Cell Biology:Protein Synthesis

Lesson 1 – Transcription and Translation (Inquiry into Life pg. 493-501)

Today’s Objectives Demonstrate an understanding of the

process of protein synthesis, including: Identify the roles of DNA, messenger RNA

(mRNA), transfer RNA (tRNA), and ribosomes in the process of transcription and translation, including initiation, elongation, and termination

Determine the sequence of amino acids coded for by a specific DNA sequence (genetic code), given a table of mRNA codons

Identify the complementary nature of the mRNA codon and the tRNA anti-codon

DNA VS RNABoth are nucleic acids made up of nucleotides.

DNA RNA (Ribonucleic Acid)(Deoxyribonucleic Acid)1. Sugar is deoxyribose Sugar is ribose

(5 carbon) (5 carbon)

2. Double stranded Single stranded

3. Found only in the nucleus Found in nucleus and cytoplasm.

4. Bases: Bases:

A – Adenine A, C, G, but not T

C– Cytosine Thymine is replaced by

G – Guanine U – Uracil

T – Thymine

Introduction

*DNA replication produces an identical DNA strand.

*Protein synthesis uses DNA to produce proteins.

Do not confuse these 2 processes!!!!!!

PROTEIN SYNTHESIS

Proteins are put together in the cytoplasm, but DNA never leaves the nucleus. A copy of the DNA must be made. This copy is called messenger RNA (mRNA).

• DNA is the master copy (or template) containing instructions for the production of proteins. (structural and functional)

mRNA travels into the cytoplasm where it is translated into proteins.

DNA-----------------> mRNA--------------> Protein

transcription translation

• Only genes for required proteins are copied into mRNA. The process of making mRNA is called transcription.

PROTEIN SYNTHESIS

The nitrogenous bases in DNA contain the instructions for making proteins. Every 3 bases in a DNA strand code for one amino acid. Many amino acids make up a protein.

Each 3 base set is called a codon.

A group of codons that together produce a specific protein is called a gene.

There are 64 possible combinations:

Example: CAA CAG

Both codons are for the A.A. glutamine.

•However, we only have about 20 amino acids, therefore most amino acids have more than 1 codon. Often the codons only differ in the last base.

The genetic code is basically “Universal.” The same codons stand for the same A.A. in all living things. This suggests that all living things came from a common ancestor.

Duplicate codons may be a way of protecting/ reducing the effects of mutations. The duplication of codons is called degeneracy.

AUG (Methionine) is a start codon, whereas UAA, UGA and UAG are stop codons.

• At the end of each mRNA strand there are long chains of adenosine (adenosine Tail, poly-A tail). This is so the body can detect that the m-RNA is still functional.

This tells the body that this particular mRNA is no longer needed to produce its protein. The cell will then digest the no longer functional m-RNA (lysosomes will be involved in this process).

• When enough protein is created the adenine tail falls off.

Types of RNA:mRNA- Messenger RNA Takes the DNA ‘message’ out of the nucleus

tRNA –Transfer RNATransfers amino acids to the ribosomes to build the protein

rRNA – Ribosomal RNATogether with proteins, forms the ribosomes where proteins are made

PROTEIN SYNTHESIS Transcription  The first step of protein

synthesis is called transcription.

Transcription is the process of taking the genetic message from DNA and converting it into mRNA.

Only RNA can leave the nucleus.

PROTEIN SYNTHESIS  A. The gene in DNA

is selected, isolated and uncoiled by enzymes. The gene codes for the protein that will be eventually produced.

B.     The gene is then unzipped by enzymes. (H bonds broken between bases)

C.     Free – floating nucleotides join complementary to the selected gene.

This DNA strand is called the template.

**Uracil joins to

adenine always in RNA. There is no thymine in RNA**

D. The Ribose – Phosphates join together to complete the backbone. The hydrogen bond that is temporarily formed breaks by enzymes and is pulled out of the DNA strand. We now have an mRNA strand. Steps A, B, C and D = Transcription

E.     mRNA is released (Enzymes break the H-bonds). DNA joins back together.

mRNA is first processed (cap & poly-A tail added, non-coding introns are removed) so it can leave the nucleus then passes through the nuclear envelope through the nuclear pores into the cytoplasm.

• F.     DNA recoils back up into its double helix shape with the help of enzymes.

transcription animation

PROTEIN SYNTHESIS Translation• The second step in protein synthesis is

called translation.

• Translation is the process of reading the code, codon by codon, and adding the corresponding amino acids to a growing polypeptide (protein). It occurs in 3 steps.

• Translation occurs at ribosomes. (cytoplasm or RER)

G. Step 1 - Initiation mRNA becomes associated with a

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

When the mRNA comes near the two subunits, the large one catches the mRNA in the smaller one.

H. The ribosome reads the first codon (first three bases – start codon methionine – AUG).

.

tRNA contains a set of bases which are complementary to the bases on the mRNA. tRNA bases are called anticodons. The ribosome bonds the codon to the anticodon.

The A.A. from the first tRNA is joined to the A.A. on the new tRNA. The first tRNA is released and goes off to find another identical amino acid.

I. Step 2 – Elongation

The ribosome moves to the next codon. A complementary tRNA arrives with an amino acid then joins to the mRNA codon.

J. Step 3 – TerminationThe ribosome continues to read the mRNA

and the polypeptide chain grows until a terminator codon (stop codon) is reached. The finished polypeptide is released.

Translation video

The mRNA will disintegrate or be read by more ribosomes. (mass production of identical proteins)

Many ribosomes may be found together, all reading the same mRNA. This group of ribosomes is called a polyribosome.

TRANSLATIONThe maximum number of tRNA on a ribosome at one time is two. MIT LEGO…

Summary of Protein Synthesis

IF the DNA strand is:

TACTTATGCTCCTAAATT  1. What are the codons on the mRNA strand?

AUG AAU ACG AGG AUU UAA

 2.  What is the sequence of amino acids produced?methionine (start codon), asparagine, threonine,

arginine, isoleucine, stop codon.