Chapter 3: Cell Division and Chromosome Heredity

1. How mitosis divides somatic cells (and stages of); remember the definition of somatic cells means it’s a body cell (aka NOT a gamete) 2. How meiosis produces cells for sexual reproduction (and stages of) 3. Chromosome theory – genes are carried on Chromosomes; X-linked vs autosomal inheritance 4. Sex determination is Chromosomal and genetic 5. Sex-linked inheritance patterns: Thomas Morgan’s Drosophila eye color X-linked discovery

Chromatin

the material of which eukaryotic chromosomes are composed; protein, RNA and DNA

Euchromatin

chromosomal material that I snot densely compacted during normal cell operation; comprises many functional (genes) parts of the genome

Heterochromatin

chromosomal material more densely packed and containing few expressed genes

Chromatid

one of the replicated structures

Sister Chromatids

the two replicated chromosomes as a unit

Cell Reproduction

• continues throughout the life of the multicellular organism

• Replaces old or died cells, and worn out blood cells

Diploid

2n chromosomes present in pairs

Haploid

1n only 1 chromosome pair

Somatic Cells

non reproductive cells, usually diploid “body cells”

• produced though mitosis

Gametes

reproductive cells, germ-line cells

• haploid

• produced meiosis

• sperm, egg cells

• not genetically identical to one another

Meiosis

produces gametes that have held the number of chromosomes as the original cell (reduction type division)

M Phase (mitotic)

• includes mitosis

  • only a small percentages of cells in a tissue are in mitosis at any given time

• cytokinesis

• DNA is tightly packed and inaccessible

• usual processes of protein synthesis are largely shut down

Cytokinesis

entire cell and its cytoplasm divide into 2 daughter cells

Mitosis

• Process of nuclear division

• replicated DNA molecules of each chromosome are parted into 2 nuclei

• accompanied by cytokinesis

• Maintains chromosome number

  • Starts with one cell with a. diploid number 2n

  • Ends with two cells with diploid complements of each chromosome

• generates new cells for organism growth, maintenance and repair

• happens in diploid or haploid cells

• 5 stages

Interphase

divided into G1 (first gap), S (synthesis) & G2 (second gap)

• carries bulk of cycle

• most DNA is unpacked and distributed throughout the nucleus

• cell prepares for mitosis, replicates DNA, performs normal metabolic functions

G₁

• Cell grows and carries out normal metabolism

• Organelles duplicate

• Active gene expression and cell activity

• Preparation for DNA synthesis

S Phase

• DNA replication and chromosome duplication

• 46 chromosomes and 92 sister chromatids

G₂

• Cell grows and prepares for mitosis

• Preparation for cell division

G₀

Terminal differentiation and arrest of cell division

• cell remains specialized but does not divide

• Eventual cell death (apoptosis)

Prophase

Duplicated chromosomes are prepared for segregation & mitotic machinery is assembled

• chromosomal material condenses to form compact mitotic chromosomes (2 sister chromatids)

  • held together wit a multi protein complex called cohesion

• cytoskeleton is disassembled and mitotic spindle is assembled

• spindle formation

• microtubules appear around each centrosome

Prometaphase

• starts with dissolution of the nuclear envelope

• spindle assembly is completed

• chromosomes are moved into position at center of cell

• Microtubules penetrate central cell region,

  • The free ends grow & shrink as if they are searching for chromosome

  • Those that contact a kinetochore are captured & stabilized

    • Kinetochore from one chromatid becomes stably associated with spindle microtubules from one pole

    • Unattached sister chromatid kinetochore captures its own MTs from opposite spindle pole

Metaphase

• chromosomes aligned at the equator plane (metaphase plate)

Anaphase

• chromosomes spilt and migrate poleward

• Chromosomes move very slow

Telophase

• Chromosomes collecting as they near their respective poles

• Daughter cells return to their interphase condition

  • Nuclear envelope reforms

  • Chromosomes disperse until they disappear from view under microscope

Meiosis

• Produces haploid gametes for sexual reproduction

• union of haploid gametes produces progeny

• followed by two divisions, no DNA replication

• recombination or crossing over

• only happens in reproductive organs

Meiosis I

separation of homologous pairs of chromosomes

• Homologous chromosomes pair

• crossing over occurs at chiasmata

• Segregation of homologous chromosomes occurs to make haploid complements

Synapsis: pairing of homologous chromosomes

• occurs in early prophase I

Prophase I

• The chromosomes condense, and the nuclear envelope breaks down

• Crossing over occurs

Metaphase I

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

Anaphase I

• Homologous chromosomes move to the opposite poles of the cell

• reduces the amount of DNA by ½

Telophase I & Cytokinesis

• Chromosomes gather at the poles of the cells

• The cytoplasm divides

Prophase II

• A spindle forms around the chromosomes

Metaphase II

• Chromosomes line at the equator

Anaphase II

• Centromere divides

• Chromatids move to the opposite poles of the cells

• reduces DNA amount by an additional ½

Telophase II & Cytokinesis

• A nuclear envelope forms around each set of chromosomes

• The cytoplasm divides

Meiosis II

• separation of sister chromatids

X-linked inheritance

• differences in male and female chromosomes in many species die to the presence of 2 X chromosomes in females and a smaller Y chromosome in males

• refers to the inheritance of genes on the X chromosome

Wild Type (WT)

the phenotype most common in a population

Sex-linked inheritance

refers to transmission of genes on sex chromosomes

Reciprocal cross

• crossing a pair of parents with the sexes reversed

Hemizygous

males having only one X that carry traits

• gene trait only found on X

XO male

sterile

Nondisjunction

no separation of gametes

• in females, gametes have either tow X chromosomes or no sex chromosomes

SRY

a gene on the Y chromosome responsible for phenotypic male-ness

• produces a protein, testis-determining factor (TDF)

  • induces the cascade of events leading to phenotypic male-ness

Chromosomal sex

the presence of chromosomes characteristics of each sex and is determined at the moment of fertilization

Phenotypic sex

the internal and external morphology of each sex, and results from differences in gene expression

X-linked inheritance patterns

• can be dominant or recessice

• more males than females have recessive X-linked phenotype

• if recessive male mates with dominant homozygous female → all are dominant

• Recessive males x heterozygous females → 50/50

• Homozygous recessive female x hemizygous dominant male → recessive males and dominant females

• affected males always yield affected daughters

Y-linked inheritance patterns

• an genes are inherited as singletons

• always dominant and only found in males

• fewer then 50 genes

Dosage compensation

• compensates for the differences

  • Genes on autosomes typically contribute protein from both copies

  • same in females

  • half for males

  • randome X-inactivation

  • Placental mammals

  • one of X chromosomes of females is converted into an inactive Barr body