GENE EXPRESSIONCH 17

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I. Basic principles of gene expression

• A. General characteristics

• process by which genetic info in DNA is converted to protein– DNA → RNA is transcription– RNA → protein is translation

• RNA is the bridge between proteins and genes that code for them

• The concept of gene is universal to all domains of life• The general process of gene expression is also

universal• The genetic code is also universal

B. The genetic code

• Language of DNA and RNA are nucleotides• Language of proteins are amino acids• The nucleotide sequence must be translated

into amino acid sequence• Nucleotide sequence is read in groups of 3

nucleotides called codons• The genetic code is redundant

II. Transcription

• Process by which the genetic info in DNA is copied into RNA

• Occurs at specific regions of DNA called genes• the basic structure of genes are the same in

all domains

Promotor: where transcription starts. Coding sequence: what is transcribed to RNATerminator: where transcription stops

A. The process of transcription• Only one strand of the

DNA is transcribed into RNA, the template strand

• RNA strand is complementary to DNA strand copied

• Enzyme is RNA polymerase

Three stages: initiation, elongation, termination

Initiation: RNA polymerase binds to promoter with the help of various transcription factors and unzips DNA.

Elongation: RNA polymerase reads template stand of DNA making RNA

Termination: transcription stops

B. Post-transcriptional processing of mRNA in eukaryotes

Before mRNA is usable it must be processed• 5’ CAP and 3’ poly A tail put on

• Purpose of CAP and tail: help RNA leave nucleus, prevent its degradation, and help ribosome bind to 5’end

• Splicing out of introns

• Introns: segments of gene that are transcribed into mRNA but don’t code for protein

• Must be cut out

• snRNPs binds to intron/exon junction

• snRNPs attract each other looping out the introns

• introns are cut out and exons are glued together

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120077/bio30.swf::How%20Spliceosomes%20Process%20RNA

III. Translation• Process by which genetic

information carried in the mRNA is converted into protein

• Requires the help of tRNA which transfers amino acids to growing protein in the ribosome

A. The structure of a tRNA

• Anticodon: a group of 3 nucleotides complementary to a codon in mRNA

• CCA site: place where amino acid is attached

• Accurate translation requires 2 steps:– There must be a correct match between tRNA

and amino acid which is done by aminoacyl tRNA synthase

– There must be a correct match between anticodon and codon

B. Ribosomes

• Where translation occurs• Facilitates interaction of tRNA and mRNA• Made of 2 subunits of rRNA• Overall structure of bacterial and eukaryotic

ribosomes are similar but many antibiotics target bacterial ribosomes without affecting eukaryotic ribosomes

Ribosomes have 3 binding sites for tRNAP site: holds the tRNA that carries growing polypeptide chainA site: holds the tRNA carrying the next amino acid to be added to growing chainE site: exit site where free tRNAs leave

C. The Process of Translation

• Occurs at the ribosome and is fundamentally the same in prokaryotes and eukaryotes but eukaryotic ribosomes are larger

• occurs in 3 stages: initiation, elongation, termination

1. Initiation

• mRNA interacts with a ribosome such that 1st AUG sits in the P site

• initiator tRNA binds to the FIRST AUG codon in mRNA

2. Elongation and translocation• 2nd tRNA with the

correct anticodon binds to the 2nd codon in mRNA in the A site

• The 2 adjacent amino acids are linked via dehydration reaction

• Ribosome moves down 3 nucleotides

• 1st tRNA leaves thru the E site

• This process continues

3. Termination

• At the stop codon:Release factor binds to stop codon in the A siteTranslation stops and protein leaves

https://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__how_translation_works.html

If a protein is destined for another location like the cell membrane or is to be secreted, it has a signal sequence that brings the ribosome to the RER

Many ribosomes can translate mRNA at the same time forming polyribosome. Can make a lot of protein quickly

• Protein can be modified after translation to make the functional protein:– 2 or more protein chains interact to form the

functional protein (quaternary structure)– Small carbohydrate chains can be added to some

proteins•

IV. Mutations

• Changes in the DNA sequence • Can be a product of mistakes made during

replication, transcription, or DNA repair• Most are caused by mutagens: agents that

damage DNA• Most change the way the protein folds,

affecting its function

• Two types of small scale mutations: substitution mutations and frameshift mutation

• Base substitution mutation: a single nucleotide is changed. Can be silent, missense, nonsense

Silent: no effect on protein due to redundancy in genetic code

Missense: mutation results in a different amino acid

Nonsense: amino acid changed to stop codon

• Insertion /deletion mutations: loss or addition of nucleotides and are most often disastrous

• http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter11/animation_quiz_3.html

• • http://highered.mcgraw-hill.com/sites/0072556781/st

udent_view0/chapter11/animation_quiz_4.html• • https://www.youtube.com/watch?v=kp0esidDr-c• • http://highered.mcgraw-hill.com/sites/0070960526/st

udent_view0/chapter18/animation_quiz_1.html• • https://www.youtube.com/watch?v=FgMKGIED4Yo•

V. Evolutionary significance of Mutations

• Mutation rate is relatively low. Keeps genome constant from generation to generation– DNA repair mechanisms– DNA polymerase proofreads– Double strandedness and coiling of DNA protect it

• However, mutations do occur. Mutations provide genetic variation for evolution to act on.