Lab 5 - Chromosomes and Their Aberrations Flashcards
How many chromosomes does all cells in the human organism contain?
23 pairs, total of 46 chromosomes. One from the mother and one from the father.
How many chromosomes does each egg cell and each sperm cell contains?
A haploid set of 23 chromosomes (1n)
How many chromosomes does a fertilized egg contain?
A diploid set of 46 chromosomes (2n)
Autosome
An autosome is a chromosome that is not an allosome (a sex chromosome).
How many pairs of autosomes is there in male and female?
22 pairs
Sex chromosomes
Males: XY
Females: XX
Centromere and its divisions
The centromere is the specialized DNA sequence of a chromosome that links a pair of sister chromatids (a dyad). It contains a repetitive-satellite DNA sequences and divides the chromosomes into a short arm p (petite) and long arm q.
Can be divided to:
Metacentric
Submetacentric
Acrocentric chromosomes
Metacentric chromosome
Centromere is positioned in the middle
Submetacentric chromosome
Centromere is positioned between the middle and the end of the chromosome
Acrocentric chromosomes
Centromere is positioned at the end, separating a normal long arm from a short arm that contains only multiple (hundreds) copies of rRNA genes (chromosomal satellites)
Sequence homology
Strictly, refers to the situation where nucleic acid or protein sequences are similar because they have a common evolutionary origin
Enables binding and recombination at meiosis, which is not only essential for the generation of haploid set of chromosomes, but also an important factor in the diversification of the species
Histones
Highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes
Core histones and Linker histone
Core histones: H2A, H2B, H3, H4
Linker histone: H1
Nucleosome
Basic packing unit in a chromosome that consist of 8 core histones and 146 base pairs of DNA
How many core histones is the DNA double strand wound around?
8 core histones, two of each core histones
How many full twists does the DNA complete?
1.75 full twists, which correspond to 146 base pairs
Chromatin
The combination of DNA and (histone and nonhistone) proteins that makes up a chromosome
Euchromatin
Relatively loosed packed chromatin that represents transcriptionally active DNA or may be recognized as light bands during chromosome analysis (G-banding)
Heterochromatin
Highly condensed chromatin that represents transcriptionally inactive (often repetitive) DNA or may be recognized as dark bands during chromosome analysis (G-banding)
Telomeres
A region of repetitive nucleotide sequences at each end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes
Telomeres repeats
TTAGGG
Euploidy
A normal diploid chromosome complement (2N) of 46 chromosomes (46,XX or 46,XY)
Aneuploidy
Changes in the total number of chromosomes
They arise from faulty segregation of the chromosomes in meiosis or mitosis
The risk is largely dependent of maternal age
Not inherited
Somatic mosaic
Somatic mosaicism occurs when the somatic cells of the body are of more than one genotype. In the more common mosaics, different genotypes arise from a single fertilized egg cell, due to mitotic errors at first or later cleavages
Polyploidy
Is the state of a cell or organism having more than two paired (homologous) sets of chromosomes.
Multiples of haploid chromosome complement of greater than 2n
Triploidy cause
Fertilization of an oocyte by two spermatozoa.
Digynic triploidy
Maternal origin. Fertilization of a diploid ovum by a haploid sperm
Growth retardation, large head, the placenta is small and fibrotic
Diandric triploidy
From the father. Fertilization of a haploid ovum by two sperm. Results in a partial mole with large cystic placenta, normal size fetus with microcephaly
Tetraploidy
A quadruple chromosome complement.
92,XXXX or 92,XXYY
Monosomies
One copy of chromosome. Result in a very early spontaneous miscarriage
Example: Monosomy X (Turner syndrome)
Most common cause of chromosome structual abnormalities?
Deletions
Other causes are ionizing radiation, viral infections, various chemicals
Balanced structural chromosome abnormality
No loss or gain in the chromosomes despite recombination, the entire genetic material is normal
Unbalanced structural chromosome abnormality
Gain or loss of chromosome segments and are clinically relevant. Structurally denoted according to the origin of their centromere, called derivative chromosomes
Deletion
When a chromosome breaks at two sites and the segment between them get lost
Contiguous gene syndrome
The presence of several distinct (dominant-inherited) monogenic disorders, caused by deletion affecting several neighboring genes
Duplication
A chromosome segment appears in two (often sequentially inserted) copies on a single homolog. Most often by a nonhomologous recombination in the first meiotic division
Inversion
When a chromosome segment between two breaks is rotated 180 degrees before reinsertion. The gene copy and number remains the same
Pericentric inversions
Include the centromere. One breakpoint is on the short arm and one on the long arm
Paracentric inversions
Do not include the centromere
Isochromosome
Unbalanced structural abnormality in which the arms of the chromosome are mirror images of each other. The chromosome consists of two copies of either the long (q) arm or the short (p) arm because isochromosome formation is equivalent to a simultaneous duplication and deletion of genetic material.
Ring chromosome
When a chromosome breaks at both ends and the ends join together.
Ex: Turner syndrome
Marker chromosome
Small fragment of a chromosome, found by FISH testing or array analysis. They develop as either isochromosomes or inverted duplications.
Ex: Pallister-Killian syndrome (isochromosomes 12p), Cat-eye syndrome (inverse duplication, 22q11)
Translocation
Exchange of chromosome segments between nonhomologous chromosome
Reciprocal translocation
Transfer of chromosomal material from one chromosome to another, and vice versa. The total number of chromosomes remains unchanged.
Robertsonian translocation
Fusion of two acrocentric chromosomes in the centromeres, with the loss of the respective short arms and the reduction of total number of chromosomes to 45
Robertsonian translocations generally occur in the five acrocentric chromosome pairs, namely 13, 14, 15, 21 and 22.
Mosaic
The occurrence of two or more genetically different cell lines within a tissue or within an organism
Germline mosacism
Mutations that occur de novo in a family may have arisen relatively early in germ cell development, resulting in a significant number of gametes that carry the mutation
Mosaicism can cause
Mosaic down syndrome (mosaic trisomy 21)
Neurofibromatosis type 1
Karyotype
Number and appearance of the chromosomes in an individuals cell
Components of a karyotype
Total number of chromosomes and the specific sex chromosomes, followed by any specific numerical abnormality and/or structural abnormality that is present
Karyotype of a male with down syndrome
47,XY,21
