Chromosome Variation (Chapter 8)

Positions on Eukaryotic Chromosomes

(My Sister Ate Tums) -Metacentric: chromosome in which the two chromosome arms are approximately the same length. -Submetacentric: Chromosome in which the centromere is displaced toward one end, producing a short arm and a long arm. -Acrocentric: Chromosome in which the centromere is near one end, producing a long at one end, and a knob, or satellite, at the other side. -Telocentric: Chromosome in which the centromere is at or very near one end.

Chromsome Translocation

*-Exchange of genetic information between NON-homologous chromosomes* -Nonreciprocal Translocation: AB-CD*EF*G & MN-OPQRS -> AB-CDG & MN-OP*EF*QRS -Reciprocal Translocation: AB-CD*EF*G & MN-OP*QR*S -> AB-CD*QR*G & MN-OP*EF*S

Effects of Chromosomes Translocation

*-Position Effect*= changes to gene expression -Translocations within genes can *alter function* *-Often associated with deletions (i.e. Robertsonian translocation) = CAN CREATE A METACENTRIC CHROMOSOME* *REFER TO PPT AND SLIDE 18

Effects of Chromosome Inversions

-A gene may be *disrupted by being split in half* -*Position Effect*=Altered gene expression by displacing regulatory elements. -*Formation of an inversion loop in meiosis (prophase 1)* -Limited crossing over due to nonviable gametes

Variations in Chromosome Number

-Aneuploidy -Polyploidy

Telomere

-Caps at the ends of a chromosome that stabilize the DNA -Free single stranded G-rich 3' end can fold over and form a T-loop -A complex called *shelterin* binds to the telomeric ends, preventing them from being "repaired" by the double-strand break repair system

Chromosome Mutations

-Chromsome Rearrangement (Duplication) 2n=6 -Aneuploidy (Trisomy) 2n+1=7 -Polyploidy (Autotriploid) 3n=9

Effects of Chromosome Deletions

-Deletions within the centromere will usually cause the chromosome to be lost during meiosis/mitosis. *-Homozygous deletions are often lethal* -For heterozygous deletions: 1. Gene dosage imbalance 2. *Pseudodominance*= Expression of recessive phenotype for +/- genes in which the WT copy is deleted. 3. Some genes require 2 functioning copies for the WT phenotype. When a single copy is not sufficient, it is called a *haploinsufficient gene.*

Chromsome Rearrangements

-Duplication -Inversion -Deletion -Translocation

Chromosome Duplications

-Have *profound effects on phenotypes* and can arise from unequal crossing over. -Wild Type Chromosomes *-Chromosomes do not align properly resulting in unequal crossing over.* -One chromosome has a Bar duplication and the other a deletion. Bar Chromosomes *-Unequal crossing over between chromosomes* containing two copies of Bar produces a chromosomes with three Bar copies (double-Bar mutation) and a wild type chromosome.

Features of Eukaryotic Chromosomes

-Humans have 23 pairs of chromosomes -At times, a chromosomes consist of a single chromatid, other times it consists of two (sister) chromatids. -The telomeres are the stable ends of chromosomes. -The centromere is a constricted region of the chromosomes where the *kinetochores form and the spindle microtubules attach.*

Types of Aneuploidy (DIPLOIDS)

-Nullisomy -Monosomy -Trisomy -Tetrasomy -Double Monosomic

Chromosome Inversions

-Occurs when a segment of a chromosome is flipped 180 degrees -A *paracentric inversion occurs outside the centromere* -A pericentric inversion contains the centromere

Types of Polyploidy

-Triploids (3n) -Tetraploids (4n) -Pentaploids (5n), etc. -Very common in plants -Autopolyploidy- all sets are from the same species -Allopolyploidy- sets are from 2 or more species

Triploids

3n

Tetraploids

4n

A diploid species has a total of 52 chromosomes. How many chromosomes would be found in a monosomy, trisomy, and autotriploid?

51, 53, 78

Pentaploids

5n

Polyploidy

A change in the *number of chromosome "sets"*

Aneuploidy

A change in the *number of individual chromosomes* (Aneuploidy can be caused by loss of the centromere or failure of homologous chromosomes to separate in meiosis *(nondisjunction)*

G Banding

A/T Rich

Autopolyploidy

All sets are from the same species

The expression of some genes is dependent upon their chromosomal position. Which kind of chromosome rearrangement would most likely cause a position-effect phenotype? a. deletion b. tandem duplication c. inversion d. all of the above

C. Inversion

Breeding and Polyploidy

Cell volume/nuclear volume/genome size relationship Thus, *polyploids are physically larger than diploids* Good for crops!

C Banding

Centromeric Heterochromatin

Paracentric Inversions

Chromosome inversion that does not include the centromere in the inverted region. (effect of chromosome inversion)

Pericentric Inversions

Chromosome inversion that includes the centromere in the inverted region. (effect of chromosome inversion)

Autopolyploidy in *Mitosis*

Diploid (2n) early embryonic cell-> replication->separation of chromatids-> nondisjunction (no cell division)-> autotetraploid (4n)

Double Monosomic/ Double Trisomic

Double Monosomic (2n-1-1) Double Trisomic (2n+1+1)

How do these breaks occur?

Double strand breaks with *faulty repair of errors in crossing over in meiosis.*

Unbalanced Gene Dosage

Duplications and other chromosomes mutations produce extra copies of some, but not all, genes which alters the relative amounts (doses) of interacting products. If the amount of one product increases but amount of other products remain the same, development problems often result. *REFER TO PPT.

R Banding

G/C Rich

Allopolyploidy Path

Generation of a rare amphidiploid that can from function gametes. *REFER TO PPT

Nondisjunction in Meiosis II (Aneuploidy)

Nondisjunction in *meiosis II results in trisomic and monosomic.* Nondisjunction in *meiosis I results in normal diploid (2n).*

Nondisjunction in Meiosis I (Aneuploidy)

Nondisjunction in *meiosis II results in trisomic.* Nondisjunction in *meiosis I results in monosomic.*

Autopolyploidy in *Meiosis*

Nondisjunction in meiosis 1 produces a 2n gamete, which then fuses with a 1n gamete to produce a *autotriploid.*

Nondisjunction in Mitosis (Aneuploidy)

Nondisjunction results in some clone of *monosomic* cells and somatic clone of *trisomic* cells.

Types of Duplications

Normal Chromosome: AB-CDEFG -Tandem Duplication: AB-CDEF*EF*G -Displaced Duplication: AB-CDEFG*EF* -Reverse Duplication: AB-CDEF*FE*G Chromosome duplications can be easily identified as looped out regions due to misfiring of chromosomes in meiosis.

Q Banding

Relative A/T vs. G/C

Translocation

Segment of a chromosome moves from one chromosome to a non homologous chromosome -OR- to another place on the same chromosome.

Deletion

Segment of the chromosome is deleted.

Duplication

Segment of the chromosome is duplicated.

Inversion

Segment of the chromosome is turned 180 degrees.

*All*opolyploidy

Sets are from 2 or more species

Tetrasomy

The gain of a homologous pair of chromosomes *(2n+2).* In humans = 48 chromosomes (4 copies of one chromosome)

Trisomy

The gain of a single chromosome *(2n+1).* In humans = 47 chromosomes (a triplet of one chromosome)

Chromosome Deletions

The heterozygote has one normal chromosome and one chromosome with a deletion. In *prophase I, the normal chromosome must loop out* in order for the homologous sequences of the chromosomes to align. Deletions can cause looping out and gene dosage issues.

Monosomy

The loss of a single chromosome *(2n-1).* In humans = 45 chromosomes

Nu*ll*isomy

The loss of both chromosomes of a homologous pair *(2n-2).* In humans n=23, so = 44 chromosomes

Triploids Are Usually Sterile

Unbalanced Gametes (good in fruits-no seeds!) *REFER TO THE PPT*

One species has 32 chromosomes and another species has 24 chromosomes. In an allotriploid of these two species, how many chromosomes would be there? a. 44 or 40 b. 56 c. 84 d. 28 e. 48 or 36

a. 44 or 40

Nondisjunction

the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei.