Chapter 6: Chromosome Variation

eukaryotic chromosomes

have a centromere, where spindles attach to separate chromosomes, and telomeres at the ends to stabilize the chromosomes

metacentric

submetacentric

acrocentric

telocentric

classifications of where the centromere is located on the chromosome

metacentric centromere

centromere is approximately in the middle creating two arms of equal lengths

submetacentric centromeres

centromere is off-center creating a long arm and a short arm

acrocentric centromere

centromere is near one end, producing a long arm and a knob at the other end

telocentric centromere

centromere is at or very near the end of the chromosome

karyotypes

complete set of chromosomes in an organism

karyotypes

prepared from pictures of metaphase chromosomes

1. actively dividing cells are treated with chemical to prevent them from entering anaphase

2. photograph is taken of these cells

3. photograph is enlarged, chromosomes cut out and arranged in pairs from largest to smallest

how to prepare a karyotype

chromosomes

paired based on size and staining pattern

regions of the chromosome

different stains can be used that stain different ?

Giemsa stain

Quinacrine mustard stain

other techniques

types of stains

giemsa stain

stains AT rich areas (G-bands)

Quinacrine mustard stain

stains CG rich areas (Q-bands) which are visible under UV light

other techniques of staining

reveal other bands in chromosomes (C bands or R bands)

G bands

AT rich areas in chromosomes

Q bands

CG rich areas in chromosome

chromosome rearrangement

alter the structure of chromosomes

aneuploidy

changes the number of chromosomes

polyploidy

changes the number of sets of chromosomes

chromosome rearrangement

often occurs after double stranded DNA breaks

death of cells

double stranded DNA breaks that occur after chromosome rearrangement can result in the ?, so cells have evolved mechanisms to reconnect the broken ends

wrong ends

when broken ends are reconnected during chromosome rearrangement, sometimes the correct ends are connected, but sometimes the ? are connected

chromosome rearrangement

can also occur as a result of errors during crossing over

duplications

deletions

inversions

translocations

chromosome rearrangement can result in ?

chromosome duplications

mutations where part of the chromosome has been duplicated

can be tandem or displaced

tandem

displaced

types of chromosome duplications

tandem chromosome duplications

the duplicated region is adjacent on the chromosome

displaced chromosome duplication

the duplicated region is some distance away from original, either on the same chromosome or on a different chromosome

homozygous for duplication

the duplication occurs on both homologous chromosomes and can pair up during meiosis

heterozygous for duplication

duplication is only on one of the homologous chromosomes

causes difficulty in prophase I of meiosis when homologous chromosomes pair up

in order for the heterozygous chromosomes to pair, the duplicated region must be looped out

looped out

in order for the heterozygous chromosomes to pair, the duplicated region must be ?

duplications

do not usually have as deleterious an affect on phenotype compared to deletions

duplications

their effect usually comes from problems in gene dosage, since with extra copies of genes, there is often more of that gene product present in the cell

balanced

for some genes, having additional amounts of the protein due to chromosome duplication can cause problems if it dose needs to be ? with respect to other proteins

duplications and deletions

can arise from unequal crossing over

duplications working with deletions

ex. red-green color blindness in humans

red opsin gene are very similar in sequence to green opsin gene

during pairing of meiosis, they can pair up incorrectly and crossover event results in one chromosome having a duplication and other has deletions

duplication of genes

duplication of genes

has evolutionary significance —> these genes are able to accumulate mutations without deleterious effect and can lead to the duplicated gene evolving a new function

deletions

part of chromosomal segment is lost

deletions

during prophase I of meiosis, heterozygotes must have one chromosome loop out to allow pairing of homologous chromosomes for this chromosomal rearrangement

phenotypic effect of deletions

depends on which genes are missing

deletions

usually remove multiple genes

homozygous lethal

if any of the deleted genes are essential, the deletion will be ?

having only one copy of a gene

any recessive mutations on whole homologous chromosome

heterozygotes for a deletion often show a phenotype due to:

having only one copy of a gene from heterozygous deletions

causes an imbalance in the amount of gene product which can sometimes cause a phenotype

can cause a phenotype if one copy cannot produce sufficient amounts of the protein product to carry out its function (haploinsufficient)

recessive mutations on whole homologous chromosomes due to heterozygous deletions

will now show their phenotype, since the deletion chromosome does not have the wild type gene to mask the recessive phenotype

inversions

a segment of chromosome is flipped 180

inversions

can include the centromere or not include the centromere

pericentric and paracentric

types of inversions

pericentric inversions

includes the centromere

paracentric inversions

does not include the centromere

inversions

there is no loss or gain of genes, just a change in order

break on one side of inversion may be a gene

movement of a gene may bring it under different regulatory control

how inversions cause phenotypes

break on one side of the inversion is a gene

results in the loss of function when an inversion happens

inversion loop

in meiosis, pairing in a heterozygote, between homologous chromosomes requires one chromosome to form an ?

heterozygotes for an inversion

have fewer progeny where recombination has occurred in the inverted area

why heterozygotes for inversions have fewer progeny

due to a single crossover between an inverted chromosome and its normal homologue will result in some abnormal gametes that do not give rise to progeny —> ex. paracentric inversion

result of crossover with paracentric inversions

produces one sister chromatid has two centromeres and one with no centromere (other two are normal) —> when homologous chromosomes separate in anaphase I, the dicentric chromosome stretched until it breaks —> chromosome without a centromere is lost

this produces two gametes: one with non-recombinant chromosomes (wild type and one with inversion) and the other with chromosomes that are missing genes and will not produce viable offspring

pericentric inversion

a single crossover with a normal homologue will give rise to gametes that form inviable progeny, because the resultant chromosomes have extra copies of some genes and are missing others

translocation

movement of part of a chromosome to another, non-homologous chromosome or to another place on same chromosome

nonreciprocal

reciprocal

types of translocation

nonreciprocal translocation

piece of chromosome moved to other chromosome without any reciprocal exchange

reciprocal translocation

two chromosomes exchange segments

Robertsonian translocation

long arms of two acentric chromosomes are exchanged —> creates one chromosome with both long arms and the other with both short arms

deletions

often accompany translocations

heterozygotes for reciprocal translocation

chromosomes must still pair up with homologous regions so they form a cross shape

fragile sites

where some chromosomes of cells in culture develop constrictions at certain locations

fragile sites

places where the chromosome can break under certain conditions

fragile site on X chromosome

has a gene with repeats of the trinucleotide CGG

increase in number of repeats on fragile site on X chromosome

leads to the silencing of this gene and Fragile X syndrome

fragile X syndrome

causes mild to moderate intellectual disability

long and narrow faces, large ears, and often problems with social interactions

aneuploidy

a change in the number of individual chromosomes

nullisomy

monosomy

trisomy

tetrasomy

four types of aneuploidy

nullisomy

type of aneuploidy no copies of a single chromosome (2n-2)

monosomy

type of aneuploidy that results in the loss of a single chromosome (2n-1)

trisomy

type of aneuploidy that results in the three copies of a single chromosome (2n+1)

tetrasomy

type of aneuploidy that results in four copies of a single chromosome (2n+2)

aneuploidy

an individual can have more than one ? mutation (ex. extra copy of two different chromosomes)

aneuploidy

can arise from a nondisjunction event in meiosis I (all chromosomes in one cell, none in the other)

aneuploidy

can arise from a nondisjunction event in meiosis II (2 normal gametes, 1 gametes with 3 chromosomes, and other with one chromosome)

aneuploidy

can arise in somatic cells by nondisjunction in mitosis

aneuploidy

affects the dosage of genes on the affected chromosome

aneuploidy

results are usually lethal, however, a duplicated chromosome is more easily tolerated than missing a chromosome

aneuploidy of sex chromosomes

has only mild effects probably due to the fact that gene dosage is already regulated on the X chromosome

additional X chromosomes

inactivated chromosomes

single X chromosome

type of aneuploidy that is fine

additional copies of Y chromosome

since this chromosome has a few genes, extra copies are well tolerated

autosomal aneuploidy

lethal in early development (few trisomies due survive)

down syndrome

trisomy 21 (extra chromosome 21)

since this chromosome is one of the smallest, it has les effect than other chromosomes

trisomy 18

trisomy 13

trisomy 8

types of autosomal aneuploidies that may survive to birth

occurrence of aneuploidies

rise with maternal age

polyploidy

presence of more than two sets of chromosomes

while a few types of animals exhibit polyploidy, it is very common in plants

autopolyploidy and allopolyploidy

types of polyploidy

autopolyploidy

extra sets of organisms own chromosomes, usually as a result of error in mitosis or meiosis

allopolyploidy

sets of chromosomes from two different species, usually as a hybridization event

autopolyploidy

in meiosis, chromosomes try to pair but fail to segregate correctly, thus most of these plants are sterile

even with even number of sets, homologous chromosomes rarely divide equally

allopolyploids

hybridization between two species results in polyploids with chromosomes from both species

hybridization

can only arise from two species that are related enough to cross-breed

allopolyploids

hybridized plant cannot produce viable gametes since the chromosomes segregate entirely at random

if in one plant, there is a NDJ, there will now be two of each chromosome —> they can pair and segregate correctly during meiosis giving rise to viable gametes

increase in cell size, taken advantage by plant breeders

benefit of allopolyploidy