NAGE Flashcards
(47 cards)
Which bases are purines and which are pyrimidines?
AG = purines CUT = pyrimidines
How many base pairs are there per helical turn and how wide is the double helix?
10 bp per helical turn
2nm
State two ways of separating the DNA strand.
Heat
Low salt concentration
What is the lowest level of packing of DNA?
Nucleosome – it is wrapped around a histone (positively charged) which strongly attract to the negatively charged backbone of DNA – 200bp – causes 7 fold condensing
What is the next level of packing of DNA?
30nm fibre
What two types of nucleotides are needed for DNA synthesis?
Oligonucleotide primer (RNA primer made by DNA primase) dNTPs (deoxynucleoside triphosphates)
Explain how the lagging strand is synthesised.
DNA polymerase can only add dNTPs to the 3’ end of an oligonucleotide primer.
The oligonucleotide primers attach and DNA polymerase moves in a 5’ to 3’ direction adding the dNTPs to the growing chain. When the newly synthesised strand reaches the oligonucleotide primer of the previous okazaki fragment, the DNA polymerase is removed. Ribonuclease removes the RNA primer and Repair DNA polymerase replaces the RNA with DNA. DNA ligase then joins the adjacent okazaki fragments.
Which three enzymes are involved in the joining of Okazaki fragments?
Ribonuclease, Repair DNA Polymerase, DNA ligase
Which strand of DNA is transcribed?
Anti-sense (it is anti-sense to the mRNA produced)
What are the roles of the three types of RNA polymerase?
I – transcribes rRNA genes
II – transcribes genes encoding proteins into mRNA
III – transcribes tRNA and 5S RNA genes
What is the role of gene promoter?
Transcription complex assembles at the gene promoter
What constitutes the basal transcription complex?
TFII (D, A, B, F, E, H, J) and RNA polymerase II
How do transcription factors affect gene expression?
By modifying histones
Hyperacetylation = gene expression
Hypoacetylation = gene repression
What is the C-value paradox?
The size of the genome is not necessarily related to its complexity.
What are non-coding RNAs? Give some examples.
Any RNA that isn’t translated into a protein (i.e. anything except mRNA)
EXAMPLES: tRNA, rRNA, siRNA
What is mRNA processing?
Splicing out introns before it exits the nucleus and enters the cytoplasm
What bases do introns begin with and end with?
GU —- AG
Describe how small ribonuclear proteins (snRNPs) bind to the mRNA.
U1 binds to the splice donor site, U5 binds to the splice acceptor site, U2, U4 and U6 bind in the middle of the intron
Describe how the intron is removed from the mRNA strand.
The RNA cleaves at the splice donor site and the first G loops round and forms a phosphodiester bond with an adenine residue in the middle of the intron (branchpoint). The mRNA then cleaves at the splice acceptor site and the intron is removed as a lariat structure.
What two structures are added during post-translational modification and why?
7-methylguanylate cap – protects the RNA from degradation – enhances translation
Poly-A-tail – occurs 11-30 bases downstream of an AAUAAA sequence in the mRNA – also acts as protection – added one base at a time
Give an example of a disease that affects one of the structures added during post-translational modification.
Poliomyelitis affects 7-methylguanylate cap recognition during translation
Give an example of a mutation in a splice site which features in human disease.
Beta Thalassemia – splice site mutation in the beta globin gene – caused by imbalances in the relative amounts of alpha and beta chains
Describe how RNA can be used to block mRNA function.
Antisense RNA can be synthesised which binds to the complementary sense RNA forming double stranded RNA thus preventing it from being translated
Describe the formation of siRNA.
You begin with viral dsRNA
DICER cuts the dsRNA into smaller pieces (21-25bp)
AGO proteins then remove the passenger strand only leaving the strand that is anti-sense to the target RNA
The RISC complex associates with the single stranded RNA and cleaves the target mRNA when bound with the siRNA.
