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Chapter 7 III. Protein Synthesis Overview Chromosome #1 Has over 3000 genes • DNA contains genes • Genes are specific sequences of nitrogenous bases. • There are many genes on one chromosome • Genes direct the synthesis of proteins. 1 Proteins • Proteins have many functions – Structure (collagen in bones) – Storage (Egg whites) – Transport (cells) – Enzymes (chemical reactions) – Defense (immune system) Proteins • A monomer of a protein is called an amino acid • Amino acids link together through dehydration synthesis • Many amino acids joined together form a protein (polypeptide) 2 Proteins • DNA codes for 20 amino acids • We must go from the language of DNA to the language of proteins. RNA • • • • RNA = Ribonucleic Acid 5' Monomers are nucleotides Has both a 5’ and 3’ side Base pairing rules differ from DNA. – Adenine binds to Uracil – Guanine binds to Cytosine 3' 3 DNA vs. RNA DNA •One type of DNA RNA •Three types of RNA –mRNA, rRNA, tRNA •Double stranded •Single stranded •Deoxyribose sugar •Ribose sugar •Has the nitrogenous bases: Adenine, Guanine, Cytosine and Thymine •Has the nitrogenous bases: Adenine, Guanine, Cytosine and Uracil Protein Synthesis • DNA is transcribed into mRNA and then translated into a protein 4 Protein Synthesis - Transcription • Transcription The base sequence of DNA is copied onto a strand of messenger RNA (mRNA) • DNA cannot leave the nucleus. • mRNA carries the information out of the nucleus to the ribosomes. Protein Synthesis Transcription • One DNA strand called the template strand is used to create mRNA. • mRNA is synthesized by RNA polymerase. • RNA polymerase begins transcription of the DNA at the promoter site. • Transcription continues until RNA polymerase encounters a terminator sequence. 5 Protein Synthesis - Transcription • Synthesis of the mRNA occurs in the 5' to 3'direction • The base pairing rules are the same as DNA except Uracil replaces Thymine. Protein Synthesis - Transcription 6 Protein Synthesis - Transcription • The newly formed mRNA is called pre-mRNA and must be modified • Introns (Intervening sequences) – noncoding segments of mRNA. • Exons (Expressed) – Coding segements of mRNA • The introns are removed leaving only the exons. Protein Synthesis - Transcription • Pre-mRNA becomes mRNA after the introns are removed. • Why mRNA is modified. – Removing Introns enables a single gene to code for more than one polypeptide. 7 Protein Synthesis - Transcription • The mRNA leaves the nucleus through the nuclear pores. The Genetic Code • Each combination of three nucleotides is called a codon. • Each codon specifies an amino acid. • Codons are read in the 5' to 3' direction along the mRNA. 8 The Genetic Code • Three base codons are enough to code for all 20 amino acids. • The genetic code is redundant Different codons can code for the same amino acid – Limits the number of transcription errors. – Protects our genes from mutations. • The genetic code is not ambiguous Each codon specifies only one amino acid. Protein Synthesis - Translation • Translation The cell interprets the mRNA and builds a protein. 9 Protein Synthesis - Translation • tRNA (transfer RNA) – transfers amino acids to a ribosome where the tRNA amino acids are joined together to form a Anti-codon polypeptide. Amino Acid – One side attaches the amino acid. – The other side is called the anticodon and attaches to correct part of the mRNA – Each tRNA carries a specific amino acid Protein Synthesis - Translation • rRNA (Ribosomal RNA) Brings the tRNA together with the mRNA and joins the amino acids to form a polypeptide. – Made up of a large subunit and a small subunit. – Three binding sites: • P site – holds the tRNA carrying the growing polypeptide. • A site – Holds the tRNA carrying the next amino acid. • E site – tRNA exits from the ribosome at this site. 10 Protein Synthesis - Translation • The mRNA is read codon by codon in the 5’ to 3’ direction. • The tRNA brings in each amino acid and the ribosome builds the growing polypeptide. Protein Synthesis - Translation 11 Protein Synthesis - Translation Interpreting the Genetic Code • This table can be used to translate a codon • Example – CAG = Gln 12 Interpreting the Genetic Code • Translation begins at the start codon and ends at a stop codon on the mRNA. • The start codon also codes for the amino acid Met • The stop codon does not code for an amino acid • The mRNA is read codon by codon from 5’ to 3’ Interpreting the Genetic Code 13 Free and Bound Ribosomes • Multiple ribosomes can translate a single mRNA. 14 Free and Bound Ribosomes • Free ribosomes – Makes proteins that remain in the cytoplasm. • Bound ribosomes (RER) – makes proteins that leave the cell (insulin, hormones, enzymes) Mutations • A mutation is a change in the DNA • Mutagens – Alter an organism’s DNA. – Chemicals, radiation such as x-rays/UV light, or mistakes during DNA replication. – Many mutagens can lead to cancer. • The sequence of codons is important in order to create the correct protein! – Example “The car was won” Start at the wrong point and you get “Hec arw asw on” 15 Mutations • • Point mutation - A single nucleotide base change. Base-pair substitution - Replacement of one nucleotide pair with another. – – – – No effect if it is the same amino acid Redundancy! Missense – codes for a different amino acid Nonsense – codes for a stop codon Can be harmful to the organism if a useless or less active protein is created. Mutations • Base-pair insertions or deletions – Addition or loss of nucleotide pairs. – Frameshift Mutation: alters the reading frame. – Can cause extensive missense and eventually nonsense. – Protein will most likely be non functional. 16