Chapter 10: Meiosis & Sexual Reproduction

A comprehensive vocabulary study guide covering chromosome numbers, the mechanics and phases of meiosis, genetic variation, gametogenesis, and chromosomal abnormalities.

Diploid ($2n$)

Cells containing two complete sets of chromosomes (one set inherited from each parent), found in somatic (body) cells.

Diploid chromosome numbers for humans

human diploid number is $2n = 46$ chromosomes ($23$ pairs)

Haploid ($n$)

Cells containing a single set of chromosomes, found in gametes (sperm and egg cells)

Haploid chromosomes for humans

human haploid number is $n = 23$ chromosomes.

Homologous Chromosomes

A pair of chromosomes (one maternal, one paternal) that are the same length, have the same centromere position, and carry genes for the same traits at the same loci.

Alleles

Alternative forms of a gene situated at the same location on homologous chromosomes.

Sister Chromatids

Identical copies of a chromosome produced during S phase, which remain attached at the centromere until separation.

Mitosis

Its purpose is for growth, tissue repair, and asexual reproduction in body cells. It occurs in somatic (body) cells throughout the organism. It divides once after S phase, forms 2 identical diploid (2n) daughter cells, shows no variation, and Synapsis & Crossing-Over do not occur

Meiosis (purpose and more)

Its purpose is for sexual reproduction and production of haploid gametes. It occurs in germline cells inside gonads (testes and ovaries). It divides twice after S phase, forms 4 genetically distinct haploid (n) cells, has high variation via crossing-over & independent assortment, as both are present during Prophase I.

Meiosis

A specialized form of cell division that reduces the chromosome number from diploid ($2n$) to haploid ($n$) through one round of DNA replication followed by two nuclear divisions (Meiosis I and Meiosis II).

Meiosis I (Reduction Division)

The division stage where homologous chromosome pairs separate, reducing the chromosome number of the cell from $2n$ to $n$.

Meiosis II (Equational Division)

The division stage where sister chromatids separate, resulting in $4$ haploid daughter cells.

Synapsis

The pairing up of homologous chromosomes tightly during Prophase I to form tetrads (bivalents).

Crossing-Over

The process occurs in Prophase I, where non-sister chromatids of homologous chromosomes exchange segments of genetic material at contact points called chiasmata. This leads to sister chromatids no longer being identical and creates entirely new, unique combinations of alleles

Chiasmata

The specific contact points between non-sister chromatids where crossing-over and exchange of genetic material take place during Prophase I.

Recombinant Chromosomes

Chromosomes produced by crossing-over that carry novel combinations of maternal and paternal genes.

Independent Assortment

The random alignment and orientation of maternal and paternal homologous chromosome pairs at the metaphase plate during Metaphase I, generating $2^{23}$ (over $8.3\text{ million}$ ) possible combinations per gamete in humans. This leads to the chromosomes being randomly distributed into the resulting gametes (sperm or egg).

Random Fertilization

The random union of a single sperm and egg, each possessing over $8.3\text{ million}$ possible chromosome combinations, yields over $70\text{ trillion}$ possible zygote combinations excluding crossing-over. Genetic diversity allows populations to adapt to changing environmental conditions, resist diseases, and drive evolutionary change over generations.

Tetrad

A complex composed of four chromatids formed by the tight pairing of homologous chromosomes during Prophase I.

Prophase I

The phase of Meiosis I where chromatin condenses, synapsis forms tetrads, crossing-over occurs between non-sister chromatids, the spindle apparatus forms, and the nuclear envelope breaks down.

Metaphase I

The phase of Meiosis I where homologous pairs (tetrads) align randomly in double files along the metaphase plate and independent assortment occurs.

Anaphase I

The phase of Meiosis I where homologous chromosome pairs separate and move toward opposite poles while sister chromatids remain attached at their centromeres.

Telophase I & Cytokinesis

The meiotic stage yielding $2$ haploid ($n$) daughter cells, each containing a haploid set of duplicated chromosomes, accompanied by nuclear envelope re-formation in some species.

Prophase II

The initial phase of Meiosis II where chromosomes condense again without prior DNA replication, a new spindle apparatus forms, and the nuclear envelope dissolves in two haploid cells.

Metaphase II

The phase of Meiosis II where duplicated chromosomes align in single file across the metaphase plate in both distinct haploid cells.

Anaphase II

The phase of Meiosis II where sister chromatids split at centromeres and move as individual daughter chromosomes toward opposite poles.

Telophase II & Cytokinesis

The concluding phase of meiosis where nuclear envelopes re-form around four separate nuclei, producing $4$ genetically distinct haploid ($n$) cells.

Steps that occur in Prophase 1 of meiosis

Homologous pairs tightly pair to form tetrads/bivalents, and crossing over occurs at chiasmata between non-sister chromatids

Stepts that occur in Prophase of mitosis

Homologous pairs act independently without pairing and crossing over does not occur

Chromosome Alignment in Meiosis I

Homologous pairs line up in double files at the metaphase plate and homologous chromosomes separate in Anaphase I.

Chromosome Alignment in Meiosis II

Individual duplicated chromosomes line up in single file and sister chromatids separate in Anaphase II.

Chromosome Alignment in Mitosis

Individual duplicated chromosomes line up in single file and Sister chromatids separate in Anaphase.

Spermatogenesis

Male gamete production occurring in the seminiferous tubules of the testes, where a diploid ($2n$) primary spermatocyte undergoes continuous meiosis from puberty onward to yield $4$ equal, functional, flagellated sperm cells ($n$) per cycle.

Oogenesis

Female gamete production occurring in the ovaries that pauses in Prophase I before birth and divides cytoplasm unequally during Meiosis I and II, producing $1$ large functional egg/ovum ($n$) and $3$ non-functional polar bodies.

Polar Bodies

Three small, non-functional cells formed during oogenesis as a result of unequal cytokinesis during meiotic divisions.

Fertilization

The fusion of haploid gametes (sperm, $n = 23$; egg, $n = 23$) that restores the diploid state in a zygote ($2n = 46$).

Roles of Mitosis & Meiosis in the human life cycle

Meiosis produces haploid gametes (sperm, n = 23; egg, n = 23). During fertilization, gametes fuse to restore the diploid zygote (2n = 46). The zygote then undergoes continuous mitosis and cellular differentiation to grow into a multicellular embryo, fetus, child, and adult.

Euploidy

The presence of the correct, normal number of chromosomes characteristic of a species ($2n = 46$ in humans).

Aneuploidy

A condition characterized by an abnormal chromosome number resulting from extra or missing individual chromosomes.

Monosomy ($2n - 1$)

An aneuploid condition in which a diploid cell is missing one chromosome, such as Turner Syndrome ($45,\text{XO}$).

Trisomy ($2n + 1$)

An aneuploid condition in which a cell contains three copies of a specific chromosome, such as Down Syndrome (Trisomy $21$).

Nondisjunction

The failure of homologous chromosome pairs or sister chromatids to separate properly during meiotic cell division.

Nondisjunction in Meiosis I

The failure of homologous pairs to separate during the first meiotic division, resulting in $100\%$ abnormal gametes ($2$ gametes with $n + 1$ and $2$ gametes with $n - 1$).

Nondisjunction in Meiosis II

The failure of sister chromatids to separate during the second meiotic division, resulting in $50\%$ normal gametes ($n$) and $50\%$ abnormal gametes ($1$ gamete with $n + 1$ and $1$ gamete with $n - 1$).