Genetics: Chromosomes, Cell Cycle, and Sex Determination

Eukaryotic chromosome

Composed of DNA, RNA, and protein; a thread-like structure of nucleic acid (DNA) around a core of protein.

Telomere

The end part of a chromosome that protects it from deterioration.

Centromere

Region of constriction in chromosomes where the spindle apparatus attaches.

Kinetochore

A protein structure on the chromosome where the spindle fibers attach during cell division.

Chromatid

One of the two identical halves of a replicated chromosome.

Monad

One chromatid; not typically visible, before replication.

Dyad

Two chromatids; visible after replication.

Diploid

Organisms that have a pair of chromosomes, called homologs or homologous chromosomes.

Tetrad

Formed from paired homologs during prophase I of Meiosis I and consists of 4 chromatids intertwined.

Chromatin

DNA and histone proteins in the nucleus which condense to form chromosomes.

Chromosome condensation

Coiling and packing of DNA into distinct packages which are easier to separate during nuclear division.

Sister chromatids

The two components of the same chromosome after DNA has been replicated; they share a common centromere and are exact copies of each other.

Homologous chromosomes

Same type of chromosome (one from each parent) that have the same length, same genes at the same loci, same banding pattern, and same centromere position, but may have different alleles for the genes.

Karyotype

The number and appearance of chromosomes in the nucleus of a eukaryotic cell.

Cell cycle

The process of cell growth and division, represented as G1→S→G2→M→C (24 hr cycle).

G1 phase

(5-10 hrs) Rapid growth phase, primary growth phase.

S phase

(7-9 hrs) Genome (DNA) replicated, formation of sister chromatids.

G2 phase

(3-4 hrs) Preparations made for genomic separation (mitosis), including replication of mitochondria and other organelles.

Mitosis

(1 hour) Separation of chromosomes during cell division.

Cytokinesis

Physical division of the cell into two daughter cells.

S-phase

DNA molecules replicate to form identical chromatids.

Mitosis (nuclear division)

Process where 2n → 2n + 2N or 1N → 1N + 1N.

Stages of Mitosis

Prophase (early) → metaphase (middle) → anaphase (toward) → telophase (end).

Prophase

Mitotic apparatus forms & condensed chromosomes become visible.

Metaphase

Centromeres align, chromosomes (pairs of sister chromatids) align along the metaphase or equatorial plate.

Anaphase

Separation of sister chromatids which move toward poles.

Telophase

Chromosomes arrive and nuclei reform; spindle apparatus is disassembled.

Animal cytokinesis

Involves 'pinching' with furrow.

Plant cytokinesis

Rigid cell wall and cell plate forms.

Products of Mitosis and Cytokinesis

One 2n cell → two 2n daughter cells; One 1n cell → two 1n daughter cells.

Daughter Cells of Mitosis

~ ½ the size of the parent cell, but nucleus has the same DNA content as the parent cell.

Equational division

One (2N) cell gives rise to two (2N) daughter cells.

Sexual Life Cycle

Life cycle that exhibits alternation of haploid and diploid chromosome number.

Meiosis

Reduction division: 2N → N + N + N + N.

Meiosis I

First round of chromosome separation (separation of homologues).

Meiosis II

Second round of chromosome separation (separation of sister chromatids).

Prophase 1

Homologs pair and exchange information.

Leptonema

2 copies of condensed chromosomes appear as a single thread.

Zygonema

Homologs pair and form tetrad (4-stranded structure), also called bivalent.

Pachynema

You can see four chromatids; crossing over occurs between homologs.

Chiasmata

Site of crossover; formation of ladder-like synaptonemal complex.

Diplonema

Homologs unpair & pull away from each other; crossing over is seen.

Diakinesis

Homologs thicken and move to center of nucleus; nuclear membrane disperses.

Metaphase 1

Paired homologs align in the middle.

Anaphase 1

Paired homologs separate & are pulled toward poles randomly.

Telophase 1

Homologs (sister chromatids intact) arrive at poles.

Leptotene

The first stage of prophase I where chromosomes condense into visible strands.

Homologous chromosome

Chromosomes that are similar in size, shape, and nature of inherited characters, with one chromosome from each parent.

Bouquet stage

A term coined by Darlington to describe the stage in which chromosomes touch the undersurface of the nuclear envelope by their telomeres pointing towards the centrioles.

Zygotene

The stage where chromosomes line up with each other into homologous chromosome pairs, also known as zygonema.

Synapsis

The pairing or coming together of homologous chromosomes during the zygotene stage.

Bivalent chromosome

Paired chromosomes during the zygotene stage that are equal in length and position of centromere.

Pachytene

The stage where chromosomal crossover occurs, involving the exchange of segments between nonsister chromatids of homologous chromosomes.

Diplotene

The stage where homologous chromosomes separate slightly, allowing some transcription of DNA, while remaining tightly bound at chiasmata.

Dictyotene stage

The suspended state of developing oocytes in human fetal oogenesis that occurs during diplotene and remains until puberty.

Synaptonemal complex

The structure that facilitates the pairing of homologous chromosomes during the zygotene stage.

Nonsister chromatids

Chromatids from homologous chromosomes that are involved in the crossing over process.

Crossing over

The process during pachytene where segments of homologous chromosomes are exchanged.

Procentric

A term describing the start of pairing at the centromere during zygotene.

Proterminal

A term describing the start of pairing at the chromosome ends during zygotene.

Intermediate

A term describing the start of pairing at any other portion of the chromosome during zygotene.

Tetrad chromosome

Another term for bivalent chromosomes, referring to the paired structure of homologous chromosomes.

Meiotic spindle

The structure that begins to form during diakinesis, similar to the spindle formed during mitosis.

Nuclear membrane

The membrane that disintegrates into vesicles during diakinesis.

Telomeres

The ends of chromosomes that cluster at one end of the nucleus during the zygotene stage.

Prophase II

Chromosomes condense.

Metaphase II

Haploid number of chromosomes (dyad) align in the middle.

Anaphase II

Separation of dyad to monad (1n to 1n).

Telophase II

2 daughter cells from each of the two previous cells.

Products of Meiosis

4 haploid cells and unduplicated (monad).

Nondisjunction

Failure of sister chromatids or homologs to separate properly in meiosis I or II.

Disjunction

The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II.

Trisomy

A condition resulting from nondisjunction, leading to gametes with too many chromosomes.

Monosomy

A condition resulting from nondisjunction, leading to gametes with too few chromosomes.

Down Syndrome

Trisomy of chromosome 21.

Patau Syndrome

Trisomy of chromosome 13.

Edward Syndrome

Trisomy of chromosome 18.

Klinefelter Syndrome

Extra X chromosomes in males (i.e., XXY, XXXY, XXXXY, etc.).

Turner Syndrome

Lacking one X chromosome in females (i.e., X0).

Triple X Syndrome

An extra X chromosome in females.

XYY Syndrome

An extra Y chromosome in males.

Risk of Down Syndrome

Increases after age 35; a woman aged 44 has a 40 times higher risk of conceiving a baby with Down Syndrome compared to younger women.

Reduction of ploidy

A key aspect of meiosis.

Mixing of chromosomes

Occurs on sides of metaphase in metaphase I; number of possible combinations in nuclei is 2^n where n=number of chromosome pairs.

Metaphase I

Chromosomes line up in pairs.

Daughter cell chromosomes in Meiosis I

Remain duplicated, containing sister chromatids.

Daughter cell chromosomes in Meiosis II

Are no longer duplicated (sister chromatids have separated from each other).

Chromosomal Theory of Inheritance

Genes are part of the chromosomes and that they deal with heredity.

Euchromatin

A lightly packed form of chromatin that is rich in gene concentration and is often under active transcription.

Heterochromatin

Tightly packed DNA that is transcriptionally inactive.

Centromeres

Regions of a chromosome that are heterochromatic.

Barr body

The inactivated X-chromosome in mammalian females.

Friedrich Miescher

A Swiss physician and biologist who first isolated nucleic acid from the nuclei of white blood cells.

Haploid

The basic chromosome number; 1 complete set of chromosomes.

Polyploidy

Multiple sets of chromosomes.

Tetraploid

4n; a type of polyploid with four sets of chromosomes.

Hexaploid

6n; a type of polyploid with six sets of chromosomes.

Octaploid

8n; a type of polyploid with eight sets of chromosomes.

N chromosome number

The number of chromosomes in a haploid set.

C amount of DNA

The amount of DNA (weight) in an N nucleus.