Protein Synthesis

Review Questions What is the function of DNA? Stores genetic information and holds the instructions for building proteins Why is DNA called the “code of life”? The sequence of nitrogen bases (A,T,C,G) act as a code that must be “decoded” or “translated” into proteins Why are proteins important? Play a vital role in most of the body’s functions Control the expression of traits in an organism Brown eyes vs. blue eyes

DNA Blueprint The process that “decodes” the instructions found in DNA, and builds proteins according to the DNA blueprint is protein synthesis Involves two stages: TranscriptionTranslation

We have a Problem DNA: Very important  housed in the nucleus We know that… Proteins are made by ribosomes in the cytoplasm. How does the DNA code get from the nucleus to the ribosomes?

RNA! There are 3 types that all play a role in protein synthesis mRNA (messenger) tRNA (transfer) rRNA (ribosomal)

Part ONE: TRANSCRIPTON

¿What is Transcription? Protein synthesis (p.s.) starts when chemical signals activate the gene on a certain chromosome This will build a specific protein RNA Polymerase attached to the location on the DNA molecule where that gene is found Causes 2 strands of DNA to separate RNA Pol makes a “copy” of the DNA by transcribing the DNA gene sequence into a complementary strand of mRNA

REMEMBER: in RNA… Uracil pairs with Adenine! Example: DNA sequence: T A C C C A T C A RNA sequence: A U G G G U A G U Certain portions of the DNA sequence can be “cut out” during transcription. These are meaningless sequences called Introns Introns are NOT transcribed because they do NOT contain instructions for building proteins

Exons vs. Introns Meaningful sequences are called: Exons They are expressed when the protein is made After introns are removed, certain enzymes will splice exons back together into one continuous mRNA strand The spliced mRNA leaves nucleus through nuclear pores and moves to the cytoplasm

PART TWO: TRANSLATION

What is Translation? Process of “decoding” the mRNA message and assembling amino acids into a protein Begins when a ribosome in the cytoplasm attaches to mRNA strand Each set of three nucleotides on mRNA is called a codon. Codons specifies a particular amino acid that will be used in the protein chain The order of amino acids in the protein chain will distinguish one protein from another, in terms of structure and function

Codons & Amino Acids Example: DNA sequence:T A C C C A T C A mRNA sequence:A U G G G U A G U METGLYSER

Codons & Amino Acids Math Challenge: Calculate how many possible codons (sets of 3) can be formed from the 4 nitrogen bases A U C G Answer: 64 = 4 3 How many different amino acids are there? Answer: 20 Where are there so many extra codons? Answer: Several different codons can specify the same amino acid (REDUNDANCIES)

List of 20 Amino Acids AMINO ACIDABBREV AMINO ACID ABBREV alanineala lysine lys arginine arg methioninemet asparagine asnphenylalaninephe aspartic acid aspprolinepro cysteine cys serine ser glutamine gln threonine thr glutamic acid glutryptophantrp glycinegly tyrosinetyr histidine hisvaline val isoleucine ile leucineleu

List all of the possible codons that specify Leucine. C U U C U C C U A C U G U U A U U G

Translation continued tRNA is vital in assembling amino acids into the correct sequence for the required protein tRNA has an anticodon at the end which has 3 nitrogen bases that are complementary to the codon of the mRNA The other end of the tRNA has a specific amino acid attached to it tRNA brings in the correct amino acid and adds it to the growing polypeptide chain in the ribosome.

How a Protein is Assembled 1.Ribosome (composed of rRNA) attaches to the mRNA strand 2.The first codon on the mRNA strand (AUG) is called the start codon This signals the beginning of the protein sequence This signals the beginning of the protein sequence It codes for Methionine It codes for Methionine 3.The (UAC) anticodon of the Met tRNA molecule attaches to the mRNA codon (AUG) and Methionine is brought in as the first amino acid in the chain 4.The ribosome moves along the mRNA strand and “reads” the second codon. The correct tRNA anticodon is matched to it and the second amino acid is brought in The correct tRNA anticodon is matched to it and the second amino acid is brought in

5.The ribosome forms a peptide bond between the first two amino acids And amino acid chain (polypeptide) begins to form And amino acid chain (polypeptide) begins to form 6.The first tRNA molecule (which is now empty) moves off and picks up another Methionine from the cytoplasm to be used again if needed 7.This sequence is repeated with the 3 rd, 4 th, 5 th … amino acids until the ribosome reaches a stop codon on the mRNA strand This signals the end of protein synthesis. This signals the end of protein synthesis. 8.The new protein is released into the cytoplasm

Overview of P.S.

P.S. Animation online.com/objects/ViewObject.aspx?ID=ap online.com/objects/ViewObject.aspx?ID=ap1302