Protein Synthesis RNA, Transcription, and Translation

RNA Three main types: Messenger RNA (mRNA): serve as messengers from DNA to the rest of the cell Ribosomal RNA (rRNA) – found in the ribosomes, the site of protein synthesis Transfer RNA (tRNA) – transfers amino acids to the ribosomes, as specified by mRNA

RNA Structure 5-carbon sugar ribose Phosphate group Nitrogen bases: Adenine Cytosine Guanine Uracil (replaces thymine from DNA) A pairs with U, C pairs with G Usually single-stranded

Proteins Made up of amino acids linked by peptide bonds 20 different amino acids exist Amino acid chains are called polypeptides Proteins coded by particular segments of DNA  GENES One gene to one polypeptide

RNA and Protein SynthesisProtein Synthesis DNA (the instructions) stays in the nucleus Proteins are made at the ribosomes RNA acts as a disposable copy of DNA RNA carries the “message” from the nucleus to the ribosomes Protein Synthesis: DNA  RNA  Proteins (Central Dogma) Consists of transcription and translation

Transcription (DNA  RNA) A complementary RNA sequence is made from a particular segment of DNA Requires the enzyme RNA polymerase Includes three stages: Inititiation, Elongation, and Termination

Transcription Initiation: RNA Polymerase binds to a promoter sequence (start) on the DNA and separates the strands Promoter tells where to start and which strand Elongation: RNA Polymerase uses one strand of DNA as a template to make mRNA, in a 5’-3’ direction Remember uracil replaces thymine This mRNA is called the primary transcript Termination: RNA polymerase reaches a terminator sequence and the mRNA is released. DNA returns to the double helix

Besides mRNAs, other types of RNA are produced by transcription: tRNA rRNA Small nuclear RNAs microRNAs RNAs may have other functions in the cell besides protein synthesis. Can sometimes act as catalysts

RNA Editing Before mRNA leaves the nucleus it is edited: 5’ cap – a modified form of guanine is added at the 5’ end Protects mRNA from degradation by enzymes Serves as a sign for ribosomal attachment Poly(A) tail – a repetition of adenine nucleotides is added at the 3 ‘ end Serves similar function as 5’ cap; also facilitates export from the nucleus

RNA splicing Not all DNA holds instructions for proteins Introns – sections that don’t code for proteins; “junk” Exons – segments that do code for proteins; “expressed” Both are copied when RNA is made introns are excised (cut out) and the exons are spliced back together

Introns and Exons

Splicing Carried out by spliceosomes Consist of snRNPs and additional proteins snRNPs – small nuclear ribonucleoproteins Composed of snRNA and proteins recognize splice sites at the ends of introns Spliceosomes cut and release the intron and immediately join the now adjacent exons

The Genetic Code Read 3 letters at a time Called codons Each codon codes for an amino acid 64 codons for the 20 amino acids One start codon – AUG and three stop codons - UAA, UAG, UGA Signify the beginning and end of the polypeptide

Genetic Code Nearly universal Shared by different organisms from bacteria to complex plants and animals The codons translate into the same amino acids for many different organisms Must have evolved early in the history of life Another example of how all living things are connected

Translation (RNA  Protein) mRNA leaves the nucleus and attaches to a ribosome As each mRNA codon moves through the ribosome, the proper amino acid is brought to the ribosome by tRNA Each tRNA carries a specific amino acid Based on the anticodon – complementary to the codon

tRNA Folds in on itself to form a 3D structure amino acid attachment site (3’) 3 letter anticodon at the end of the loop Only 45 different anticodons Base pairing not as strict on the third base – called wobble Some have the base inosine (I) which can bond with U, C, or A tRNAs become “charged” when they bind to an amino acid

Ribosome Each ribosome has a binding site for mRNA, and three binding sites for tRNA A site – holds tRNA carrying the next amino acid P site – holds tRNA carrying the growing polypeptide chain E site – where discharged tRNA leave

Translation Initiation – ribosomal subunit binds to mRNA and initiation (AUG) codon signals the first amino acid, methionine, which is brought by tRNA Elongation – amino acids are added one by one Codon recognition – anticodon on tRNA binds to codon on mRNA at the A site Peptide bond formation – bond between amino acids Translocation –tRNA to is moved from A to P site, moving mRNA with it; tRNA in the P site is moved to the E and released N terminus is the first amino acid added and C terminus is the last Termination – a stop codon is reached and protein is released

More on translation… Many ribosomes may read the same mRNA sequence, often at the same time – polyribosome Once complete, the final polypeptide my be modified and take on conformational shape (secondary and tertiary structure) Signal sequences target proteins for particular functions – membrane, secretory, etc.

Gene mutations Point mutation – caused by changes to individual nucleotides Substitution (a base incorrectly paired) Changes one amino acid Missense – altered codon still codes for an amino acid Nonsense – change an amino acid codon to a stop codon Deletion or Insertion (nucleotides subtracted or added) changes many amino acids Causes frame-shift – a change in the reading of 3 letter codons Mutagen – any physical or chemical agent that interacts with DNA and causes mutations