Chapter 10 Meiosis and Sexual Life Cycles

Heredity

The transmission of traits from one generation to the next

Variation

Differences between members of the same species

Genetics

The scientific study of heredity and heredity variation

Genes

Discrete units of hereditary information consisting of specific nucleotide sequences in DNA (or RNA, for some viruses)

Somatic cell

Any cell in a multicellular organism except a sperm or egg or their precursors

Gamete

A haploid reproductive cell, such as an egg or sperm, which unite during sexual reproduction to produce a diploid zygote

Locus

A specific place along the length of a chromosome where a given gene is located

Sexual reproduction

A type of reproduction in which two parents give rise to offspring that have unique combinations of genes inherited from both parents via the gametes

Asexual reproduction

The generation of offspring from a single parent that occurs without the fusion of gametes , by budding, division of a single cell, or division of the entire organism into two or more parts

Clone

A lineage of genetically identical individuals or cells

Life cycle

The generation to generation sequence of stages in the reproductive history of an organism

Karyotype

A display of the chromosome pairs of a cell arranged by size and shape

Homologous chromosomes

A pair of chromosomes of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One is inherited from the organism's mother, and one is inherited from the organism's father.

Sister chromatids

Two copies of a duplicated chromosome attached to each other by proteins at the centromere and, sometimes, along the arms. While joined, they make up one chromosome. They are eventually separated during mitosis or meiosis II

Sex chromosome

A chromosome responsible for determining the sex of an individual

Autosome

A chromosome that is not directly involved in determining sex; not a sex chromosome

Diploid cell

A cell containing two sets of chromosomes (2n), one set inherited from each parent

Haploid cell

A cell containing only one set of chromosomes (n)

Sperm

The male gamete

Egg

The female gamete

Fertilization

The union of haploid gametes to produce a diploid zygote

Zygote

The diploid cell produced by the union of haploid gametes during fertilization; a fertilized egg

Meiosis

A modified type of cell division in sexually reproducing organisms consisting of two rounds of cell division but only one round of DNA replication. It results in cells with half the number of chromosome sets as the original cell

Alteration of generations

A life cycle in which there is both a multicellular diploid form, the sporophyte, and a multicellular haploid form, the gametophyte: characteristic of plants and some algae

Meiosis I

The first division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the number of chromosome sets as the original cell

Meiosis II

The second division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the number of chromosome sets as the original cell

Prophase I

Each chromosome comes near its replicated chromosome pair. Nucleus dissolves at this stage. Crossing over (exchange of genetic material) occurs at this phase.

Synapsis

The pairing and physical connection of duplicated homologous chromosomes during prophase I of meiosis

Crossing over

The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis

Chiasmata

The X-shaped, microscopically visible region where crossing over has occurred earlier in prophase I between homologous nonsister chromatids. They become visible after synapsis ends, with the two homologs remaining associated due to sister chromatid cohesion

Metaphase I

Pairs of homologous chromosomes move to the equator of the cell.

Anaphase I

Homologous chromosomes are pulled to opposite ends of the cell. Note that chromatids do not separate - each duplicated chromosome still has two chromatids.

Telophase I

A nuclear membrane forms around each cluster of chromosomes

Cytokinesis I

Cytoplasm divides and cell separates into two cells during the first stage of meiosis

Prophase II

The first phase of meiosis II. Prophase II is identical to mitotic prophase, except that the number of chromosomes was reduced by half during meiosis I.

Metaphase II

The duplicated chromosomes move to the center of the cell. Each centromere attaches to two spindle fibers instead of one.

Anaphase II

The chromatids separate and move to opposite ends of the cell. Each chromatid is now an individual chromosome. Anaphase II is identical to mitotic anaphase except that the number of chromosomes was reduced by half in meiosis I.

Telophase II

The fourth and final phase of meiosis II. Telophase II is identical to mitotic telophase, except that the number of chromosomes was reduced by half during meiosis. I.

Cytokinesis II

The final phase of meiosis II where the cells split into four haploid daughter cells.

Genetic variation

Differences among individuals in the composition of their genes or other DNA segments

Independent assortment

Each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs

Recombinant chromosomes

A chromosome created when crossing over combines DNA from two parents into a single chromosome

Random fertilization

The combination of each unique sperm with each unique egg increases genetic variability