nucleic acids Flashcards
(42 cards)
what are 3 experiments that gave evidence that DNA is genetic material?
Griffith experiment (mice and smooth/rough strains of streptocuccus pneumoniae)
Avery (out of purified DNA, RNA, protein, lipid and carbohydrate, only DNA from the heat killed strain could induce viruence on non virulent strain)
Herschey and chase (radioactively labelled phages to show DNA carries genetic information)
describe the structure of a DNA molecule
anti parallel strands form a double helix
sugar phosphate backbone
phosphodiester bonds run in 5’ to 3’ direction to give directinality
base pairs join complementary strands together by hrydrogen bonding
2 H bonds between AT nd 3 H bonds between CG
packaging is tightly coiled with help of histones to fit into the nucleus
DNA+histones = chromatin (interphase)
active genes are more loosely coiled than silent genes
what experiment was used to show that DNA replication is semi-conservative?
Meseleson-stahl experiement - growing e-coli in a medium containing heavy isotope nitrogen 15 and after many generations switching to light nitrogen 14, then measuring the density of DNA by density gradient centrifugation, separating DNA into strands
how does DNA replicate?
DNA helicase unzips strands by breaking H bonds between base pairs
leading strand:
RNA primer binds to 3’ end (made by DNA primase)
DNA polymerase binds to strand at primer and begins adding new base pairs in 5’ to 3’ direction
lagging strand:
binding multiple primer, each primer is only several basses apart. DNA polymerase adds DNA = okasaki fragmnets between the primers = fragments
exonuclease removes RNA primers and replaced by appropriate bases by DNA polymerase. DNA ligase joins the okasaki fragments together
what reduces error frequency in replication?
DNA polymerases editing function that removes incorrectly insterted bases
other enzymes check
methylated C in DNA can mutate to T?
5-methylcytosine (5Mc) is caused by post-synthetic modification of cytosine residues mainly in CpG doubles. it is a mutable site and can undergo spontaneous deamination to thymine
What are DNA repair proteins?
they repair damage in DNA before replication
Base excision repair proteins cut out damaged bases (specific to certain types of damage)
Nucleotide excision repair proteins are less specific and cut out sections of damaged DNA strand
in both cases DNA polymerase replaces DNA removed by copying the intact strand and DNA ligase seals the gap
how many nucleotides are there compared to genes
3x10^9 nucelotides
19,000-20,000 genes
the human genome is much bigger than might be expected to accomodate genes of known function why?
much DNA is noncoding introns
what is synteny?
long DNA sequences are present in the same order across sepcies
single nucleotide polymorphisms can be associated with disease risk
Can result in amino acid substitution or introduce a stop codon (nonsense mutation0 resulting in a truncated proteins
what types of single nucleotide mutations are there?
point mutation
insertion
deletion
what are VNTR?
variable number tandem repeats, multiple copies of short sequences, unique length and number of repeats in individuals
what are VNTRs used for and how?
paternaty and forensic testing. because VNTRs are unique they can be amplified by PCR and compared by electrophoresis
3 differences between DNA and RNA?
RNA has ribose whereas DNA has deoxyribose
RNA contains uracil whereas DNA has thymine instead
RNA is single stranded, DNA is double stranded (if doubles stranded RNA is detected alarm response is set off as may suggest a viral infection)
describe DNA transcription
RNA polymerase binds to a sequence of DNA called the promoter, found near the beginning of a gene. Each gene has its own promoter. Once bound, RNA polymerase separates the DNA strands, providing the single stranded template needed for transcription
The template strand acts a template for RNA polymerase, it reads one base at a time 5’ to 3’, building up a RNA molecule by CBP, this carries the same information as the coding strand but has Uracil instead of Thymine
Terminator sequences – stop codon signal RNA transcript completetion. Causing the release from RNA polymerase.
what is the TATA box and what does it do?
Different promoter elements in eukaryotes including the TATA box. Short run of T and A bases (lowest energy base-pairs so easiest to unwind) that vary slightly from gene to gene and is probably the best characterised promoter element. Transcription factors bind to promoter to initiate transcription by binding the RNA polymerase to the region of the gene where transcription begins. Some promoter elements are several kB away from transcription site but TAT box is very close
what is and what does the CpG islands do?
Stretches of DNA where there are multiple points at which C is followed by G, occurring upstream of many genes, they are unmethylated regions of the genome associated with 5’ end of many genes and appear to have promoter activity of housekeeping genes (essential for general cell function)
how is eukaryotic mRNA modified by capping?
5’ end contains free triphosphate group since it was the first incorporated nucleotide in the chain the capping process replaces the triphosphate group with another structure called the cap, added by guanyl transferase. This enzyme catalyses the reaction between the 5’ end of the RNA transcript and guanine triphosphate (GTP) molecule
how is eukaryotic mRNA modified by polyadenylation?
Transcription continues past the point where a polyadenylation sequence is present. The mRNA is cut near the polyA sequence and a polyA tail is added to 3’ end
what does polycistronic mean and what kind of mRNA can be polycistronic?
when mRNA can code for multiple proteins (multiple genes per transcript), in prokaryotic mRNA
what are introns
non coding regions of mRNA
how are introns removed?
by splicing at specific sites at the start and end of each intron which are recognised by the spliceosome
what can alternative splicing do?
can generate related proteins from a single gene.
can generate subtly different mRNAs in related tissues
