Chromosome Types

Cells contain different types of chromosomes that often come in pairs.
Male: XY
Female: XX

Homologous Chromosomes

Homologous chromosomes may contain different alleles of the same gene.
A gene is a segment of DNA that influences one or more hereditary traits.
Different versions of the same gene are called alleles.

Haploid vs. Diploid Genomes

Haploid (N):
- One copy of genetic material subdivided into chromosomes.
Diploid (2N):
- Two copies of genetic material subdivided into chromosomes.

Interpreting Terminology

Ploidy: $x$ (Different types of chromosomes) = Total # of chromosomes.
Example: Humans:
- are diploid, so $2n = 46$

Cell Cycle

New cells arise from division of preexisting cells.
Two main parts of cell cycle:
- Interphase (G1 + S + G2): the period of time between two consecutive M phases.
- M-phase (mitosis or meiosis).

Chromosome Replication

Each chromosome replicates before undergoing meiosis.
When replication is complete, each chromosome has two identical sister chromatids.

Terms for Describing Chromosomes

Homologous Chromosomes (Homologs):
- Chromosomes that have the same genes in the same position and are the same size and shape.
- Alleles of particular genes are often different between the homologs.
Non-sister Chromatids:
- Chromatids on different members of a homologous chromosome pair.
- One of the chromatids is on one homolog, and the other chromatid is on the other homolog.
Bivalent / Tetrad:
- Homologous replicated chromosomes that are joined together during prophase I and metaphase I of meiosis.
- Two homologous chromatids that are bound together.
Unreplicated Chromosome:
- A chromosome that consists of one double-helical molecule of DNA packaged with proteins for compactness.
Replicated Chromosome:
- A chromosome after DNA replication.
- Consists of two identical chromatids, each containing one double-helical DNA molecule packaged with proteins for compactness.
Sister Chromatids:
- The two identical chromatid copies in a replicated chromosome.

Meiosis

Parent cell is diploid $(2n)$ and contains a homologous pair of replicated chromosomes.

Meiosis I

Homologs separate.
Daughter cells are haploid $(n)$ and contain just one homolog.

Meiosis II

Sister chromatids separate.
Four daughter cells contain one unreplicated chromosome each $(n)$ .
In animals, these cells can develop into gametes.

Meiosis Details

Another type of nuclear division.
Only occurs in diploid and polyploid organisms.
Consists of two nuclear divisions:
- Meiosis I (reductional division $[2n \rightarrow n]$ ):
- Chromosomal copy number is halved.
- Meiosis II
- Daughter cells are genetically different from the mother cell.

Prophase I

Nuclear envelope begins to break down.
Chromosomes condense.
The spindle apparatus begins to form.
Synapsis and crossing over occur.

Synapsis and Crossing Over

Key Events of Prophase I
1. Condensation
2. Synapsis (bivalent formation)
3. Crossing over and chiasma formation
4. Partial separation of homologs
Structures Involved
- Bivalent/Tetrad
- Crossover between non-sister chromatids
- Chiasma
- Cohesin proteins
- Kinetochores and Kinetochore microtubule
- Synaptonemal complex

Metaphase I

Tetrads (bivalents) remain paired.
Line up on the metaphase plate.
Different homologous pairs align independently of each other.

Anaphase I

Homologous chromosomes move to opposite poles.
Chromatids are still connected (dyads).

Telophase I

Nuclear membranes reform.
Chromosomes decondense.
Cytokinesis occurs.
Daughter cells are now haploid.

Meiosis stages

Prophase I
Metaphase I
Anaphase I
Telophase I

Meiosis II

By the end of Meiosis I, two haploid daughter cells are produced.
Meiosis II is like mitosis in a haploid cell.
Meiosis II can be divided into four stages:
1. Prophase II
2. Metaphase II
3. Anaphase II
4. Telophase II

Prophase II

Chromosomes condense.
No synapse.
The spindle apparatus reforms.
Nuclear membrane disintegrates.
Chromosomes are loaded onto the spindle apparatus by kinetochores.

Metaphase II

Dyads line up along the metaphase plate.

Anaphase II

Sister chromatids separate.
The resulting daughter chromosomes begin moving to opposite sides of the cell.

Telophase II

Chromosomes arrive at opposite sides of the cell.
Nuclear membrane forms.
Spindle apparatus breaks down.
Chromosomes decondense.
Cytokinesis occurs.
Four haploid cells, genetically different from each other and from the parental cell are produced.

Fertilization

Fertilization restores a diploid set of chromosomes.
Female gamete (egg) (n)
Male gamete (sperm) (n)
Zygote (2n)
Diploid offspring contains homologous pair of chromosomes

Full Meiosis stages

Interphase
Early prophase I
Late prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II

Meiosis vs Mitosis

Meiosis

Mitosis vs Meiosis

Mitosis:
- Diploid parent cell
- Chromosome replication
- Prophase
- Metaphase
- Anaphase and Telophase
- Sister chromatids separate; nuclear envelope re-forms.
- Produces two diploid daughter cells $(2n)$
Meiosis:
- Diploid parent cell
- Chromosome replication
- Prophase I
- Metaphase I
- Anaphase I and Telophase I
- Meiosis II
- Four haploid daughter cells of meiosis $(n)$

Mitosis vs. Meiosis Detailed Comparison

Feature	Mitosis	Meiosis
Synapsis	Homologs do not synapse	Homologs synapse
Anaphase	-	Homologs separate in anaphase I
Daughter Cells	Produces two genetically identical diploid daughter cells	Produces four genetically distinct haploid daughter cells
Ploidy	$2n \rightarrow 2n$	$2n \rightarrow n$

Meiosis Promotes Genetic Variation

Meiosis results in four gametes with a chromosome composition different from each other and from the parent cells.
Factors contributing to genetic variation:
- Crossing over during meiosis I (prophase I).
- Independent shuffling of maternal and paternal chromosomes (metaphase I).
- Fertilization introduces variation as haploid sets of chromosomes combine to make a unique offspring.

Genetic Variation

Figure 13.8 Crossing Over and Independent Assortment of Homologous Chromosomes
-- Prophase I - Crossover

Genetic Variation in Metaphase I

Figure 13.8 Crossing Over and Independent Assortment of Homologous Chromosomes
-- Metaphase I – random assortment of tetrads
-- Different combinations of chromosomes in daughter cells

Meiosis and Fertilization

The changes in chromosomes produced by meiosis and fertilization are significant.
Asexual reproduction produces clones that are genetically identical to one another and to the parent.
Sexual reproduction produces offspring with unique chromosome complements.
Only sexual reproduction results in a shuffling of the alleles of the parents into the offspring.

Karyotype

Karyotype refers to the full set of chromosomes that are visible during metaphase.

Nondisjunction

Nondisjunction leads to nuclei with abnormal chromosome numbers.

Meiosis I starts normally. Bivalents line up in the middle of the cell.
Nondisjunction occurs with one set of homologs.
Chromosomes align at metaphase II.
Sister chromatids separate normally in meiosis II.
Aneuploidy results. All gametes have too many or too few chromosomes.