GENE3200 - Mitosis and Meiosis (class 2)

telomeres and centromeres

-classified as heterochromatin (transcriptionally silenced)

-highly repetitive DNA, clusters of 100,000s to millions of tandem repeats < 10bp length, also called satellite DNA

telomeres

-consist of specific repetitive DNA sequences and don’t contain genes

-tandem repeats of A or T followed by several G’s

-sequence of repeats is specific to each species

-maintain integrity of chromosomal ends and protect the ends of the chromosomes to prevent chromosome fusion

specific structures occur at the telomeres

the G-rich 3’ overhang folds over and forms a loop at the end of the telomere (t-loop) by intermolecular H-bonding with the complementary strand

chromosomes are differentiated by their length and centromere position

metacentric, submetacentric, acrocentric, telocentric

metacentric

centromere is in the exact middle of a chromosome

submetacentric

centromere is off-center

acrocentric

centromere is almost to the telomere, but the small arms are still visible

telocentric

centromere is close enough to the telomere that the small arms are not visible

eukaryotes have two major portions of their cell cycle

-mitosis is the portion where chromatin condenses into thread-like chromosomes are visible, align and separate into the nascent cells

-interphase is the time between one mitosis and the next, in interphase, the chromatin is diffuse

the cell consists of interphase and mitosis

-interphase chromosomes are relaxed and cannot be visualized in a microscope

-s-phase: DNA replication (chromosome duplication)

-mitotic and meiotic chromosomes are highly condensed and can be visualized in a microscope

after s-phase

-chromosomes consist of a pair of sister chromatids

-identical copies of each chromosome joined at the centromere

-each chromatid consists of double-strand DNA (ex: A and a and B and b are two different alleles of the same gene)

the mitotic cycle

-G1 + S + G2 = interphase

-mitosis = prophase + prometaphase + metaphase + anaphase + telophase

-cytokinesis = cell division

G₀ phase

stable, nondividing period of variable length

G₁ phase

growth and development of the cell; G₁/S checkpoint

s phase

synthesis of DNA

G₂ phase

preparation for division; G₂/M checkpoint

prophase

chromosomes condense and mitotic spindle forms

prometaphase

nuclear envelope disintegrates, and spindle microtubules anchor to kinetochores

metaphase

chromosomes align on the metaphase plate; spindle-assembly checkpoint

anaphase

sister chromatids separate, becoming individual chromosomes that migrate toward spindle poles

telophase

chromosomes arrive at spindle poles, the nuclear envelope re-forms, and the condensed chromosomes relax

cytokinesis

cytoplasm divides; cell wall forms in plant cells

gamete fusion…

combines one set of human chromosomes with another

gamete fusion

-humans have 2 sets of 23 chromosomes (diploid) (1 from mom, 1 from dad)

-sperm and egg each contain 1 set of the we chromosomes (haploid)

-meiosis is the process of separating homologous chromosomes to create gametes

mitosis vs meiosis (mitosis)

-2 identical daughter cells are produced from 1 parent cell

-daughter cells have full set of homologous chromosomes

mitosis vs meiosis (meiosis)

-4 non-identical daughter cells from 1 parent cell

-daughter cells have half the parent cell’s chromosomes

-haploid daughter cells have no homologous chromosomes

mitosis =

equational division

meiosis =

reductional division

homologs and chromatids

-identical copies of each chromosome present after s-phase and held together at their centromeres

-each chromatid consists of double-strand DNA

meiosis I

-reductional division

-separation of homologous chromosomes (sister chromatids remain intact)

-number of chromosomes per cell reduced by half

meiosis II

-equational division

-separation of sister chromatids

-similar events as mitosis except the cell starts with half the number of chromosomes

crossing-over…

-during meiosis I results in genetic variation

-non-sister chromatids recombine (cross over) and create new combinations of alleles

meiosis I reductional division

-separation of homologous chromosomes

-chromsomes that have exchanged information are known as recombinants

somatic cells

-cells other than germ cells

-most types are diploid (non are haploid)

-never go through meiosis

-can undergo mutations (including recombination) and can have abnormal chromosome segregation (nondisjunction) during mitosis

germ cells

-precursors to gametes

-develop from specialized diploid cells

-located only in the gonads (testis and ovary)

-go through mitosis and meiosis only at specific stages of development

-the only cells that go through meiosis

spermatogenesis is…

-the process of sperm formation

spermatogenesis

-mitosis and meiosis occur continuously throughout adult life

-each testis contains thousands of spermatogonia and spermatocytes that enter and progress through mitosis and meiosis respectively

oogenesis is…

the process of egg cell formation

oogenesis

-mitosis occurs only during embryonic development and meiosis begins during gestation, but arrests in prophase I until menarche

-during ovulation, one 1°oocyte per month completes meiosis I to form a secondary oocyte and the 1st polar body (2°oocyte arrests in meiosis II, 1st polar body is degraded)

-meiosis II of secondary oocyte: resumes if sperm/egg fusion occurs and produces an ovum and the 2nd polar body and is not completed if fertilization does not occur (disintegrates)

nondisjunction

-abnormal chromosome segregation that results in abnormal chromosome numbers after cell division

-can occur in mitosis, meiosis I, or meiosis II

-can rest in inviable cells/gametes or inviable embryos

-can also result in abnormal chromosome numbers in viable individuals (trisomy21, down syndrome)