13.2 Ribosomes & Protein Synthesis

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13.2 Ribosomes & Protein Synthesis

Introduction:

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A specific sequence of bases in DNA carries the directions for forming a polypeptide, a chain of amino acids (there are 20 different types of amino acid).

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RNA contains a code for making proteins. The types and order of amino acids in a polypeptide determine the properties, shape, and function of the protein.

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There is one gene codes for one enzyme. We now know that one gene codes for one polypeptide.

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Experiments during the 1960s demonstrated that the DNA code was a three- base code.

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The three-base code in DNA or mRNA is called a codon.

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The code is used by cells to translate messenger RNA (mRNA) sequences into protein.

Key Point: The genetic code is read three "letters" at a time, so that each "word" is three bases long and corresponds to a single amino acid.

The Genetic Code

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The directions for making proteins are found in the order of the four bases – A, U, C, G (Adenine, Cytosine, Guanine, Uracil)

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The genetic code is read three letters at a time, called a codon

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Each codon, or group of three nucleotides, stands

for a particular amino acid

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Some amino acids are specified by more than one

codon

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One codon is a start signal for translation

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There are three codons that can signal the end of a protein How to Read a Codon Chart:

 Because there are 4 different bases, there are 64 possible codons in the genetic code (4*4*4=64)

 Many amino acids can be specified for by more than one codon

Steps (using an example):

1. To decode the codon CAC, fined the first letter in the set of bases at the center of the circle.

2. Find the second letter of the codon A, in the "C" quarter of the next ring.

3. Find the third letter, C, in the next ring, in the "C-A" grouping

4. Read the name of the amino acid in that sector - in this case histidine.

Practice:

1) What amino acid is coded by AUG?

_______

2) What amino acid is coded by AAA?

_______

3) Tyrosine is specified by what codon(s)?

_______ and _______

4) What amino acids are coded for by the message:

AUG AAC UCU? _______ _______

_______

5) What is coded by UGA? _______

What is the purpose of this?

____________________

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Start and Stop Codons:

 Any message needs punctuation marks so you know where to start and stop a sentence

 The genetic code has its own system to let the cell know where to “start”

and “stop” protein synthesis

o The methionine codon (AUG) also serves as the “START” CODON – following the “start” codon, the mRNA is read, three bases at a time, until it reaches one of the three different “STOP” CODONS, which end translation – polypeptide is complete.

So...

 proteins = amino acids joined into long chains called polypeptides

 properties of proteins determined by order of their amino acids

 genetic code is 4 letters – U, A, G, C (read 3 bases at a time)

 codon = 3 consecutive nucleotides that specify a single amino acid

 64 possible codons

o some amino acids coded for by more than one codon o AUG = start codon

o 3 stop codons

Translation: decoding of an mRNA message into a polypeptide chain (proteins)

 occurs on ribosomes

 Remember: mRNA sequence codes for amino acids to be joined to form a protein…

 Just like assembling a complex toy or furniture, you must first read the instructions and then put the parts together – in the cell, a tiny factory—

the RIBOSOME—carries out both these tasks.

Key Point: Ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains

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 process:

o mRNA molecule attaches to ribosome o begins with AUG = start codon (methionine)

o tRNA has an anticodon whose bases are complementary to codons on the mRNA

o ribosome positions start codon to attract its anticodon

o ribosome also binds next codon and a second tRNA molecule o ribosome forms peptide bond between first and second amino

acids and breaks bonds holding them to the tRNA molecules

o process continues down length of mRNA until stop codon reached o ribosome releases newly formed protein and mRNA

 proteins are very important - keys to almost everything that living cells do, catalyze and regulate chemical reactions that control growth,

appearance, etc.

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 In Eukaryotic cells, transcription goes on in the cell’s nucleus – translation is carried out by ribosomes after the transcribed mRNA enters the cell’s cytoplasm

Roles of tRNA & rRNA in Translation:

 All three forms of RNA come together in the ribosome during translation:

o mRNA carries the coded message that directs the process of translation

o tRNA molecules deliver the exact amino acid called for by each codon on the mRNA

 tRNA molecues are like “adaptors” that enable the ribosome to “read” the mRNA’s message accurately

o rRNA help with the ribosome’s structure and help locate the “start codon” on the mRNA

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Molecular basis of Heredity:

 Proteins are responsible for all of your traits – color of a flower, shape of a leave, sex of a newborn baby, etc.

 The “Central Dogma” of molecular biology is that information is transferred from DNA to RNA to PROTEIN

o GENE EXPRESSION is how DNA, RNA, and proteins are involved in putting genetic information into action in living cells

o Exception to the dogma are viruses, who transfer info from RNADNA