Genetics Chapter 2

Reproduction

The production of daughter cells from a mother cell or the production of offspring from parents.

Chromosome

The structures within living cells that contain the genetic material. Genes are physically located within the structure of chromosomes. Biochemically, chromosomes contain a very long segment of DNA, which is the genetic material, and proteins, which are bound to the DNA and give it structure.

Chromatin

The complex between DNA and proteins found within eukaryotic chromosomes.

Prokaryotes

Another name for bacteria and archaea. The term refers to the fact that their chromosomes are not contained within a separate nucleus in the cell.

nucleoid

A darkly staining region that contains the genetic material of mitochondria, chloroplasts, or bacteria.

Eukaryotes

One of the three domains of life. A defining feature of these organisms is that their cells contain a nucleus bounded by a membrane. Some simple eukaryotic species are single-celled protists and yeast; more complex multicellular species include fungi, plants, and animals.

organelle

A large, specialized structure within a cell, which is surrounded by a single or double membrane.

nucleus

A membrane-bound organelle in eukaryotic cells where sets of chromosomes are found.

cytogenetics

The field of genetics that involves the microscopic examination of chromosomes.

cytogeneticist

A scientist who studies chromosomes using microscopy.

somatic cell

Any cell of the body except for gametes and the germ-line cells that give rise to gametes.

gamete

A reproductive cell (usually haploid) that can unite with another reproductive cell to create a zygote. Sperm and egg cells are types of gametes.

karyotype

An organized representation of a micrograph of all the chromosomes within a cell. It reveals how many chromosomes are found within an actively dividing somatic cell.

diploid

Refers to an organism or cell that contains two sets of chromosomes.

homolog

One of the chromosomes in a pair of homologous chromosomes.

allele

An alternative form of a specific gene.

homozygous

Describes a diploid individual that has two identical alleles of a particular gene.

heterozygous

Describes a diploid individual that has two different alleles of the same gene.

locus (plural- loci)

The physical location of a gene or other DNA segment within a chromosome.

asexual reproduction

occurs when an organism makes more of itself without exchanging genetic information with another organism.

Multicellularity

Refers to the phenomenon that an organism may be composed of more than one cell.

binary fission

The physical process whereby a bacterial cell divides into two daughter cells. During this event, the two daughter cells become divided by the formation of a septum.

cell cycle

In eukaryotic cells, a series of phases through which a cell advances in order to divide. The phases are G for gap, S for synthesis (of the genetic material), and M for mitosis. There are two G phases, G1and G2.

Interphase

The series of phases G1, S, and G2, during which a cell spends most of its life.

G1 phase

A gap phase of the eukaryotic cell cycle during which a cell may prepare to divide; it precedes the S phase.

restriction point

A point in the G1 phase of the cell cycle at which a cell becomes committed to cell division.

S phase

A phase of the eukaryotic cell cycle during which the chromosomes are replicated.

chromatid

Following chromosomal replication in eukaryotes, the two copies that remain attached to each other as sister chromatids.

centromere

A region of a eukaryotic chromosome that provides an attachment site for the kinetochore.

sister chromatids

Pairs of replicated chromosomes that are attached to each other at the centromere. Sister chromatids are genetically identical.

dyad

A pair of sister chromatids.

monad

A single chromatid within a pair of sister chromatids; can also refer to a chromosome that has not replicated.

kinetochore

A group of cellular proteins that attach to the centromere during meiosis and mitosis.

G2 phase

A gap phase of the eukaryotic cell cycle during which a cell accumulates the materials that are necessary for nuclear and cell division; it precedes the M phase.

M phase

A general name for nuclear and cellular division during mitosis or meiosis. Nuclear division is divided into prophase, prometaphase, metaphase, anaphase, and telophase. Nuclear division is usually followed by cellular division or cytokinesis.

mitosis

A type of nuclear division into two nuclei, such that each daughter cell receives the same complement of chromosomes. The two daughter cells that result are genetically identical (except for rare mutations)

mitotic spindle apparatus

The structure responsible for organizing and sorting the chromosomes during mitosis; also known as the spindle apparatus or mitotic spindle.

microtubule-organizing center (MTOC)

A structure in a cell from which microtubules begin to grow.

centrosome

A cellular structure from which microtubules emanate.

spindle pole

During cell division in eukaryotes, one of two sites in the cell where microtubules originate.

centrioles

In animal cells, a pair of cylindrically shaped structures found within the centrosome.

decondensed

The state of chromosomes in which they are less tightly compacted.

prophase

The first stage of M phase. The chromosomes have already replicated and begin to condense. The spindle apparatus starts to form. Nuclear envelope begins to dissociate. Nucleolus (site of ribosome assembly) disappears

condense

To form a more compact structure, as chromatids do during prophase.

prometaphase

The second stage of M phase. During this stage, the nuclear membrane vesiculates, and the spindle apparatus is completely formed. Centrosomes move to opposite ends of the cell. Kinetochore MTs capture chromatids - sister chromatids tugged back and forth between the two poles

metaphase plate

The plane at which chromosomes align during metaphase.

metaphase

The third stage of M phase. The chromosomes align along the center of the spindle apparatus. Each pair of chromatids is attached to both poles by kinetochore microtubules. Now chromatids can be equally distributed into two daughter cells

anaphase

The fourth stage of M phase. The connection between sister chromatids breaks. Kinetochore MTs shorten and chromatids move to each pole. As anaphase proceeds, half of the chromosomes move to one pole, and the other half move to the other pole.

telophase

The fifth stage of M phase. The chromosomes have reached their respective poles and decondense. Nuclear membrane reforms to produce two separate nuclei. Organelles segregate

cytokinesis

The division of a single cell into two cells. The two nuclei produced in M phase are segregated into separate daughter cells during cytokinesis. In animals- Formation of a cleavage furrow. In plants- Formation of a cell plate

cleavage furrow

A constriction that precedes the division of two animal cells during cytokinesis.

cell plate

The structure that forms between two daughter plant cells and leads to the separation of the cells by formation of an intervening cell wall.

meiosis

A form of cell division in which the sorting process results in the production of haploid cells from a diploid cell.

haploid

Containing half the genetic material found in somatic cells. For a species that is diploid, a haploid gamete contains a single set of chromosomes.

leptotene

The first stage of prophase of meiosis I. Replicated chromosomes begin to condense

zygotene

The second stage of prophase of meiosis I. Via the process of synapsis, homologous chromosomes recognize each other and align, forming the synaptonemal complex

synapsis

The event in which homologous chromosomes recognize each other and begin to align themselves along their entire lengths.

synaptonemal complex

A complex of proteins that promotes the interconnection between homologous chromosomes during meiosis

pachytene

The third stage of prophase of meiosis I. Homologs are aligned. Pairs of sister chromatids- four total- are called bivalents or tetrads. Crossing over is most likely to occur in this stage (enhances genetic variation)

bivalent

A structure in which two pairs of homologous sister chromatids have synapsed (i.e., aligned) with each other.

tetrad

The association of four sister chromatids during meiosis.

crossing over

A physical exchange of chromosome pieces that most commonly occurs during pachytene of prophase of meiosis I.

chiasma (pl. chiasmata)

The site where crossing over occurs between two chromosomes. It resembles the Greek letter chi, χ.

diplotene

The fourth stage of prophase of meiosis I. Synaptonemal complex has mostly disappeared. Chromatids within bivalent pull slightly apart

diakinesis

The fifth stage of prophase of meiosis I. Synaptonemal complex completely disappears

sexual reproduction

The process whereby parents make gametes (e.g., sperm and egg) that fuse with each other in the process of fertilization to begin the life of a new organism.

gametogenesis

The production of gametes (e.g., sperm or egg cells).

isogamous

Describes a species that makes morphologically similar gametes.

heterogamous

Describes a species that produces two morphologically different types of gametes (i.e., sperm and eggs).

sperm cell

A male gamete, also known as a sperm. Sperm are small and usually travel relatively far distances to reach the female gamete.

egg cell

A female gamete, which is usually very large and nonmotile; also known as an ovum or an egg.

ovum

A female gamete, also known as an egg cell.

spermatogenesis

The production of sperm cells.

oogenesis

The production of egg cells. In female animals, it occurs in the ovaries. Early in development, diploid oogonia produce diploid primary oocytes. The primary oocytes initiate meiosis I, however, they enter into a dormant phase- they are arrested in prophase I until sexual maturity (~ age 13). At sexual maturity, primary oocytes are periodically activated to progress through meiosis I. The division in meiosis I is asymmetric producing two haploid cells of unequal size. A large secondary oocyte and a small polar body (only one of the four cells produced in this meiosis becomes an egg). The secondary oocyte enters meiosis II but is quickly arrested. It is released into the oviduct (ovulation). If the secondary oocyte is fertilized- Meiosis II is completed. A haploid egg and a second polar body are produced. The haploid egg and sperm nuclei then fuse to create the diploid nucleus of a new individual

gametophyte

The haploid generation of a plant.

sporophyte

The diploid generation of a plant.

pollen grain

The male gametophyte of flowering plants.

embryo sac

In flowering plants, the female gametophyte that contains an egg cell.

Prometaphase of Meiosis I

Pairing and crossing over are completed at the end of prophase. In prometaphase I, the spindle apparatus is formed. Chromatids are attached to the spindle via kinetochore microtubules

Metaphase of Meiosis I

At metaphase, the bivalents (or tetrads) are organized along the metaphase plate. Pairs of sister chromatids form a double row (not a single row as in mitosis). Arrangement of sister chromatids is random with regard to which parent they came from. Sister chromatids may be aligned in a very large number of possible ways. 2n possible alignments. One pair of sister chromatids is linked to one pole, and the homologous pair is linked to the opposite pole (in mitosis a pair of sister chromatids is linked to both poles)

Anaphase of Meiosis I

The two pairs of sister chromatids within a bivalent separate from one another. However, the connection between the sister chromatids does not break. The joined pair of sister chromatids moves to the pole (the two dyads within a tetrad separate and move to opposite poles)

Telophase and Cytokinesis of Meiosis I

The dyads have separated to opposite poles. The chromatids may decondense and the nuclear membrane may re-form. This is a reduction division, and the cells are considered haploid, because they only carry one set of homologous chromosomes

Meiosis II

The sorting events that occur during meiosis II are similar to those that occur during mitosis, (the starting point is different- mitosis start as diploid; meiosis II start haploid). Meiosis II proceeds through prophase, prometaphase, metaphase, anaphase, and telophase

OUTCOMES Mitosis versus Meiosis

Mitosis produces two diploid daughter cells that are genetically identical.

Meiosis produces four haploid daughter cells that are NOT genetically identical- The daughter cells contain only one homologous chromosome from each pair and contain many different combinations of the single homologs

OUTCOME of Meiosis in animals vs. plants

Animals produce gametes by meiosis, while Plants produce spores by meiosis. The spores develop into gametophytes. The haploid gametophyte becomes multicellular by mitotic cell divisions. The multicellular gametophyte then goes on to produce certain specialized cells as gametes