Protein Synthesis The Making of Proteins Using the Genetic Information Stored in DNA

Central Dogma of Molecular Biology Genetic information flows in one direction – from DNA to RNA to proteins.

Relationship between DNA, Genes, Proteins Genetic information passed from parent to offspring is DNA

Each DNA molecule has hundreds of sections known as genes Gene - section of DNA that provides the instructions for making a protein

 Protein molecules are polymers formed by joining amino acids

 The type of protein made is determined by the order of amino acids.

 1,000’s of different protein molecules are made by the cell every minute

Protein Synthesis - process of using DNA’s code to make proteins.  Two-part process: 1. Transcription 2. Translation

Part 1 Transcription - DNA code used to make RNA Occurs in the cell nucleus. DNA is too big to leave the nucleus. So its “message” is rewritten in the language of RNA molecules which are smaller and can leave the nucleus

How Transcription Works  Enzymes unwind DNA to expose bases in a specific gene. start site nucleotides transcription complex

Transcription RNA polymerase attaches complementary RNA bases to the exposed DNA strand RNA polymerase bonds the nucleotides together to form RNA. DNA RNA polymerase moves along the DNA

Transcription – rewriting DNA into RNA

Transcription The newly made RNA strand detaches from the DNA after the gene is transcribed. The small RNA strand exits the nucleus through pores in the nuclear membrane RNA

Transcription – DNA code rewritten in the “Language” of RNA DNA RNA C G G C T A A U Uracil replaces Thymine in RNA

Transcription – DNA code rewritten in the “Language” of RNA GTC ABC ATG ACT AAT GCT AC DNA CAG UCG UAC UGA UUA CGA UG RNA DNA RNA C G G C T A A U Uracil replaces Thymine in RNA

Transcription makes 3 types of RNA: Messenger RNA (mRNA) –carries DNA’s protein making code to ribosomes Ribosomal RNA (rRNA) –forms ribosomes, the cell’s “protein factories” Transfer RNA (tRNA) – carries amino acids to the ribosome to make proteins.

Messenger RNA Carries the message of a DNA’s gene needed to make a protein. Each group of 3 nucelotides in mRNA is called a codon. Each codon specifies a particular amino acid. A codon must be read in the right order sequence called a reading frame.

8.4 Transcription Transcription compared to replication. Transcription and replication both involve enzymes and complementary base pairing. The two processes have different end results. –Replication copies all the DNA; transcription copies a gene. –Replication makes one copy; transcription can make many copies. growing RNA strands DNA one gene

8.4 Transcription Part 2: Translation – mRNA’s code is used to make proteins (polypeptides)

8.4 Transcription Amino acids are coded by mRNA base sequences. A codon is a sequence of three nucleotides on mRNA that codes for an amino acid. codon for methionine (Met) codon for leucine (Leu)

8.4 Transcription The genetic code matches each codon to its amino acid or function. –three stop codons –one start codon, codes for methionine The genetic code matches each RNA codon with its amino acid or function.

8.4 Transcription

 Site of protein synthesis – ribosomes in the cytoplasm.

8.4 Transcription Ribsomes are made from rRNA and proteins

8.4 Transcription Ribosomes consist of two subunits. –The large subunit has three binding sites for tRNA. –The small subunit binds to mRNA.

Transfer RNA (tRNA) carries amino acids to the ribsome. tRNA has a 3 nucleotide sequence called an anticodon tRNA’s anticodon is complementary with mRNA’s codon to deliver the correct amino acid to make the protein.

8.4 Transcription For translation to begin, tRNA binds to a start codon and signals the ribosome to assemble. –A complementary tRNA molecule binds to the exposed codon, bringing its amino acid close to the first amino acid.

tRNA’s anticodon matches with mRNA’s codon to deliver the correct amino acid needed to make the protein. The Ribosome positions the tRNA molecules close enough so that peptide bonds can form between the amino acids.

8.4 Transcription Once the stop codon is reached, the ribosome releases the protein and disassembles.

Central Dogma

Transcription is similar to Replication SIMILARITIES Replication (making DNA) andTranscription (making RNA) both * Make a nucleic acid * Occur in the nucleus (eukaryotes) * Occur in all living organisms * Require a number of enzymes to carry out process * Involve complimentary base pariring DIFFERENCES The two processes have different end results. _ Replication produces DNA; Transcription produces RNA – Replication copies all the DNA; transcription copies an individual gene on the DNA. – Replication makes one copy; transcription can make many copies.