Chapter 8 Chromosome Structure & Number

Molecular Genetics Lecture

what is cytogenetics

microscopic study of chromosomes

what is a micrograph

picture taken with a microscope

during what stage are chromosomes usually analyzed for karyotypes

metaphase

what is the centromere

chromosome constriction where the kinetochore forms and microtubules attach

what are the two chromosome arms called

p & q

what the does the p arm stand for

petit

what does the q arm represent

long arm of chromosome

which centromere position produces equal arm lengths

metacentric

which chromosome type has an extremely small p arm

telocentric chromosome

what are the four major centromere positions

metacentric

submetacentric

acrocentric

telocentric

what is chromosome banding pattern

light and dark staining pattern seen on chromosomes

what is G banding

giemsa staining that binds preferentially to AT rich DNA

what is R banding

reverse giemsa staining that binds preferentially to GC rich DNA

how are chromosome locations written

chromosome number, arm, then band location

what does the location 15q24.1 indicate

chromosome 15, long arm (q), region 24, band 1 (CYP1A, caffeine metabolism gene)

what is a deletion

loss of a chromosome segment

what is duplication

repetition of a chromosome segment

what is an inversion

reversal of a chromosome segment

what is a translocation

movement of chromosome material between chromosomes

what is a reciprocal translocation

exchange of chromosome segments between chromosomes

what is a terminal deletion

loss of the end of a chromosome following a break

what is an interstitial deletion

loss of an internal chromosome segment between two breaks

what can interfere with proper crossing over

repetitive DNA sequences

what is unequal crossing over

crossing over between misaligned homologous chromosomes

what happens to one chromatid after unequal crossing over

gains a duplicated segment

how are some paralogs produced

through unequal recombination

what are paralogs

homologous genes produced by duplication

why can duplicated genes evolve new functions

they experience reduced evolutionary constraints

what allows duplicated genes to diverge over time

accumulation of mutations

what does CNV stand for

copy number variant

what are CNV

naturally occurring differences in DNA copy number among individuals

what are insertions and deletions often called collectively

indels

approximately what percentage of the human genome may differ between unrelated individuals due to CNV

up to 0.4%

what colors are commonly used to label comparison DNA samples

red and green fluorescent probes

what happens if no CNV exists

equal hybridization creates a yellow signal

what does a strong bias toward one color indicate

copy number variant

why do labeled probes bind to chromosomes

complementary base pairing

what is an indel

insertion or deletion of DNA/chromosome segments

how can insertions and deletions arise during meiosis

through unequal crossing over

what often causes unequal crossing over

misalignment of repetitive DNA sequences

what chromosome abnormalities can unequal crossing over create

duplications and deletions

what is an inversion

a chromosome segment reversed in orientation

do inversions usually change gene dosage

no

are genes usually deleted or duplicated in inversions

neither, gene order is reversed

can inversion cause phenotypic abnormalities

yes

approximately what percentage of humans have detectable inversions

about 2%

how many exons does the Factor Vlll gene contain

26

how many introns does the Factor Vlll gene contain

25

what promotes the inversion shown in the Factor Vlll

repetitive sequences

why can inversions create problems during meiosis

homologous chromosomes must pair based on sequence similarity

what structure forms when homologs pair around an inversion

inversion loop

what are two major inversion types

pericentric and paracentric

which inversion includes the centromere

pericentric

which inversion excludes the centromere

paracentric inversion

what can happen if crossing over occurs inside an inversion loop

abnormal recombinant chromosomes may form

what chromosome can result from a crossover in a paracentric inversion

dicentric chromosome

what is a dicentric chromosome

chromosome with two centromeres

what chromosome fragment may also form

acentric fragment

what is an acentric chromosome

chromosome fragment lacking a centromere

what happens to acentric fragments

they are easily lost

what are telomeres

protective ends of eukaryotic chromosomes

why are telomeres important

they protect chromosome ends from degradation

what replication issue do telomeres help solve

replication of chromosome ends

what enzyme maintains telomeres

telomerase

what is translocation

movement or exchange of chromosome segments between chromosomes

how can translocation arise

through chromosome breakage and incorrect repair

how can nonhomologous crossing over cause a translocation

crossing over occurs between different chromosomes

what is a balanced translocation

translocation in which no genes are lost or duplicated

does a balanced translocation alter gene content

no

can an individual with a balanced translocation appear normal

yes

why can balanced translocations still cause abnormal offspring

meiosis may produce unbalanced gametes

what forms during meiosis in translocation carriers

translocation cross

why are meiotic outcomes variable in translocation carriers

segregation depends on spindle attachment

what is ploidy

number of chromosome sets

what is euploidy

having an exact multiple set of chromosomes

what is polyploidy

have more than two chromosome sets

what is triploid

three chromosome sets

what is tetraploid

four chromosome sets

what is octoploid

eight chromosome sets

and is aneuploidy

chromosome number variation that is not a complete set

what is nullisomy

missing a homologous chromosome pair

what is monosomy

missing one chromosome

what is trisomy

one extra chromosome

what is tetrasomy

two extra chromosomes

why are aneuploid cells often harmful

they create major gene dosage imbalances

what happens to many aneuploid embryos

they fail to implant or spontaneously abort

what is nondisjunction

failure of chromosomes to separate properly

what process commonly causes aneuploidy

nondisjunction during meiosis

when does mammalian female meiosis pause

near the end of prophase 1

when does mammalian male meiosis begin

at puberty

what is haplodiploidy

a system where one sex is haploid and the other is diploid

what are polytene chromosomes

giant chromosomes formed by repeated dna replication without cell division