Chapter 8 Flashcards
Chromosome mutation
Variations in chromosome number and or structure do periodically arise
Chromosome morphology
Position of the centromere on the chromosome
Metacentric
centromere in middle of chromosomes
Submetacentric
centromere so placed that it divides the chromosome into two arms of unequal length
Acrocentric
centromere is very close to one end of chromosome
Telocentric
centromere is placed very close to the end of the chromosome
Karyotyping
- Chromosomes prepared from actively dividing cells
- Halted in metaphase preventing from going into anaphase
-Chromosomes arranged according to size
-helps identify abnormalities
Three processes of mutation
- Chromosome rearrangements: structural
- Aneuploidy: Number of chromosomes altered ( one or more individual chromosomes are added or deleted)
- Polyploidy: Number of chromosomes ( seen in plants One or more complete sets of chromosomes are added, 3n,4n…)
Chromosome Rearrangements do..
Alter the structure of the chromosomes.
deleted or duplicated chromosomes
Double-stranded breaks in DNA often cause cell death.
Mechanisms help repair breaks, but they are sometimes incorrect! (Leads to chromosome rearrangement!)
Four basic types of rearrangements:
1.Duplications
2.Deletions
3.Inversions
4.Translocations
Chromosome rearrangements are
chromosome mutations that change the structures of individual chromosomes.
Chromosome rearrangement can also arise through
errors in crossing over or when crossing over occurs between repeated DNA sequences.
Chromosome Duplication
Tandem – duplicated segment immediately adjacent to the original segment (i.e. AB CDEFEFG)
Displaced duplication – duplicated segment is some distance from the original (either on the same or different chromosome) (i.e. AB CDEFGEF)
Reverse duplication – duplicated region is inverted
segmental duplications
duplications greater than a thousand base pairs.
Most segmental duplications are intrachromosomal (two copies found on the same chromosome) but others are interchromosmal (two copies found on different chromosomes).
Effects of Chromosome Duplication
When an individual has a duplication on one chromosomes (heterozygous for duplication) – pairing can arise at prophase I of meiosis.
Chromosome Deletions
loss of a chromosomal segment (i.e. AB CDEFG undergoes deletion EF to become AB CDG)
Large deletions easily detected - chromosome is noticeably shorter!
Effects of Deletions
Phenotypic consequences depend on which genes are located in the deleted region!
If deletion includes centromere – chromosome will not segregate in meiosis or mitosis.
Many deletions are lethal in the homozygous state - essential genes in that region are lost.
Imbalances in gene product may occur.
Expression of a normally recessive gene may occur (pseudodominance).
Some genes require two copies for normal function. When a single copy of a gene is not sufficient to produce a wild-type phenotype, that gene is said to be haploinsufficient.
Inversion
chromosome segment is inverted – turned 180 degrees. (i.e. and inversion in ABCDEFG could be ABCFEDG)
Paracentric inversion –
does not include the centromere (i.e. AB*CFEDG)
Pericentric inversion
– does include the centromere (i.e. AC*BDEFG)
Effects of Inversions
An inversion may break a gene into two parts.
One part may move to a new location, destroying the function of that gene!
Many genes are regulated in a position-dependent manner – if position changes, their expression may be altered – referred to as position effect.
Inversions in meiosis
Individuals homozygous: no problems arise during meiosis
Individuals heterozygous:
Homologous sequences align only if the two chromosomes form an inversion loop
Nonreciprocal translocation
– genetic material moves from one chromosome to another without any reciprocal exchange.
Consider AB CDEFG and MN OPQRS;
if EF moves and creates chromosomes AB CDG and MN OPEFQRS
Reciprocal translocation
– two way exchange of segments between chromosomes
Consider AB CDEFG and MN OPQRS;
If EFG exchanges with QRS, so now - AB CDQRS and MN OPEFG
Translocation
a segment of a chromosome that moves from one chromosome to a nonhomologus
Effects of Translocations
Translocations can physically link genes that were formally located on different chromosomes.
New linkage may affect gene expression
The chromosome break that brings about translocations may take place within a gene and disrupt its function.
Robertsonian translocation
– long arms of two acrocentric chromosomes become joined to a common centromere through a translocation.
Fragile Sites
– Sites that develop constrictions or gaps when the cells are grown in culture and are prone to breakage under certain conditions.
Common sites
– present in all humans and are normal features of chromosomes.
Rare sites
– found in few people and exhibit Mendelian inheritance.
Copy-number variations
– the number of copies of a particular gene varies from person to person
Structural variants
– chromosome rearrangements and copy number variations
Aneuploidy
change in the number of individual chromosomes
Polyploidy
– increase in the number of chromosome set
Causes of aneuploidy:
Deletion of centromere during mitosis and meiosis
Robertsonian translocation
Nondisjunction
Types of Aneuploidy
Nullisomy: loss of both members of a homologous pair of chromosomes; 2n − 2
i.e humans 2n = 46, nullisomic zygotes has 44 chromosomes.
Monosomy: loss of a single chromosome; 2n − 1
Trisomy: gain of a single chromosome; 2n + 1
Tetrasomy: gain of two homologous chromosomes; 2n + 2
Effects of aneuploidy in humans
Turner syndrome; XO
Klinefelter syndrome; XXY
Autosomal aneuploids:
Trisomy 21: Down syndrome
genetic mosaicism
Regions of the tissue with different chromosome constituents
Autopolyploidy
Accidents of mitosis or meiosis that produce extra sets of chromosomes, all derived from single species
Allopolyploidy
Hybridization between two species, the resulting polyploid carries chromosome sets derived from two (or more) species
The significance of polyploidy:
Increase in cell size
Larger plant attributes
Evolution: may give rise to new species
