Lecture 14

Question 1

Crick’s three concepts

Answer 1

the role of nucleic acids is to inform the synthesis of protein

the flow of information is unidirectional

the information is a sequence

Question 2

Cricks central dogma theory

Answer 2

The central dogma of molecular biology is a theory stating that genetic information flows only in one direction, from DNA, to RNA, to protein, or RNA directly to protein

Question 3

One gene, one enzyme hypothesis

Answer 3

Beadle knew that X-rays induced mutants from his previous work in drosophila and with Tatum they generated 3 new mutants in neurospora which couldn’t grow without certain vitamins

Neurospora can reproduce sexually and Beadle and Tatum showed that the mutants had Mendelian inheritance

this means that genes controlled biochemical reactions later dubbed one gene one enzyme theory

Question 4

Crick, Barnett, Brenner & Watts-Tobin

Answer 4

used mutagens that either insert or delete, not change, a base pair

they used phage T4 ‘point’ mutants that could not lyse E coli normally and reverted the phenotype with additional rounds of point mutagenesis

they observed that insertions could be reverted with deletions and vice versa but most importantly that 3 insertions would revert a single insertion

Question 5

Nirenberg and Matthaei

Answer 5

Brenner had shown an RNA intermediate between DNA and protein

Heppel and Singer were able to synthesise polyuridine using the newly described Polynucleotidephosphorylase

they devised a method to lyse E coli but retaining the ribosome activity

they treated their extracts with RNAse and DNAse and then added synthetic poly-U leading to synthesis of poly-Phenylalanine

the first codon was determined

Question 6

Features of the genetic code

Answer 6

continuous, non-overlapping
degenerate
4 bases can distinguish 20 amino acids in combinations of three
unambiguous- 1codon, one amino acid
universal with a few exceptions

Question 7

using the genetic code

Answer 7

dsDNA is copied into ssRNA
the coding trend is the non-template
-the RNA sequence is identical to the DNA coding sequence
-codons are read in the 5’-3’ direction

Question 8

what is the start codon

Answer 8

AUG on RNA
ATG on DNA

Question 9

Start codon

Answer 9

indicates the first codon of a mRNA to be translated by a ribosome

codes for methionine also in internal positions of the protein

bacteria and organelles incorporated N-formylmethionne at start AUG by modifying the normal tRNA-Met complex to tRNA formylMet
Met are often removed post translationally

Question 10

What are the stop codons

Answer 10

UAG
UAA
UGA

Question 11

Stop codons

Answer 11

do not code for any amino acid
have a different efficiency of termination
UAA>UAG>UGA
are usually followed by other accessory sequences

Question 12

mRNA basic structure

Answer 12

5’UTR (untranslated region)- governs interaction with the ribosome
3’UTR (can be regulatory)

Question 13

Open reading frame

Answer 13

a portion of a DNA sequence that does not include a stop codon

Question 14

Wobble base-pairs

Answer 14

only the first two bases of a codon have precise Watson-crick pairing with the corresponding bases of the anticodon of tRNA

four non-standard interactions may take place between the base at the 3’ end of the codon and the first 5’ base of the tRNA

this wobble effect allows different codons to be recognised by the same tRNA

these pairings are essential for secondary structure of many RNAs, some organisms do not have tRNAs for all synonym codons

Question 15

Codon usage bias

Answer 15

the genetic code is universal but synonym codons allow that each species has favourite codons for certain amino acids

this has implications for translation efficiency of individual genes

Question 16

Deciphering codon tables

Answer 16

the universality of the genetic code underlies the fact that all life on earth has a common evolutionary history

it also enables scientists to combine genes from different species and express their products in a novel combination and in a new host

Question 17

Mutation types

Answer 17

Spontaneous
- from replication errors
-very low frequency

Induced
- caused by external agents
chemical: alkylating agents, intercalating agents
physical: ionising radiation
biological: pieces of exogenous DNA inserting into genome

Question 18

Point mutation types

Answer 18

substitution- change of identity of base pair

insertion- addition of nucleotide pair

deletion- removal of a nucleotide pair

Question 19

Base substitutions

Answer 19

transition
- Pu to pu or Pyr to Pyr
transversion
- Pu to Pyr

Question 20

Silent mutations

Answer 20

degeneracy of the code implies that some base changes will lead to synonymous codons

Question 21

Missense mutations

Answer 21

base changes lead to codons for a different amino acids

Question 22

Nonsense mutations

Answer 22

base substitutions lead to termination codon

nonsense mutation lead to C-terminal truncations of protein structure

Question 23

Indels

Answer 23

in coding regions these mutations produce frame shift

Question 24

Frame-shift mutations

Answer 24

single nucleotide deletion in the squalene epoxidase1 gene leads to a frame shift that changes the sequence of protein

Question 25

Mutation type by lineage

Answer 25

in multicellular organisms, mutations have different effects depending on which cells harbour them
gremlin mutations are found in oocytes/sperm cells. when transmitted to the offspring all cells will have it .. can cause heriditary diseases

somatic mutations- arise in non-germ cells, they are not passed on to offspring and lead to mosaicism, cancer arises from somatic mutation