How cellular information is altered Flashcards
What is recombinant DNA/ gene splicing?
The ability to isolate a given piece of DNA out of millions of nucleotides and to be able to generate new DNA molecules in the test tube and to introduce the custom made material back into the living organisms
What is an error in DNA replication called?
A mutation
What is an example of inherited disease illustrates the importance of protecting reproductive cells (germ cells) against mutation
A permanent change in a single nucleotide in the sequence coding for one of the two types of protein chains in haemoglobin (b-globin chain) causes the disease sickle-cell anaemia. The sickle-cell haemoglobin is less soluble than normal hemoglobin and forms fibrous precipitates which lead to the characteristic sickle-cell shape of affected red blood cells.
A mutation in a reproductive cell will be passed on to all the cells in the body of the multicellular organism that develops from it, including the germ cells for production of the next generation.
How many errors are made in DNA replication
1 error per 10^7 nucleotides
How does the cell correct errors made in DNA replication?
A system called DNA mismatch repair
What percentage of errors made in DNA replication does the DNA mismatch repair system correct?
99%
Give another example of how DNA can be damaged
ultraviolet radiation in sunlight is also damaging to DNA. It promotes covalent linkage between two adjacent pyrimidine bases, forming, for example, a thymine dimer. Skin cells that are exposed to sunlight are especially susceptible to this type of DNA damage.
Give the method of repairing mutated DNA
The basic pathway for repair of these types of damages involves three basic steps. Instep 1 (excision), the damage is cut out by one of a series of nucleases, each specialized for a type of DNA damage.
In step2 (resynthesis), the original DNA sequence is restored by a repair DNA Polymerase, which fills in the gap created by the excision events.
In step 3 (ligation), DNA ligase seals the nick left in the sugar-phosphate backbone of the repaired strand.
Nicksealing, which requires energy from ATP hydrolysis, remakes the broken phosphodiester bond between the adjacent nucleotides. For some types of DNA only a single nucleotide needs to be replaced, for the replacement of other types of damage, such as thymine dimers, a longer stretch of 10-20 nucleotides is removed from the damaged strand.
Why do changes in the DNA accumulate very slowly in the course of evolution?
High fidelity of DNA maintenance allows closely related species to have proteins with very similar sequences
How similar are human and chimpanzee DNA sequences?
Thus humans and chimpanzees, after about 5 million years of divergent evolution, still have DNA sequences that are at least 98% identical.
What are DNA recombinations
DNA rearrangements are caused by a class of mechanisms called genetic recombination.
What is the most fundamental genetic recombination mechanism, and how does this work?
Homologous recombination
Two double-stranded DNA that have regions of very similar (homologous) DNA sequences align so that their homologous sequences are in register. The DNA molecules can then “cross over”: in a complex reaction, both strands of each double helix are broken and the broken ends are rejoined to the ends of the opposite DNA molecule to re-form two intact double helices, each made up of parts of the two different DNA molecules. No nucleotide sequences are altered at the site of exchange; the cleavage and rejoining events occur so precisely that not a single nucleotide is lost or gained.
Where is the site of exchange in homologous recombination
The site of exchange (that is, where one double helix is joined to the other double helix), can occur anywhere in the homologous nucleotide sequences of the two participating DNA molecules.
What are the 3 steps of homologous recombination in bacteria?
Conjugation - transfer of genetic material between bacteria through direct cell-to-cell contact
Transduction - DNA is transferred from one bacterium to another by a virus
Transformation - genetic alteration of a cell from the uptake, genomic incorporation, and expression of foreign genetic material (DNA) (other than the methods above).
What does the diversity of species depend on?
Balance between the conservative accuracy of genome replication that enables progeny to inherit the virtues of their parents, and the creative errors of genome replication and maintenance that enable the progeny to acquire novel features and evolve new capabilities.
Explain mutation within a gene
An existing gene can be modified by mutations that change a single nucleotide or that delete or duplicate one or more nucleotides in its DNA sequence. These so called point mutations are usually a result of rare “mistakes” made during DNA replication or to a failure in DNA repair following DNA damage
Explain gene duplication
An existing gene, a larger segment of DNA, or even a whole genome can be duplicated, creating a set of closely related genes within a single cell. As this cell and its progeny divide, these duplicated genes can then undergo additional mutations and assume functions distinct from those of the original genes.
Explain gene deletion
Individual genes, or whole blocks of genes can be lost through chromosome breakage and failures to repair.
Explain exon shuffling
Two or more existing genes can be broken and rejoined to make a hybrid gene containing DNA segments that originally belonged to separate genes. Because the breaking and rejoining often occurs within intron sequences in eucaryotic cells, the process does not have to be precise to result in a functional gene.
Explain horizontal gene transfer
A piece of DNA can be transferred from the genome of
one cell to that of another- even to that of another species. This process differs from the usual vertical transfer of genetic information from parent to progeny
What’s the process that bacteria horizontally transfer DNA called?
Conjugation
