Unit 5: Molecular Genetics

DNA

Transcription

Translation

Trait

RNA Protein

The “Central Dogma” of Molecular Genetics

When does it begin? • When mRNA leaves the nucleus Where does translation take place? • In the cytoplasm, on a ribosome

The information on

The mRNA istranslated fromlanguage of RNA(nucleotides) to

thelanguage of

proteins ( amino acids)

Each ‘word’ is calledA codon.

Codon - A sequenceof three nucleotideson mRNA that codefor one amino acid

There are 20 different types

of amino acids.

There are 4 bases ( A, U, G, C ). This means that there are 64 groups of three that are possible

  Scientists have discovered which

triplet of three bases codes for each amino acid. Ex: AUG codes for Methionine

 

Most of the 64 possible triplets code for an amino acid.

 Some amino acids may be specified by two, four, or even six different codons.

Example: UCA and AGU both

code for the amino acid serine.

Others require a single codon before they are added to an amino acid chain

  Example: AUG is the only codon

that codes for methionine.

 They act like punctuation marks in

a sentence.  They provide the start signal for

protein production, as well as the stop signal, when the protein is complete

Take 2 minutes to summarize what youhave learned regarding the informationthat a mRNA molecule actually contains.

What Translation Accomplishes

In translation, information present in the mRNA is read by the ribosome to synthesize a protein (polypeptide).

The sequence of amino acids determines the structure, and therefore the function, of a protein.

Translation Is Complicated

Translation requires:

ribosomes

mRNA

tRNA

amino acids

Each tRNA is foldedinto a compact shape;has a specificamino acid onone end, and anANTICODON on theother end.

Anticodon - a 3nucleotidesequence on tRNAthat is complementaryto an mRNA codon.  

 

 

 

 

Ribosomes are madeof rRNA and protein.

Each ribosome temporarily holds

1 mRNA molecule 2 tRNA molecules

* There are 3 binding sites that can be occupied at any given time **

mRNA “start” codon AUG signals the beginning of assembly of the protein chain.

tRNA carries methionine to the start codon, where it binds. This spot on the ribosome is called the P site.

Codon in the region of the ribosome called the A site is ready for the next tRNA.

tRNA with the complementary anticodon binds to the codon. It has the specific amino acid on the other end.

A peptide bond forms between the amino acids that are next to each other when both the P site and A site are occupied

tRNA molecule at P site moves to the E site, while the one at the A site slides over to the P site. (everyone shifts to the left)

The tRNA at the E site then detaches and leaves the new polypeptide chain (protein) attached to the tRNA at the P site.

The tRNA molecule then moves away from the molecule

A new codon is present at the A site ready to receive the next tRNA and its amino acid

A amino acid is carried to the A site by the tRNA and is bonded to the growing peptide chain

Steps 3-4 arerepeated until astop codon isreached.

(UAG,UAA,UGA)

Since the A site remains unoccupied, no more amino acids are added and protein synthesis stops

The newly made protein is released into the cell.

Translation Is a Cyclic, Multistep Process

Translation Animation

Take 2 minutes to summarize howproteins are assembled during

translation.

Basic Genetic Mechanisms are Universal

The storage of genetic information in DNA, the use of an RNA intermediate that is read in three letter words, and the mechanism of protein synthesis are essentially the same in all organisms.

It appears that all life forms have a common evolutionary ancestor with a single genetic code.

Among other things, this means cancer can be studied productively in flies or yeast.

A universal code also means that human genes can be expressed in a plantor mouse genes in a yeast.

A tobacco plant expressing the firefly luciferase gene.

Take 2 minutes to summarize what youhave learned by answering the following

questions:

What does it mean when we say thegenetic code is ‘universal’?

How does this help humankind makeadvancements in science?