Protein Synthesis

Central Dogma

After discovering the double helix structure Crick went on to study how DNA serves as the hereditary molecule of life.

Both he and Watson reasoned that DNA must code for proteins.

Central Dogma

One puzzle that they had to figure out, though, was how DNA in the nucleus could code for proteins in the cytoplasm…

Central Dogma

Central Dogma

Crick proposed the ‘central dogma’ of biology which states that RNA serves as the carrier molecule between the DNA in the nucleus and the ribosomes in the cytoplasm.

DNA and RNA both have a sugar phosphate backbone.

However, DNA and RNA have a different sugar -

DNA and RNA also each have 4 nitrogenous bases– However…

• DNA’s bases are A, T, C, G • RNA’s bases are A, U, C, G

Further, RNA is single stranded while DNA is a double helix!

Crick Animation

http://www.dnaftb.org/dnaftb/21/concept/index.html

Go to note sheet and do transcription

TRANSCRIPTION of DNA

How does RNA polymerase know where a gene starts and stops? Enzyme binds to places with specific DNA sequences

called _______________. PROMOTERS tell _________________ where to start.

promoters

http://images2.clinicaltools.com/images/gene/dna_versus_rna_reversed.jpg

RNA polymerase

RNA

At this point we know RNA is the key. Information flows from DNA to RNA to PROTEIN. But how?– How is RNA made?– How does RNA make proteins?

Animation of Transcription

Types of RNA

Messenger RNA (mRNA)

Transfer RNA (tRNA)

Ribosomal RNA (rRNA)

How & Where is mRNA formed? All RNA is formed in the nucleus in a process

called transcription.– Once mRNA is formed it is edited and sent out to

the ribosome…– The mRNA transcript has a cap and a tail added.

– The mRNA transcript has the region coding for introns removed.

» DNA is composed of both introns and exons.

» Exons are segments of DNA that are expressed as genes.

» Introns are ‘junk DNA.’ They are not expressed and must be removed before the mRNA leaves the nucleus.

What is the typical shape of mRNA?

– mRNA (single short strand of RNA)

How and where does mRNA function?

mRNA – serves as a messenger carrying messages from the DNA in the nucleus to the ribosome in the cytoplasm– mRNA functions in the nucleus and

cytoplasm (remember it carries the message)

mRNA requires editing before leaving the nucleus…

Image by Riedell

Exons Introns

“expressed”

Exons Exit the nucleus!

Think break – wait, WHY WASTE IT?

May allow same gene to be used indifferent ways in different kinds of cells.

May have a role in evolution… allows smallchanges in genes to have a big effect.

How & Where is tRNA formed?

All RNA is formed in the nucleus in a process called transcription.– Once tRNA is formed it heads right to the

cytoplasm

What is the typical shape of tRNA?

– tRNA (folded strand of RNA, forms a t shape)

How and where does tRNA function?

tRNA – transfers amino acids from the cytoplasm to the ribosome– tRNA functions in the cytoplasm

(remember it gathers the amino acids)

How & Where is rRNA formed?

All RNA is formed in the nucleus in a process called transcription.– Once rRNA is formed it goes to the

nucleolus to be modified into a ribosome.

What is the typical shape of rRNA?

– rRNA (folded strand of RNA, forms a ribosome)

– Consists of 2 subunits

How and where does rRNA function?

rRNA – functions as the ribosome (makes proteins) – rRNA functions in the cytoplasm

(remember it is the ribosome)

Animation of Translation

Images modified from © Pearson Education Inc, publishing as Pearson Prentice Hall. All rights reserved

Amino Acid

tRNA

anticodon

mRNA strand

rRNA

Step 2: Translation

The 3 Types of RNA work together to make proteins…

Let’s check it out! (overview of t & t)

• This analogy often helps people remember the roles of RNA –

• Think of 3 people baking a cake…– mRNA – brings the recipe– tRNA – brings the ingredients– rRNA – puts the ingredients together using the

recipe / it is the chef!

Protein (polypeptide)

Translation

Transcription

DNA replication

DNA

Nucleus

mRNA

codon

rRNA

 The m-RNA Code

• 64 possible codons

• Some amino acidshave more than onecodon.

Codons code for different amino acids

UCG-CAC-GGU

Serine- Histidine- Glycine