Meiosis Overview and Chromosomes

Study Notes

Meiosis is a type of nuclear division that halves the chromosome number.
It produces gametes (sperm and eggs in animals).
Gametes combine during fertilization to form a zygote, restoring the full chromosome number.
Meiosis consists of two cell divisions (Meiosis I and Meiosis II).
Meiosis I separates homologous chromosomes, resulting in two haploid daughter cells.
Meiosis II separates sister chromatids, resulting in four haploid daughter cells.

Every organism has a unique chromosome number.
Sex chromosomes determine the sex of an individual (e.g., XX for females, XY for males in many animals).
Autosomes are non-sex chromosomes.
Homologous chromosomes (or homologs) are chromosome pairs with the same genes in the same order.
A gene is a segment of DNA that codes for a particular trait.
Different forms of a gene are called alleles.
A karyotype displays the number and types of chromosomes in a species.
Diploid (2n) cells have two sets of chromosomes.
Haploid (n) cells have one set of chromosomes.
Humans are diploid (2n = 46).

Early prophase I: Chromosomes condense, nuclear envelope breaks down, homologous chromosomes pair (synapsis). The paired chromosomes are called a bivalent or tetrad.
Late prophase I: Homologous chromosomes begin to separate but remain connected at points called chiasmata. Crossing over occurs between non-sister chromatids.
Metaphase I: Paired homologous chromosomes line up at the metaphase plate. The alignment is random.
Anaphase I: Homologous chromosomes separate and move to opposite poles of the cell.
Telophase I and Cytokinesis: Homologous chromosomes complete their movement to opposite poles and the cell divides into two daughter cells; each daughter cell has one set of replicated chromosomes.

Crossing over involves the exchange of genetic material between non-sister chromatids, creating recombinant chromosomes.
Independent assortment is the random orientation of homologous chromosomes during metaphase I. This produces different combinations of maternal and paternal chromosomes in daughter cells.

Prophase II: Spindle fibers form, attaching to sister chromatids.
Metaphase II: Replicated chromosomes line up at the metaphase plate (individual chromosomes).
Anaphase II: Sister chromatids separate and move to opposite poles.
Telophase II and Cytokinesis: Chromosomes arrive at opposite poles, nuclear envelope reforms, and the cell divides, resulting in four haploid daughter cells.

Errors in meiosis, such as nondisjunction, can produce gametes with extra or missing chromosomes.
Nondisjunction can lead to conditions like Down syndrome (trisomy 21).
Maternal age is a risk factor for errors in oogenesis.

The varying allele combinations resulting from crossing over and independent assortment create genetic diversity in offspring. This offers a variety of phenotypes, aiding survival in changing environments.
It's important to understand meiosis has a role in the cycle of life for sexually reproducing organisms.