(UNIT 5) Chapter 13 - Meiosis and Sexual Life Cycle

Heredity (inheritance)

 Transmission of traits from one generation to the next.

Genetics

Study of both heredity and inherited variation. 

Genes

Segments of DNA that code for basic units of heredity.

• Written in the language of DNA (four nucleotides: A = T & G=-C) 

Chromosome (big to small order)

• Chromosomes → Chromatids →  DNA → Genes

Chromosome structure

Consists of a single long DNA molecule, elaborately coiled in association with various proteins, includes several hundred to a few thousand genes.

Locus

 Gene’s specific location along the length of a chromosome.

Asexual reproduction vs Sexual Reproduction

Asexual Reproduction: One parent passes copies of all its genes to its offspring, making exact genetic copies of themselves.

• Single-celled eukaryotic and multicellular organisms.

• Clone: An individual or group of individuals that are genetically identical to the parent. 

  • Mutations cause genetic differences. 

• Fast and energy efficient.

Sexual reproduction: Two parents give rise to offspring that have unique combinations of genes (chromosomes) inherited from two parents. (Slower and energy consumptive.)

Examples of asexual reproduction

• Mitotic cell division (binary fission), in which DNA is copied and allocated equally to two daughter cells.

• Budding: Localized mass of mitotically dividing cells growing off the organism that eventually detaches from the parent. 

Life cycle

Generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.

Karyotype

Display of condensed chromosomes arranged in pairs. Can be used to screen for defective chromosomes or abnormal numbers of chromosomes.

How are karyotypes done?

Isolated somatic cells are treated with a drug to stimulate mitosis and then grown in culture for several days. Cells arrested when the chromosomes are most highly condensed (metaphase) are stained and then viewed with a microscope equipped with a digital camera. An image of the chromosomes is displayed on a computer monitor, and digital software is used to arrange them in pairs according to their appearance. 

Example of karyotype in use

amniocentesis

How do they pair chromosomes?

• The two chromosomes of a pair have the same length, centromere position, and staining pattern.

The same traits will be located at the same locus.

Sex chromosomes

23rd pair of chromosomes that determine sex.

• Only small parts of the X and Y are homologous. Most of the genes carried on the X chromosome do not have counterparts on the tiny Y, and the Y chromosome has genes lacking on the X.

Autosomes

The first 22 pairs of chromosomes.

Homologous chromosomes

(homologs) Paired chromosomes. 

Diploid vs Haploid

• Diploid Cell: Cells with two sets of chromosomes. 

• Haploid cells: Single set of chromosomes. 

Fertilization

Human life cycle begins when sperm fuses with the egg, fusion of their nuclei. 

• Zygote (fertilized egg): Dipload, contains two haploid sets.

  • Meiosis → Fertilization

How are gametes produced?

• Gametes are produced by specialized cells called germ cells in the gonads (ovaries in females and testes in males). 

Meiosis

Type of cell division that forms gametes. Reduces half of chromosomes. This counters the doubling in fertilization. 

Types of alternations

• In humans: Gametes are produced by meiosis, go through fertilization, then mitosis.

• Plants and some species of algae: A second type of life cycle called alternation of generations.

  • Multicellular means there the organism is made of multiple cells with only one set of chromosomes. 

  • The first stage is sporophyte (multicellular diploid stage). Meiosis produces haploid cells called spores that fuse with another cell and divide mitotically. 

  • A haploid spore produces gametophyte (haploid multicellular phase), which gives rise to gametes by mitosis. Fertilization occurs. 

  • The sporophyte generation produces a gametophyte as its offspring, and the gametophyte generation produces a sporophyte. 

• In most fungi and some protists, including some algae 

  • After gametes fuse and form a diploid zygote, meiosis occurs without an offspring developing. 

  • Meiosis doesn’t produce gametes but instead haploid cells that then divide by mitosis and give rise to either unicellular descendants or a haploid multicellular adult organism. 

  • The organism carries out further mitosis, producing the cells that develop into gametes. 

Meiosis Process Steps

Meiosis 1: Separates homologous chromosomes

Prophase 1

• Centrosome movement and spindle formation.

• Before this stage, each chromosome pairs with its homolog, crossing over occurs.  

• Tetrad: 4 sister chromatids 

• Each pair has one or more X-shaped regions called chiasmata, where crossovers have occurred. 

• Later, microtubules from a pole attached to the kinetochores. (The two kinetochores on the sister chromatids of a homolog are linked together by proteins and act as a single kinetochore.)

• Microtubulus moves the pairs toward the metaphase plate.

Metaphase 1

• Pairs are lined up independently.

Anaphase 1

• Breakdown of proteins that are responsible for sister chromatid cohesion along chromatid arms, allowing separation. (This sister chromatid cohesion is caused by crossing over.)

• Move toward opposite poles.

• Sister chromatid cohesion persists at the centromere.

Telophase 1 and Cytokinesis

• One or both sister chromatids include regions of nonsister chromatid DNA.

• Cytokinesis usually occurs with telophase 1, forming two daughter cells.

• In some species, chromosomes decondense and nuclear envelope forms.

• No chromosome duplication occurs between meiosis I and meiosis II.

Meiosis 2: Separates sister chromatids, creates gametes

Prophase 2

• Spindle apparatus forms.

• Later, chromosomes associate at the centromere and are moved toward the metaphase 2 plate.

Metaphase 2

• Chromosomes at plate.

• The two sister chromatids are not genetically identical (crossing over).

• Kinetochores are attached to microtubules.

Anaphase 2

• Breakdown of proteins holding the sister chromatids, (sister chromatid cohesion) separating them.

• They are now individual chromosomes.

Telophase 2 and Cytokinesis

• Nuclei form, the chromosomes begin decondesing and cytokinesis occurs.

• Produces four daughter cells that are genetically distinct (different).

Crossing Over and Synapsis During Prophase 1

• The DNA molecules of nonsister chromatids are broken (by proteins) and are rejoined to each other (certain genes “recombine” creating new genes and variety not from your parents.)

• This occurs after interphase (duplicated DNA).

1. Two members of a homologous pair associate loosely along their length , sister chromatids held together by cohesins. The DNA is then broken by specific proteins at matching points. 

2. Formation of a zipper-like structure called synaptonemal complex holds one homolog tightly to another.

3. Synapsis occurs, the DNA breaks are closed up so that each broken end is joined to the corresponding segment of the nonsister chromatids. 

1. These points become visible as chiasmata (crossing over and sister chromatid cohesion) after the synaptomemal complex disassembles and the homologs move slightly apart from each other. The homologs remain attached because sister chromatids are still held together by sister chromatid cohesion. At least one crossover per chromosome must occur in order for the homologous pair to stay together as it moves to the metaphase I plate. Condensation occurs throughout this whole process.

A Comparison of Mitosis and Meiosis

Mitosis	Meiosis
Produces two cells. Cells are genetically identical to the parent cell and each other.  No synapsis and crossing over Individual chromosomes lined up at the metaphase plate.  Sister chromatids separate.	Produces four cells with half the chromosomes. Cells are genetically different from parent cells and each other. Synapsis and crossing over occur Alignment of homologous pairs at the metaphase plate Separation of homologs.