Topic 1: Nucleic Acids Flashcards
Describe the primary structure of DNA
chains of nucleotides held together by nucleotide bonds
each DNA molecule is arranged into one chromosome
Describe the secondary structure of DNA
base pairing between the bases in the nucleotides
the two DNA strands run in opposite directions (anti-parallel)
What is base pairing?
pair between purine and pyrimidine
specific pairs depending on number of hydrogen bonds
Which baes are purines?
Adenine
Guanine
Which bases are pyrimidines?
Cytosine
Thymine
Uracil
Describe DNA replication
1) DNA strands are unwound by DNA helicase
2) Single-stranded binding proteins bind to each strand to prevent reassembling
3) RNA primase makes a short strand of complementary RNA which acts as the primer
4) DNA polymerase III reads parent strand in 3’ to 5’ direction
5) The lagging strand is made by the synthesis of Okazaki Fragments
6) DNA polymerase I then replaces the RNA primer with DNA
7) DNA ligase joins the fragments together
Which protein controls the accuracy of DNA replication?
DNA polymerase III (checks nucleotides and replaces them if they are not correct)
What are the two mechanisms that contribute to DNA repair?
1) DNA glycosylase (small lesions)- checks DNA for mispaired bases, modified bases or excess bases; then fixes them
2) DNA helicase and DNA ligase (larger lesions)- helicase removes section of DNA containing damage, ligase attaches a new section which had been made by polymerase
What is the approximate size of the human genome?
30,000 genes
3 trillion base pairs
Why is the genetic code similar for all sepcies?
the genetic code is degenerate- the same codons code for the same amino acids in all organisms. (some amino acids may be represented by multiple codons, some only by one)
What are VNTRs?
Variable Number Tandem Repeats
Have repeated sequences that are tens of base pairs long (minisatellites)
Repeated sequences that are 2-6 base pairs (microsatellites)
What can occasional polymerase slippage of VNTRs cause?
- addition/elimination of a sequence
- disease e.g. Huntington’s
What can be associated with disease risk?
Single Nucleotide Polymorphisms (SNPs)
can have a very varied effect based on polymorphism
What can VNTRs be useful in?
forensic and paternity testing
How is DNA transcription started?
- antisense strand is read from 3’ to 5’ end
- RNA polymerase binds to initiation site (normally TATA boxes)
- promoter regions (upstream and downstream of gene) control rate of transcription through binding with trans-acting factors
How is RNA polymerase bound during DNA transcription?
- binding of RNA polymerase separates strands of DNA
- first exposed base binds with complementary free nucleotide
- RNA polymerase travels along the DNA adding complementary bases to the mRNA chain
How is DNA transcription terminated?
RNA polymerase encounters a stop codon and is dislodged from DNA
mRNA has been formed
What is capping of mRNA?
5’ end of mRNA is capped by the addition of 7-methylguanosine to the triphosphate group
Capping is thought to protect the mRNA from destruction by nuclease enzymes
What is polyadenylation of mRNA?
the 3’ end is cleaved at the poly-A site (sequence AAUAAA or AUUAAA)
a sequence of up to 300 A nucleotides is then added to the 3’ end
What are CpG islands?
Unmethylated C and G rich regions
associated with 5’ ends
