Lecture 5 - Chromosomes

Testes Determining Factor (TDF)

• Produced by the SRY (sex determining region on Y) gene that prevents ovaries from developing (female is the default sex)

Indels

Insertions or deletions

Testicular feminization syndrome

Mutation that makes male development incomplete even if SRY is functional

Heterogametic

When there are two different gametic chromosomes

Homogametic

When there are two of the same gametic chromosome

Environmental sex determination

When the environment determines the sex of an unborn organism. Happens with some reptiles and turtles (temperature)

Hyplodiploidy

Sex determining system where males are developed through unfertilized eggs that are haploid. Common in anys, bees, and wasps

X chromosome inactivation

X chromosomes are inactivated at random in the developing embryo → similar expression of males

They are reactivated during the formation of eggs

Oogenesis

The reactivation of X chromosomes durning the formation of eggs

Hyperactivation

When male X chromosomes’ gene expression is doubled and mimics a female’s two X chromosomes. Common in fruit flies

Barr body

The condensed, inactivated X chromosome that attaches itself to the inner surface of the nuclear membrane

Why do tortoise shell cats have patterns on their fur?

They have an X-linked coat color gene. DIfferent patches of skin cells have different X chromosomes inactivate and produces different patterns, EVEN in twins

Tetrad

Duplicated pairs of homologous chromosomes

Cytological analysis

A way to visualize chromosomes

1. Cell cycle is stopped at mitosis

2. Colchicine disables mitotic spindle at prophase

3. Chromosomes ara condensed at this phase and can be viewed under the light microscope

Colchicine

A compound that inhibits microtubule polymerization by binding to tubulin

• Cancer cells are more vulnerable to colchicine poisoning BUT it is also toxic against normal cells and its use is limited

Karyotype

Photographed chromosomes that are cut and pair with homologues (align by size)

G Bands

Giemsa stain is usedin cytological analysis and results in alternating light and dark bands (regions)

Polyploidy

Presence of more than 2 sets of chromosomes in complete extra sets

• Triploid, tetraploid, hexaploid

• Common in plants and often results in sterility (odd numbered ploidy)

• In plants, more chromosomes → larger cells → larger plants

Euploidy

Normal set of chromosomes (46 in humans)

Aneuploidy

Change in the number of single chromosomes

Chromosomal arrangements

Position changes/additions/deletions of chromosome regions BUT still have correct number of chromosomes

How is colchicine used in plants?

Used for inducing polyploidy in plant cells by inhibiting chromosome segregation during meiosis

• Half the resulting gametes contain no chromosomes

• The other half contain 2N (diploid instead of haploid)

What is the origin of hexaploid bread wheat?

It is a combination of three different genomes from 3 different species

Describe aneuploidy in humans

• miscarriages (embryo or live birth) from chromosomal abnormalities

• Chromosomal abnormalities are usually not survivable

What is the cause of down syndrome

• Usually due to abnormalities in eggs. Risk increases as mothers age (older eggs)

• Associated with short life expectancy, heart abnormalities, impaired mental abilities