RNA, & PROTEIN

SYNTHESIS 7th Grade, Week 4, Day 1

Monday, July 15, 2013

The Central Dogma

RNA vs. DNA

Ribonucleic Acid • RNA is required for translation of genetic

information stored in DNA into protein products

• Transcribed from DNA

• Contains ribose sugar instead of deoxyribose

• Single stranded instead of double stranded

• Uracil instead of

thymine

Types of RNA

• Precursor mRNA (pre-mRNA): an immature form of

messenger RNA that contains introns and exons

• Messenger RNA (mRNA): contains only exons that form

the code for the sequence of amino acids that makes up a

protein

• Transfer RNA (tRNA): decodes the message contained in

mRNA and allows for the synthesis of proteins

• Ribosomal RNA (rRNA): forms part of the structure of

ribosome

Transcription

• Making RNA from DNA

• Occurs in the nucleus

• RNA Polymerase

synthesizes pre-mRNA

from DNA in the 5’ 3’

direction

• Happens in much the same

way that DNA polymerase

synthesizes new strands of

DNA during replication

Transcription • Template (antisense) strand is transcribed, while the other

(sense) strand remains inactive

• Uracil bonds with Adenosine in place of Thymine

• Initial pre-mRNA made contains both exons and introns

Splicing

• In order for the pre-mRNA

to leave the nucleus and

travel to the ribosomes in

the cytoplasm for

translation, it must be

made into mature mRNA

• This happens by removing

the introns so that only

exons are left

• An enzyme complex called

a spliceosome performs

this task

Alternative Splicing

• Not all exons are always

left – sometimes they get

spliced out as well

• Therefore, the same pre-

mRNA can be spliced

differently to get different

gene products (proteins)

and create diversity

RNA Processing

• Addition of a poly-adenosine (poly-A) tail and a 5’ cap before the mRNA can exit the nucleus and move into the cytoplasm

• The cap: is a modified guanine (G) • protects the RNA from

being degraded by enzymes that degrade RNA from the 5′ end;

• serves as an assembly point for the proteins needed to recruit the small subunit of the ribosome to begin translation.

Proteins

• DNA acts like a blueprint that determines the structure of

every protein made in your body

• Every protein is made up of amino acids

• There are 20 amino acids:

• Essential: must be supplied in the diet

• Non – essential: synthesized de novo

• We obtain most of our amino acids by digesting proteins

taken in with our food.

• The digestive process breaks the protein chains down

into individual amino acid molecules which are then

absorbed by the blood and transported to the individual

body cells.

Proteins • During protein synthesis, the separate amino acids are

reassembled into new chains. Each kind of protein has its

own particular sequence of amino acids, which differs

from the sequence in every other kind of protein.

• Just the way the order of letters in a word give it its own

specific form and meaning, it is the order of the amino

acids in the chain that determines the protein's structure

and function.

• The code for ordering the amino acids of a protein is

written as a sequence of bases in the DNA in the nucleus.

The Genetic Code

• Triplet code: codons are

made of 3 nucleotide

bases which are non-

overlapping

• The system is redundant –

amino acids are encoded

by more than one codon

• Practice – Translate:

5’ – AUG ACU AAU GCU

UAA – 3’

Translation

• mRNA is translated into

amino acids using the

genetic code, which are

then assembled into a

protein

• This process takes place

in the cytoplasm on

ribosomes

• Both mRNA and tRNA are

necessary for this process

Steps in Translation • Ribosomes bind mature

mRNA

• There are about 32 different tRNA molecules

• Each tRNA molecule has an anticodon that is complementary to a codon on mRNA coding for a certain amino acid (so most amino acids have more than one tRNA that will code for them)

• The tRNA will then retrieve that amino acid and bring it to the ribosome for protein assembly

Ribosomes

• Made up of rRNA

• Composed of two subunits

– one large and one small

• Ribosomes can be free in

the cytosol or membrane

bound to the endoplasmic

reticulum (ER) called the

"rough ER“

• Where the process of

protein assembly is carried

out

A U G G G C U U A A A G C A G U G C A C G U U

It brings an amino acid to the first three bases

(codon) on the mRNA.

Amino acid

tRNA molecule

anticodon

U A C

A transfer RNA molecule arrives.

The three unpaired bases (anticodon) on the

tRNA link up with the codon.

A U G G G C U U A A A G C A G U G C A C G U U

Another tRNA molecule comes into place,

bringing a second amino acid.

U A C

Its anticodon links up with the second codon on

the mRNA.

A U G G G C U U A A A G C A G U G C A C G U U

A peptide bond forms between the two amino

acids.

Peptide bond

A U G G G C U U A A A G C A G U G C A C G U U

The first tRNA molecule releases its amino acid and

moves off into the cytoplasm.

A U G G G C U U A A A G C A G U G C A C G U U

The ribosome moves along the mRNA to the next

codon.

A U G G G C U U A A A G C A G U G C A C G U U

Another tRNA molecule brings the

next amino acid into place.

A U G G G C U U A A A G C A G U G C A C G U U

A peptide bond joins the second and third

amino acids to form a polypeptide chain.

A U G G G C U U A A A G C A G U G C A C G U U

The polypeptide chain gets longer.

The process continues.

This continues until a termination (stop)

codon is reached.

The polypeptide is then complete.

Rules of Translation

• The start codon is AUG which codes for methionine

• What does this mean about the first amino acid in every protein?

• Chain elongation continues on the ribosome as it reads

the mRNA strand

• Translation continues until a stop codon is reached

• The stop codons, unlike the start codon, do not encode amino

acids

• Completed protein is then folded with help from

chaperones

But what happens when this process

goes wrong?

Mutations

• Frame shift mutation: adding or deleting one base causes

a change in the reading frame…why?

• Missense mutation: a base change that results in

substituting one amino acid for another

• Nonsense mutation: a base change that results in

substituting a stop codon in place of an amino acid

• This results in early termination of the protein

• Silent mutation: a base change that results in no change

in the encoded amino acid (or stop codon)

• Why could this happen?

• Are these still dangerous? Why or why not?