Chapter 8 Part 1 Flashcards
Metacentric
The centromere is located exactly in the middle
Submetacentric
The centromere was displaced towards one location
p-arm
The shorter arm of the chromosome
q-arm
The longer arm of the chromosome
Acrocentric
The centromere is near one end, producing a long arm and a knob, or satellite, at the other end
Telocentric
The centromere is at or very near the end of the chromosome
Karyotype
The complete set of chromosomes passed by an organism
– usually presented in metaphase
Colchicine ** extra not too important
Prevents chromosomes to go to anaphase to see karyotypes
Extra Stuff – Click on to see what it is!
Giesma – reveals G bands which distinguish areas of DNA that are rich in adenine-thymine base pairs
Q-bands – reveal by staining chromosomes with quinacrine mustard
C-bands – which are regions of DA occupied by centromeric heterochromatic
R-bands –which are rich in cytosine-guanine base pairs
What are the different types of chromosome mutations?
- Chromosome Rearrangement
- Aneuploidy
- Polyploidy
Chromosome rearrangement
Mutations that change the structure of the chromosome
There are four basic types of rearrangements:
1. Duplication
2. Deletions
3. Inversions
4. Translocations
Occur…when double stranded breaks occur in the DNA molecule found within a chromosome
Aneuploidy
The number of chromosomes is altered: one or more individual chromosomes are added or deleted
For example…
- Trisomy 21 which results from adding have three 21 chromosomes
- People with this issue get Down syndrome
Polyploidy
One or more complete sets of chromosomes are added
A polyploidy is any organism that has more than two sets of chromosomes
– 3n,4n,5n, and more
How does double-strand repair and mutation occur?
Double-stranded breaks in DNA often cause cell death
If the two broken ends are rejoined correctly, the original chromosome is restored
Sometimes the wrong ends are connected…this causes mutations
Chromosome rearrangements cal also rise through errors in crossing over or when crossing over occurs between repeated DNA sequences
Chromosome Duplication
Is a mutation in which part of the chromosome has been doubled….
Consider a chromosome with segments AB*CDEFG
* = centromere
A duplication of the allele E and F can result in
AB*CDEFEFG –> So notice that the chromosome is larger in this case
Tandem Duplication
This type of duplication, in which the duplicated segments is immediately adjacent to the original segment
Ex. EFEF
Displaced Duplication
If the duplicated segment of alleles of one chromosomes are located some distance away from the original location on the same chromosome
Ex. ABEF*CDEFGH
Reverse Duplication
A duplication where the orientation of the alleles has been inverted
AB*CDEFFEG
EF switched to FE
Unequal Crossing Over
Chromosomes misalign during crossing over and results in duplication and deletions
Ex. Human colour Blindness
Abnormal gene dosing
The amount of a particular protein synthesized by a cell is often directly related to the number of copies of that gene
Pseudo-dominance
Normally recessive mutations on the homologous chromosomes lacking the deletion maybe expressed when the wild-type allele has been deleted and no longer masks the recessive allele
Haplo-insuffiecient
Some genes must be present in two copies for normal function, and this situation occurs when we are missing the other gene
Paracentric Inversion
Inversions that occur before or after the centromere
AB*DCEFG
Pericentric Inversion
Inversion that occur between the centromere
ADB*CEFG
Dicentric Chromatid
Chromatid that has two centromeres; produced when crossing over takes place within a paracentric inversion
Acentric Chromatid
Chromatid that lacks a centromere; produced when crossing over takes place within a paracentric inversion
Dicentric Bridge
Structure produced when the two centromeres of a dicentric chromatid are pulled toward opposite poles, stretching the dicentric chromosomes across the centre of the nucleus…
Eventually, the dicentric bridge breaks as the two centromeres are pulled farther apart
How do individuals heterozygous for inversions cross over during prophase I of meiosis? – paracentric
- The heterozygous chromosomes – homologous pairs or homologs – posses one wild type – the correct sequence – and one chromosome with a paracentric inversion
- In prophase I, an inversion loop forms when a single crossing over occurs within the inverted region
- This results in an unusual structure…where two chromosomes form with four chromatids and two centromeres…and one chromosome that lacks a centromere –> formation of dicentric chromatids
- In anaphase I, the centromeres separate stretching the dicentric chromatid, which breaks…the chromosome lacking a centromere is lost into the cytoplasm
- The two gametes contain non-recombinant chromosomes one wild type – normal and one with inversion…the other two contain recombinant chromosomes that are missing some genes; these gamester will not produce viable offspring
How do individuals heterozygous for inversions cross over during prophase I of meiosis? – pericentric
- The heterozygote possesses one wild-type chromosome and one chromosome with a pericentric inversion
- In prophase I, an inversion loop forms…if crossing over takes place within the inverted region…two of the resulting chromatids have too many copies of some genes and no copies of others
- Unlike in paracentric, we still have two chromosomes each with two chromatids and one centromere
- The chromosomes separate in Anaphase I, and than later in Anaphase II
- Four gametes are produced:
- Normal non recombinant gamete
- Nonviable recombinant gametes X 2
- Nonrecombinanat gamete with pericentric inversion
Nonreciprocal translocation
Movement of a chromosomes segment to a non homologous chromosome or chromosomal region without any – or with unequal – reciprocal exchange of segments
ABCDEFG and MNOPQRS
ABCDG and MNOPEFQRS
Reciprocal Transaction
Reciprocal exchange of segments between two non homologous chromosomes
ABCDEFG and MNOPQRS
ABCDQRS and MNOPEFG
What are the effects of a phenotype in translocation?
- They can physically link genes that were formerly located on different chromosomes…these new linkage relations may affect gene expression – a position effect – as genes translocated to new position effect, as genes translocated to new locations may come under the control of different sequences or other genes that affect their expression
- Second, chromosome breaks that bring about translocations may take place within a gene and disrupt its function –
Ex. Neurofibromatosis, a genetic disease characterized by numerous fibrous tumours of the skin and nervous tissue, results from an autosomal dominant mutation.
Robertsonian Translocation
The long arms of two afrocentric chromosomes become joined to a common centromere, generating a metacentric chromosome with two long arms and another chromosome with two very short arms
– some causes of Down syndrome disorder
– the short arm is a fragment that often fails to segregate and is lost
Which type of segregation in reciprocal or non-reciprocal translocation produce viable gametes?
Alternate Segregation
N1,N2 || T1,T2.
Which types of segregation in reciprocal or non-reciprocal translocation produce inviable gametes?
Adjacent-1 Segregation and Adjacent-2 Segregation
missing genes or too many genes occur on the gene
Adjacent-2 gametes are rare because homologous chromosomes with adjacent centromere do not segregate
Adjacent-1 gametes are often found too…but they produce 50% viable and 50% non-viable offspring
Alternate Segregation
Type of segregation that takes place in a heterozygote for a translocation:
If the original, non-translocated chromosomes are N1 and N2 and the chromosomes containing the translocated segments are T1 and T2, then alternate segregation takes place when N1 and N2 move toward one pole and T1 and T2 move towards the opposite pole
Adjacent-1
One translocated and one non-translocated chromosomes separate…
***Homologous chromosomes segregate in Meiosis I
Adjacent-2
One translocated and one non-translocated chromosomes separate…
***Homologous chromosomes DO NOT segregate in Meiosis II
How does reciprocal transformation occur in meiosis?
- An individual heterozygous for the reciprocal translocation possesses one normal copy of each chromosome and one translocated copy
- Because each chromosome has sections that are homologous to two other chromosomes, a cross like configuration forms in prophase I of meiosis
- In anaphase I, the chromosomes separate in three different ways
- Alternate Segregation
- Adjacent-1 and Adjacent-2 Segregation
Fragile Sites
Sites that develop constrictions or gaps when the cells are grown in culture and that are prone to breakage under entrain conditions
Ex. Common health condition as a result is fragile-X syndrome
