Transcription & Translation Protein Synthesis Transcription & Translation

Protein Synthesis: An Overview “The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins.” Proteins are the link between genotype (AAC) and phenotype (eye color) Protein synthesis is the process of transferring the information encoded in DNA into proteins Step 1: Transcription Step 2: Translation

Transcription Transcription is the synthesis of messenger RNA (mRNA) from DNA Occurs in the nucleus DNA does not leave the nucleus Analogy: blueprints do not get used at the job site when constructing a house

DNA vs. RNA DNA RNA Contains: Adenine, guanine, cytosine, thymine Adenine, guanine, cytosine, uracil 5-carbon sugar is deoxyribose 5-carbon sugar is ribose Double-stranded Single-stranded

Transcription: Initiation Promoter = a specific sequence of DNA (starting point); consists of initiation site and TATA box) RNA polymerase binds to the promoter on the DNA molecule Transcription factors (proteins) bind to promoter region to help RNA polymerase find the starting point. RNA polymerase then separates the 2 DNA strands

Transcription: Elongation As RNA polymerase moves along the DNA, it untwists the double helix and separates the strands RNA polymerase adds nucleotides to the 3’ end of the mRNA molecule Follows the base-pairing rules

Transcription: Termination Termination signal = sequence of bases in DNA molecule that tell RNA polymerase to stop transcription “finish line” in a race mRNA is released from the DNA Most common termination signal = AATAAAA

RNA Modifications In eukaryotes, mRNA is edited before it’s sent out of the nucleus 2 major types of modifications: Alteration of mRNA ends RNA splicing

Alteration of mRNA Ends 5’ end receives a 5’ cap Modified form of a guanine nucleotide 3’ end receives a poly-A tail 30 – 200 adenine nucleotides Protects the ends from being degraded by enzymes in the cytoplasm Shoelace analogy

RNA Splicing Genes have stretches of nucleotides that don’t code for anything “junk DNA” Noncoding sequence = introns “intervening sequences” Coding regions = exons During RNA splicing, the introns are removed by enzymes and the exons are joined together by SLICEOSOMES (made of snRNPs + proteins)

Translation Big Picture: Occurs in the cytoplasm, on a ribosome The building of a polypeptide (protein) from the information encoded in mRNA Occurs in the cytoplasm, on a ribosome

tRNA Transfer RNA (tRNA): Cloverleaf shape Contains anticodon (triplet of bases, complementary to mRNA) Carries specific amino acid to ribosome during translation

Ribosomes Made up of 2 subunits composed of rRNA & proteins: Large subunit Small subunit Have special binding sites: P site: holds tRNA with amino acid chain A site: holds “next” tRNA The “green room”

Translation Consists of 3 stages: Chain Initiation Chain Elongation Chain Termination

Translation: Chain Initiation mRNA and tRNA join Codon + complementary anticodon mRNA is AUG (start codon) tRNA is UAC Large and small ribosomal subunits join, forming a functional ribosome

Translation: Chain Elongation Amino acids are added one by one by the following process: mRNA codon bonds with anticodon on tRNA Peptide bond is formed between new amino acid and the last one tRNA moves over from A site to P site

Translation: Chain Termination Elongation continues until a stop codon is reached (UAA, UAG, UGA) Water is added instead of an amino acid Polypeptide (protein) is released from ribosome

Point Mutations (aka gene mutations) Just one change in a nucleotide in a single gene Can seriously affect phenotype  genetic disorder Sickle cell anemia results from one nucleotide substitution.

Point Mutations Normal Hemoglobin sequence: Base-pair substitution CTT GAA Glutamic Acid Sickle Cell Sequence: CAT GUA Valine Base-pair substitution Replacing one nucleotide with another Example: instead of G, it’s C Can cause a noticeable change in protein structure Can be a silent mutation with no actual change in amino acid sequence. Can change only one a. acid Non sense mutations change sequence to a stop codon.

Point Mutations Base-pair insertions or deletions Normal Sequence: AAG GCG TAG  UUC CGC AUC  Phenylalanine-arginine-isolecine Insertion: AAG GGC GTA G  UUC CCG CAU C  Phenylalanine-proline-histidine Base-pair insertions or deletions Adding or losing a base-pair Changes the whole “reading frame” Where, within a gene, would this be the biggest problem?

Frame Shift Mutations Insertions- extra nucleotide is added; moves reading frame up one nucleotide & changes every amino acid after that point. Deletions- one nucleotide is removed; moves reading frame down one base. Both are more harmful if at the beginning of a gene.

Chromosomal Mutations A chunk of a chromosome is changed. Addition or duplication: A piece of a chromosome is added so genes appear twice. Deletion: part of the chromosome is lost (genes lost) Inversion: piece of the chromosome breaks twice and broken piece inserts upside down (reverses order of genes) Translocation: two nonhomologous chromosomes break & exchange genes.