RNA and Protein Synthesis

Protein Synthesis Protein Synthesis is the process that cells use to produce protein. - it involves 2 distinct phases Transcription – occurs in the nucleus involves the creation of mRNA Translation – occurs in the cytoplasm at a ribosome – the protein recipe is “read” and the correct protein is made

Proteins are chains of amino acids amino acid + amino acid + amino acid = protein The order of amino acids determines protein shape Shape determines function DNA recipe consists of the order of amino acids for each protein - the recipes are known as genes

Recipe has to get from DNA to the ribosome which builds the protein

Transcription: makes a copy of the protein recipe This is necessary because: DNA cannot leave the nucleus!!! Proteins are made on ribosomes in the cytoplasm. mRNA provides the solution Messenger ribonucleic acid mRNA is a copy of the protein recipe that can leave the nucleus

Knowledge Check How do transcription and translation work together to produce proteins? What is necessary for protein synthesis to occur? CRITICAL THINKING: What is the one time when DNA does leave the nucleus? (It is not in strands that can be copied at this time)

The Structure of RNA There are three main differences between RNA and DNA: The sugar in RNA is ribose instead of deoxyribose. RNA is generally single-stranded. RNA contains uracil in place of thymine.

Types of RNA There are three main types of RNA: messenger RNA ribosomal RNA transfer RNA

Messenger RNA (mRNA) carries copies of instructions for assembling amino acids into proteins, leaving the nucleus and traveling to the ribosome

Ribosomes are made up of proteins and ribosomal RNA (rRNA).

During protein construction, transfer RNA (tRNA) transfers each amino acid to the ribosome.

Critical Thinking Based on their descriptions, hypothesize why you think each type of RNA is required for proteins synthesis to occur.

Transcription: Initiation The Process Begins The enzyme RNA polymerase finds the beginning of a protein recipe called the promoter - promotor = a series of nucleotides that indicate the start of a protein recipe The RNA polymerase opens the DNA molecule at the promoter

Transcription: Initiation The RNA polymerase uses one DNA strand as a template to build the mRNA - only one of the DNA strands contains the protein recipe - the strand with the recipe is the template strand - the strand without the recipe is the non-template strand - it is not copied

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Transcription: Elongation Building the mRNA Molecule RNA polymerase brings RNA nucleotides to the template strand -pairs them with their complements on the original DNA molecule -this follows the base pairing rules except that uracil replaces thymine - Adenine on DNA is paired with Uracil (U) on the new mRNA

Transcription: Termination The Process Ends the RNA polymerase continues to add new nucleotides until it reaches the terminator - the terminator is a sequence of nucleotides that indicates the end of the recipe the mRNA drops off the DNA -this is pre-mRNA it needs further processing before it can be translated

Knowledge Check How does RNA polymerase assist in transcription? What role does the promoter play? Summarize the major steps of transcription (you should have at least 3)

RNA Editing Introns – segments of nucleotides not involved in coding for proteins Exons – DNA sequences that code for proteins Introns get cut out and exons get spliced (added) together

Transcription

Genetic Code The genetic code is the “language” of mRNA instructions. The code is written using four “letters” (the bases: A, U, C, and G). A codon consists of three consecutive nucleotides on mRNA that specify a particular amino acid.

The Genetic Code “Start” codon – AUG (methionine) 3 “stop” codons – do not code for amino acids; signify the end of a protein molecule

Knowledge Check What is the amino acid for AUC? What is the amino acid for CUG? Explain (in words) how you determine the amino corresponding to each codon.

Translation Overview

Translation Translation is the decoding of an mRNA message into a polypeptide chain (protein). Translation takes place on ribosomes. During translation, the cell uses information from messenger RNA to produce proteins.

Translation mRNA molecule attaches to a ribosome As each codon of the mRNA molecule moves through the ribosome, the proper amino acid is brought to the ribosome by the tRNA In the ribosome, the amino acid is transferred to the growing protein molecule

Translation Each tRNA molecule carries only one kind of amino acid Each tRNA has 3 unpaired bases Anticodon – complementary to the one mRNA codon

Knowledge Check Where does translation occur? Summarize the major steps of translation (you should have at least 3). Critical Thinking: How does the anti-codon relate to the original DNA template strand?

The ribosome binds new tRNA molecules and amino acids as it moves along the mRNA, beginning with the “start” codon

The process continues until the ribosome reaches a stop codon.

DNA mRNA Protein