DNA RNA Protein

Flow of Information DNA  RNA  Protein DNA REPLICATION

Flow of Information DNA  RNA  Protein TRANSCRIPTION

Flow of Information DNA  RNA  Protein TRANSLATION

Defintions Replication – DNA synthesis – Occurs in the nucleus Transcription – RNA synthesis – Occurs in the nucleus Translation – protein synthesis – Occurs in the cytoplasm at a __________

Eukaryotic gene expression (overview)

RNA Structure – review Single strand of covalently bonded RNA nucleotides – RNA nucleotides: Ribose – 5 carbon sugar Phosphate group bonded to carbon 3 or carbon 5 of ribose Nitrogenous base bonded to carbon 1 of ribose – Adenine-- Uracil (replaces thymine) – Guanine-- Cytosine

RNA nucleotides 5 C Sugar = ribose Phosphate Base – A, G, C, or U purine vs pyrimidine? RNA BASES ADENINE GUANINE CYTOSINE URACIL 5’ 3’ 1’

Transcription RNA is made complementary to a short segment of DNA – Section of DNA transcribed is called a gene DNA opens up at the location of a gene RNA polymerase attaches to a promoter sequence (TATA sequence) on one of the DNA strands

Transcription RNA is made complementary to the sense strand of DNA by RNA polymerase – No primer sequence is needed – RNA is made in the 5’  3’ direction – RNA is made anti-parallel to the DNA strand

Transcription Newly synthesized RNA detaches from the chromosome DNA reforms the double helix

Fate of the RNA RNA may be one of three types: – mRNA = messenger RNA Needs to be processed before leaving the nucleus – rRNA = ribosomal RNA Used to make a ribosome – tRNA = transfer RNA

Processing of mRNA mRNA is modified before leaving the nucleus 1. “Cap” is added to the 5’ end – cap is often a phosphate and/or methyl group 2. Poly-A tail is added to the 3’ end – The longer the tail the longer the mRNA can be used to direct protein synthesis See page 194

Processing of mRNA 3. Segments called introns are removed and the remaining mRNA segments, called exons, are joined – page 194 – Splicing of exons may occur in more than one way – Each results in synthesis of a different mRNA and a different protein

RNA Processing 5’

mRNA Function: order of the bases in the mRNA determines the primary structure of a protein Primary structure = ____________________ – mRNA strands are fairly long, ~ 1, ,000 nucleotides long

Ribosome Review – The ribosomal subunits are made at the _________ – Ribosomes function in the ______________

rRNA Function: structural component of a ribosome Ribosomes: – Made of 2 subunits, large and small Each is made up of rRNAs and proteins Two subunits come together in the cytoplasm during protein synthesis

tRNA Function: each tRNAs brings a specific amino acid to the ribosome during protein synthesis.

tRNA There are 61 different tRNA in the cell – There are only ____ amino acids – Therefore, most amino acids can be brought to the ribosome by more than one tRNA Each tRNA is ~ 80 bases long and folds into a cloverleaf formation

tRNA At the bottom of the cloverleaf is a 3 base sequence called the anticodon A specific a.a. attaches to the top of the cloverleaf at the sequence ACC – The anticodon specifies which a.a. the tRNA will carry

tRNA

mRNA tRNA AMINO ACID mRNA Bases H bond With tRNA Bases The codon on the mRNA is correlated to the amino acid on the tRNA – see chart on page 192

trNA, mRNA, and aa The anticodon on tRNA hydrogen bonds to the complementary codon on mRNA mRNA codon:A U G tRNA anticodon:_________ Amino acid on tRNA: __________

Anticodon, tRNA Codon, mRNA Amino acid UAC CAC GUA

Anticodons There are 61 different tRNA/anticodons each carrying a specific amino acid – Yet there are 64 possible anticodons…..

Translation Translation – protein synthesis – Occurs in the cytoplasm at a ribosome – Involves: Ribosomal subunits tRNA’s with their appropriate amino acids attached mRNA, to be read in the 5’ to 3’ direction

Translation Three stages of translation: – Initiation – Elongation – Termination

Eukaryotic Initiation – page 197, 10.13B Small subunit of the ribosome binds to the mRNA at the 5’ end of the mRNA – The small subunit has the tRNA with an anticodon complementary to AUG attached – This tRNA carries the aa _______________ Small subunit with the tRNA travels down the mRNA to the first AUG – AUG = start codon for all protein synthesis

Initiation Large subunit of ribosome binds Initiation is now complete.

More about ribosomes A ribosome has two tRNA binding sites – “P” site Binds the tRNA attached to the growing protein chain – “A” site Binds the tRNA with the next amino acid to be added to the growing protein

Elongation tRNA with anticodon complementary to the next codon on the mRNA enters the “A” site of the ribosome and H bonds to the mRNA

Elongation The ribosome is now full – The tRNA for methionine is in the “P” site – The tRNA carrying the second amino acid is in the “A” site

Elongation The two amino acids in the ribosome are joined by a peptide bond – The reaction is catalyzed by an enzyme in the large subunit of the ribosome The bond between the initiator a.a. and its tRNA breaks

Elongation The ribosome moves down 3 bases on the mRNA – the initiator (first) tRNA falls out of the ribosome – the tRNA bonded to the peptide bonded amino acids is transferred into the P site – The A site is now empty

Elongation The tRNA with an anticodon complementary to the next codon enters the A site of the ribosome

Elongation A peptide bond forms between the 3 rd amino acid and the dipeptide – Bond between the tRNA in the P site and the growing protein chain breaks – This tRNA falls out of the ribosome as the ribosome moves down three bases

Elongation The next tRNA enters the A site A peptide bond forms between the 4 rd amino acid and the tripeptide – Bond between tRNA in the P site and the growing protein chain breaks – This tRNA falls out of the ribosome as the ribosome moves down three bases Process repeats and repeats……until…..

Termination ….. The ribosome reaches a stop codon – Stop codon = codon on the mRNA for which there isn’t a tRNA with a complementary anticodon There are 3 stop codons See page 198

Termination In response to the stop codon a release factor enters the ribosome and attaches to the A site As a result the following are released: – Protein detaches from the tRNA in the P site – Ribosomal subunits fall off the mRNA – Last tRNA falls out of the ribosome

Translation The mRNA can be used over and over again to direct protein synthesis – Each time it is read a little of the poly A tail is destroyed – When the tail becomes too short, the mRNA is no longer functional and targeted for destruction

Eukaryotic gene expression (overview)

Translation Since mRNA is fairly long it can be read by more than one ribosome at a time – Called a polysome when many ribosomes are reading the same mRNA – n/index.htm

Mutations Mutation – permanent change in the DNA – Base substitution mutation – Addition/deletion mutations Frameshift mutations

BASE SUBSTITUTION MUTATIONS Can Have Variety of Impacts on the Protein Made # 1 - Silent Mutation # 2 and 3 – One amino acid changed in the protein, variable effects # 4 – Devastating, protein is truncated

ADDITION/ DELETION mutations-> frameshift A frameshift mutation results in a change in the codons read by the ribosome Can Be Very Disruptive; Especially if Early in Sequence Correct DNA: TAC,TTC,AAC,CGT,TT Correct mRNA: AUG,AAG,UUG,GCA,AA…. Mutated mRNA: AUG,AAG,UUU,GGC,UAA Mutation A added to the DNA

Mutagens – Ionizing radiation (x-rays, radiation) – Nonionizing radiation (UV light) Creates thymine-thymine dimers – Chemotherapy and radiation treatment mutate the DNA of cells during DNA replication Mutates and potentially kills both cancer cells and healthy cells