Chromosomal abnormalities Flashcards
DNA
A DNA moleculeis composed of two polynucleotide chains ( DNA strands) held together by hydrogen bonds between the paired bases. The arrows on the DNA strands indicate thepolarities of the two strands, which run antiparallel to each other in the DNA molecule. -> a double helix
What are the 3 major components (building blocks) of a nucleotide?
nitrogen-containing base and a five-carbon sugar, to which is attached one or more phosphate groups
Name the 4 nucleotides that are in DNA
adenine (A), cytosine (C), guanine (G), or thymine (T)
What is it about a DNA strand that gives it polarity, i.e. a 3’ and 5’ end?
linkage at 3-OH and 5-Phosphate to the sugar
Base pair
Each purine–pyrimidine pair is called a base pair
(A)-(T)
(G)-(C)
DNA Functions
- Encoding genetic information/instructions
- storage of information
Genome
the total genetic information carried by all the chromosomes in a cell or organism constitutes its genome.
Chromosome
- In eukaryotic cells, very long double-stranded DNA molecules are packaged into chromosomes.
- Chromosomes comprise of both DNA and protein. DNA is packaged by the protein component and is allowed to fit and organized within the cell.
- Some of the proteins play a role in replication , repair and transcription.
Chromatin
- The complex of DNA and protein is called chromatin
Karyotype
An ordered display of the full set of 46 human chromosomes is called the human karyotype
Gene
- segment of DNA that contains the instructions for making a particular protein or RNA molecule.
- functional units of DNA
- Majority encode for proteins
Diploid
each cell contains two sets of chromosomes—one inherited from each parent (germ-line cell)
Haploid
they each contain only one set of chromosomes ( gametes)
Meiosis generates haploid
Sister chromatids
The twin copies of each duplicated chromosome
Homologous chromosomes/homologs
the marternal and parternal chromosomes of a pair are called homologs.
all of which are ‘identical’
Recombination/crossing-over
a process in which two identical or very similar nucleotide sequences exchange genetic information
during meiosis
2 mechanisms that are generated by meiosis
- Independent assortment
- Crossing over
Change in the amount of genetic material in a cell leads to
Deletions and duplications
Rearrangements of genetic material within or between chromosomes
inversions and translocations
Heterochromatin
condensed region, shows up as dark bands (more difficult to transcribe)
Euchromatin
Light bands; less condensed region (easier to transcribe)
Nonhomologous chromosomes
paired sex chromosomes, X = maternal and Y = paternal, they are not ‘identical’
Independent assortment
After replication in meiosis I, allocation of maternal or paternal homologs to each daughter cell is random = reshuffling of maternal and paternal chromosomes into different combinations
Crossover (recombination)
Crossing-over occurs between the maternal and paternal chromatids – they swap homologous sections between the chromatids. The locations at which crossingover occurs are held together by chiasmata.
Crossover (recombination) between non‐sister chromatids in the bivalent in meiosis I allows for genetic variation of gametes.
Euploidy
term used to describe a cell which contains the same number of chromosomes within each set of chromosomes and that is normal for that organism (diploid organisms contain 2 chromosomes per set).
Aneuploidy
is when one or more of the sets of chromosomes within a cell contains a different number of chromosomes compared to the other sets. Aneuploidy can occur during meiosis I, meiosis II (and during mitosis).
- when 1 or more sister chromatids fail to segregate properly ⇒ nondisjunction
- or due to chromosome loss because sister chromatid not properly attaching to the mitotic spindle, i.e. to a kinetochore microtubule, and because it is unattached it is rapidly degraded
Polyploidy
is when all of the sets of chromosomes have additional chromosomes to what is considered ‘normal’ for a particular organism, for example a human cell, which should be diploid, that contains 4 of each chromosome per set. Polyploidy results when cytokinesis does not occur.
5 types of chromosomal abnormalities
deletion, duplication, inversion, simple translocation, reciprocal translocation ⇒ these all occur because of ds breaks in DNA
- Deletions and duplications result in a change in the amount of genetic material in a cell,
- inversions and translocations result in rearrangements of genetic material within or between chromosomes
Can daughter somatic cells be aneuploidy?
Yes. Because Nondisjunction and loss of chromosomes can also occur during mitosis
Chromosomal abnormalities : variation in chromosome structure
Causes: Recombination errors and DNA damaging agents, e.g. x-rays, that cause double strand breaks in DNA
Complementary base pairing
- G forms 3 H-bonds with C
- A forms 2 H-bonds with T/ (Or U in RNA)
Human somatic cells
- contain 2 copies of each chromosome (they are diploid); 1 maternal and 1 paternal homolog.
- They are generated via mitosis.
Human germ cells (gametes; sperm and egg cells)
have only 1 copy of each chromosome (they are haploid), they are generated via meiosis.
telomeres
The DNA located at the end of chromosomes
centromere
The DNA sequence that links sister chromatids and ensures each is apportioned correctly to daughter cells
nucleosome
DNA that is packaged around histones
first step in meiosis
- chromosomes are replicated to generate sister chromatids
- there are 2 maternal chromatids and 2 paternal chromatids for each chromosome that the organism has.
- During meiosis I the replicated maternal and paternal chromosomes line up adjacent to each other on the spindle and form bivalents
5’‐GCATCA‐3’
complement sequence is; 3’‐CGTAGT‐5’
Reverse complement sequence is; 5’‐TGATGC‐3’
