Nucleic Acids and Genetics A Language of Its Own. DNA Structure and Replication. In the mid-1900s, scientists knew that chromosomes, made up of DNA ( deoxyribonucleic acid ) and proteins , contained genetic information.
Various reseachers showed that DNA was the genetic material when they performed an experiment with a T2 virus.
By using different radioactively labeled components, they demonstrated that only the virus DNA entered a bacterium to take over the cell and produce new viruses.
In the early 1950s, Rosalind Franklin and her associates began to test X-ray beams with DNA. The X-ray scattering produces a pattern that provides important clues to the structure of many molecules.
This X-ray diffraction photograph of DNA was taken by Franklin. The X-shaped pattern in the center indicates that the structure of DNA is helical.
Watson and Crick showed that DNA is a double helix in which A is paired with T and G is paired with C.
This is called complementary base pairing because a purine (A and G) is always paired with a pyrimidine (T and C).
The sides of the ladder are the sugar-phosphate backbones, and the rungs of the ladder are the complementary paired bases.
The two DNA strands are anti-parallel – they run in opposite directions.
New nucleotides move into complementary positions are joined by DNA polymerase.
The process is semiconservative because each new double helix is composed of an old strand of nucleotides from the parent molecule and one newly-formed strand.
Some cancer treatments are aimed at stopping DNA replication in rapidly-dividing cancer cells.
Transcription makes an RNA molecule complementary to a portion of DNA.
Translation occurs when the sequence of bases of mRNA directs the sequence of amino acids in a polypeptide.
A ribosome has a binding site for mRNA as well as binding sites for two tRNA molecules at a time.
As the ribosome moves down the mRNA molecule, new tRNAs arrive, and a polypeptide forms and grows longer.
Translation terminates once the polypeptide is fully formed; the ribosome separates into two subunits and falls off the mRNA.
Several ribosomes may attach and translate the same mRNA, therefore the name polyribosome.
If a codon is changed to a stop codon, the resulting protein may be too short to function; this is a nonsense mutation.
If a point mutation involves the substitution of a different amino acid, the result may be a protein that cannot reach its final shape; this is a missense mutation.
An example is Hbswhich causes sickle-cell disease.
3) may be too short to function; this is a Proto-oncogenes are normal genes that stimulate the cell cycle and tumor-suppressor genes inhibit the cell cycle; mutations can prevent normal regulation of the cell cycle.
4) Telomeres are DNA segments at the ends of chromosomes that normally get shorter and signal an end to cell division; cancer cells have an enzyme that keeps telomeres long.
In eukaryotes, the control of gene expression occurs at all stages, from transcription to the activity of proteins.
Gene mutations vary; some have little effect but some have a dramatic effect.
Loss of genetic control over genes involved in cell growth and/or cell division cause cancer.