Meiosis & Sexual Life Cycles

Genetics: The study of heredity and variation.
Heredity: The passing on of traits through generations.
Offspring typically resemble parents more than unrelated individuals.
Variation: Differences in appearance shown by offspring from parents and siblings.

Genes: Units of heredity made up of DNA segments.
- Passed to the next generation through gametes (sperm and egg cells).
- DNA is packaged in chromosomes; human somatic cells have 46 chromosomes.
- Gene locus: Specific location of a gene on a chromosome.

Asexual Reproduction: Involves a single individual passing genes to offspring without the use of gametes (i.e., a clone).
Sexual Reproduction: Involves two parents producing offspring with unique combinations of genetic information.
Life Cycle: The sequence of stages in reproductive history across generations of an organism.

Somatic Cells: Have 23 pairs of chromosomes.
Karyotype: Ordered display of chromosome pairs.
Homologous Chromosomes: Two chromosomes in a pair, they are of the same length and shape and control the same genetic traits.

Autosomes: Make up pairs 1-22 and follow the standard homologous chromosome patterns in human somatic cells.
Sex Chromosomes: Determine gender.
- Males: XY (non-homologous)
- Females: XX (homologous)

Each homologous chromosome pair consists of one chromosome from each parent.
In humans: Diploid cells (2n) have 46 chromosomes (2 sets of 23).
After DNA synthesis (S phase), chromosomes are replicated, and sister chromatids become visible.

Gametes: Haploid (n), containing a single set of chromosomes (n = 23 in humans).
- Composed of 22 autosomes and one sex chromosome.
- Egg cells carry X from mother; sperm can carry either X or Y from father.

Fertilization: Union of gametes resulting in a zygote that is diploid as it receives genetic material from both parents.
Gamete production occurs through meiosis producing one set of chromosomes per daughter cell.

Alternation of meiosis and fertilization is common among sexually reproducing organisms.
Three main types:
- Haploid (n)
- Diploid (2n)

Animals: Diploid multicellular organism produces haploid gametes through meiosis.
Plants: Alternation of generations, where the diploid sporophyte produces haploid spores by meiosis.
Fungi and some protists: The only diploid stage is the single-celled zygote, which produces haploid cells through meiosis.

Meiosis: Involves two rounds of divisions, producing four daughter cells with half the chromosome number of the parent cell.
Meiosis I: Separation of homologous chromosomes.
Meiosis II: Separation of sister chromatids leading to haploid cells.

Interphase: Chromosomes duplicate prior to meiosis I.
Meiosis I Stages:
- Prophase I: Homologous chromosomes pair up and may undergo crossing over.
- Metaphase I: Pairs line up at the metaphase plate.
- Anaphase I: Homologous chromosomes separate.
- Telophase I and Cytokinesis: Results in two haploid cells.
Meiosis II Stages: Similar to meiosis I but separates sister chromatids.
- Prophase II, Metaphase II, Anaphase II, Telophase II and Cytokinesis yield four genetically unique haploid cells.

Occurs during Prophase I: Non-sister chromatids exchange DNA segments via chiasmata.
Synapsis: Homologous chromosomes form a complex that facilitates crossing over.
- Synaptonemal Complex: A zipper-like structure formed during synapsis.

Property	Mitosis	Meiosis
DNA Replication	Occurs during interphase	Occurs during interphase
Number of Divisions	One	Two
Synapsis of Homologous Chromosomes	Does not occur	Occurs (crossing over)
Number of Daughter Cells	Two identical cells	Four genetically different cells
Role	Growth, repair, and asexual reproduction	Produces gametes or spores

Mutations: Provide the original source of genetic diversity, creating different versions of genes (alleles).
Genetic variation is achieved through the shuffling of alleles during reproduction.
Natural Selection: Accumulates favorable variations based on environmental pressure.

Behavior of chromosomes during meiosis and fertilization contributes significantly to variation.
- Mechanisms include:
- Independent Assortment: Random alignment and separation of chromosomes during metaphase I.
- Crossing Over: Exchanges between non-sister chromatids during synapsis.
- Random Fertilization: Any sperm can fuse with any egg, creating diverse zygotes.

Human haploid number (n) = 23, leading to over 8 million possible combinations (2^23).

Results in a zygote with about 70 trillion unique diploid combinations.
Each zygote represents a unique genetic identity, further enhanced by crossing over events.