Protein Synthesis Section 3 Transcription and Translation Watson and Crick discovered the structure of DNA, but what remained a mystery is how DNA served as a genetic code for the making of proteins. Animation

DNA’s Purpose DNA has genes that code for the synthesis (creation) of specific PROTEINS Here’s the problem… Where is DNA located? Nucleus Where does Protein Synthesis occur? At ribosomes in the cytoplasm Can DNA ever leave the nucleus? No.

Differences between DNA and RNA Structure: Single-stranded Sugar: Ribose Bases: Adenine Guanine Cytosine Uracil DNA Structure: Double stranded Sugar: Deoxyribose Bases: Adenine Guanine Cytosine Thymine

RNA Ribonucleic acid Single-stranded Sugar is ribose Thymine is replaced by URACIL 3 major types in living cells Messenger RNA (mRNA) carries information from DNA to ribosome Transfer RNA (tRNA) Carries amino acids to ribosome to make protein Ribosomal RNA (rRNA) Makes up ribosomes; site of protein synthesis. located on Rough ER and in cytoplasm

Bring amino acids to ribosome RNA can be Messenger RNA Ribosomal RNA Transfer RNA also called which functions to also called which functions to also called which functions to mRNA Carry instructions rRNA Combine with proteins tRNA Bring amino acids to ribosome from to to make up DNA Ribosome Ribosomes

How do you get from DNA to Proteins? TRANSCRIPTION – the synthesis of RNA under the direction of DNA TRANSLATION – the actual synthesis of a protein, which occurs under the direction of mRNA

Transcription From DNA to RNA

Transcription- Where does this happen? Where is the DNA?

Transcription- how RNA is made location= nucleus RNA polymerase runs along DNA strand in nucleus and makes RNA (mRNA) mRNA = messenger RNA (sends message outside of nucleus) mRNA leaves nucleus through a nuclear pore and meets up with a ribosome (rRNA) in the cytoplasm

Translation From RNA to Protein

Translation- Where does this happen? Where is the DNA? Protein synthesis – the manufacture of proteins Where are proteins made in the cell?

Translation- how proteins are made location= ribosome in cytoplasm Once mRNA is at a ribosome (rRNA), amino acids are assembled to make proteins Transfer RNA (tRNA) brings the appropriate amino acid to the growing protein chain

Genetic Code Genetic code – the language of mRNA instructions (blueprints) Read in three letters at a time Each letter represents one of the nitrogenous bases: A, U, C, G Codon found on mRNA; consists of three bases (one right after the other) 64 codons for 20 amino acids mRNA carries the codon (three base sequence that codes for an amino acid) tRNA carries the anticodon which pairs up with the codon tRNA brings the correct amino acid by reading the genetic code

Codon (cont’d) For example, consider the following RNA sequence: UCGCACGGU The sequence would be read three base pairs at a time: UCG – CAC – GGU The codons represent the amino acids: Serine – Histidine - Glycine

Codons (cont’d) AUG – start codon or Methionine UAA, UAG, UGA – stop codons; code for nothing; like the period at the end of a sentence

Protein formation Amino acids link together to form a protein The new protein could become cell part, an enzyme, a hormone etc.

Translation CODON ANTI CODON

SO: Methionine-Aspartic acid-Glutamine-stop Say the mRNA strand reads: mRNA (codon) AUG–GAC–CAG-UGA tRNA (anticodon) UAC-CUG-GUC-ACU tRNA would bring the amino acids: Methionine-Aspartic acid-Glutamine-stop

SUMMARY 1)mRNA is transcribed in the nucleus and leaves the nucleus to the cytoplasm 2) mRNA attaches to the ribosome 3) The codon on the mRNA is read by the anticodon on the tRNA 4) tRNA brings the amino acid as it reads mRNA 5) The amino acids are joined together to form a polypeptide (protein) 6) When a stop codon is reached (UAA, UAG, UGA) protein synthesis stops

What if things go wrong? MUTATION!!! If transcription or translation were to copy the wrong sequence, the incorrect amino acid could be added This would change the overall protein structure and could make the protein ineffective Example: Sickle cell anemia is caused by a single amino acid difference in the hemoglobin protein sequence

Gene Mutations Point Mutations – only occur at a single point in the DNA sequence – only changes a few amino acids Frameshift Mutations – shift the entire “reading frame” – change ALL the amino acids Substitution – one base replaces another Insertion – an extra base is inserted Deletion – loss of a single letter (makes entire base disappear!)

Deletion Substitution Insertion

Chromosomal Mutations Change in the number or structure of chromosomes Ex. – Deletion, Duplication, Inversion, and Translocation

Deletion Duplication Inversion Translocation