Lecture 3: Meiosis and Genetic Diversity

These flashcards cover key vocabulary related to meiosis, genetic diversity, and related processes that are critical for understanding the material presented in the lecture.

Meiosis

is a specialized type of cell division in sexually reproducing organisms that reduces the chromosome number by half. It involves two rounds of division to produce four genetically distinct haploid cells (gametes) from a single diploid parent cell, ensuring genetic diversity and proper chromosome number for fertilization.

Ploidy

refers to the number of complete sets of chromosomes in a cell or organism. For example, diploid (2n) organisms, like humans, have two sets of homologous chromosomes (one from each parent), while haploid (n) cells, like gametes, contain only one set.

Homologous Recombination

also known as crossing over, is the crucial exchange of genetic material between non-sister chromatids of homologous chromosomes. This event occurs during Prophase I of meiosis, creating new combinations of alleles on chromosomes and significantly contributing to genetic diversity among offspring.

Independent Assortment

is the process where homologous chromosome pairs orient and separate randomly into daughter cells during Metaphase I and Anaphase I of meiosis. This random pairing and separation of maternal and paternal chromosomes leads to countless different combinations of alleles in the resulting gametes, increasing genetic variation.

Nondisjunction

an error that occurs during cell division (either Meiosis I, Meiosis II, or Mitosis) when homologous chromosomes fail to separate during anaphase I, or sister chromatids fail to separate during anaphase II (or mitotic anaphase). This improper segregation leads to daughter cells with an abnormal number of chromosomes (aneuploidy).

Aneuploidy

describes a chromosomal abnormality where a cell has an abnormal number of chromosomes, meaning it has either too many or too few chromosomes compared to the normal diploid number for that species. This condition typically results from nondisjunction during meiosis and is a common cause of genetic disorders and developmental abnormalities.

Chiasma (chiasmata)

is the point of contact, visible under a microscope, between two non-sister chromatids belonging to homologous chromosomes. It is at these specific sites that homologous recombination (crossing over) occurs during Prophase I of meiosis, facilitating the exchange of genetic material.

Gametes

are haploid reproductive cells that contain only one set of chromosomes (n). They are produced through meiosis and are essential for sexual reproduction, as they fuse during fertilization to form a diploid zygote (2n).

Zygote

is the diploid cell (2n) that results from the fusion of two haploid gametes (sperm and egg) during fertilization. It is the first stage in the development of a new individual organism, containing a complete set of chromosomes from both parents.

Sex Chromosome Aneuploidy

is a condition where an individual has an abnormal number of sex chromosomes (X or Y), rather than the usual XX for females or XY for males. These conditions, such as Turner syndrome (X) or Klinefelter syndrome (XXY), typically arise from nondisjunction during meiosis and can lead to various developmental and physiological effects.

Down Syndrome (Trisomy 21)

a genetic disorder caused by the presence of all or part of an extra 21st chromosome. This condition almost invariably results from nondisjunction during oogenesis (egg formation), leading to characteristic physical features, intellectual disability, and increased risk of certain health problems.

Spermatogenesis

is the complex biological process occurring in the male testes, where diploid spermatogonia undergo meiosis to produce four functional, haploid sperm cells (spermatozoa). This continuous process begins at puberty and ensures a constant supply of male gametes for fertilization.

Oogenesis

is the process of egg cell (ovum) production in the female ovaries. Unlike spermatogenesis, it begins before birth and is interrupted, continuing at puberty. It results in a single large, functional haploid egg cell and smaller polar bodies, ensuring the egg is well-equipped to support embryonic development after fertilization.

Non-sister chromatids

are the chromatids of homologous chromosomes that are not identical to each other (i.e., one is from the paternal chromosome and the other from the maternal chromosome). These are the specific chromatids that participate in homologous recombination (crossing over) during Prophase I of meiosis, facilitating genetic exchange.