From Gene to ProteinFrom Gene to Protein

Genes code for...Genes code for...

Proteins

RNAs

Remember...Remember...

a protein is made of monomers called amino acids

amino acids bond together to form a polypeptide - the primary structure of a protein

DNA vs RNADNA vs RNA

Gene expression is Gene expression is accomplished through two accomplished through two

basic processes...basic processes...Transcription = DNA to mRNA

Translation = mRNA to a polypeptide

Some of the RNAs Some of the RNAs involved in gene involved in gene

expression...expression...

mRNA (messenger)

tRNA (transfer)

rRNA (ribosomal)

TranscriptionTranscriptionTranscription factors bind to the TATA box on the DNA strand

RNA polymerase pries the two strands of DNA apart

RNA polymerase joins RNA nucleotides together that are complementary to the DNA template

RNA polymerase works only in the 5' to 3' direction - just like DNA polymerase

TranslationTranslation

mRNA transcript is brought to the ribosome

Initiation = the rRNA, mRNA transcript, and tRNA carrying methionine bind together

Elongation = amino acids are added one by one to create the polypeptide

Termination = when a stop codon is reached on the mRNA

tRNA

What two processes ensure that the correct amino acid is added to a growing

polypeptide chain?

There are 64 codons. Three are stop codons, so we can say there are 61 amino acid coding codons. However, there are only 45 tRNA molecules that exist. How is this possible?

The need for The need for molecular molecular

recognition... recognition...

Pairing of the tRNA anticodon and mRNA codon, but wobble allows for some mistakes

Pairing of the tRNA with the amino acid, accomplished by aminoacyl-tRNA synthetases

The Genetic CodeThe Genetic Code

mRNA is read three bases at a time

3 mRNA bases = a codon

The Genetic CodeThe Genetic Code

The Genetic CodeThe Genetic Code

RibosomesRibosomes

Bring mRNA and tRNAs together

3 sites = A, P, and E

P= holds the tRNA that is attached to the growing polypeptide chain

A = holds the tRNA carrying the next amino acid to be added to the chain

E = is where discharged tRNAs leave the ribosome

How can human cells make 75,000- 100,000 different proteins, given that there are about 20,000 human genes?

Transcript ProcessingTranscript Processing

5' Cap and Poly A Tail5' Cap and Poly A Tail

5' Cap and Poly A tail5' Cap and Poly A tail

Facilitate the export on mRNA from the nucleus

Help protect the mRNA from degradation from hydrolysis enzymes in the cytoplasm

Help ribosomes attach to the 5' end of the mRNA prior to translation

RNA SplicingRNA Splicing

The average transcription unit is 27,000 bases long.

The average protein is 400 amino acids long, requiring only 1,200 RNA bases.

This means that most mRNA transcripts are initially too long and contain a lot of non-coding segments.

RNA Splicing RNA Splicing continued...continued...

Non-coding segments (introns) of the mRNA transcript are dispersed among the coding segments (exons)

Introns are removed from the mRNA transcript prior to it leaving the nucleus.

This forms a mRNA transcript with a continuous coding sequence

RNA Splicing RNA Splicing continued...continued...

The signal for RNA splicing is a short nucleotide sequence at each end of an intron

snRNPs (small nuclear ribonucleoproteins) recognize these sequences

SplicesomesSplicesomes

Includes multiple snRNPs and proteins

Cuts out introns and pastes together exons

A Note...A Note...A single gene can code for more than one type of polypeptide

This is because of alternative RNA splicing

Whether mRNA segments are treated as introns or exons depends on the polypeptide product being expressed

For this reason the number of protein products an organism produces can be much greater than its number of genes

Mutations!Mutations!The ultimate source of new genes and, therefore, genetic diversity.

Types of mutations...Types of mutations...

Mutations are caused Mutations are caused by...by...

Physical mutagens (e.g. radiation)

Chemical mutagens (e.g. tobacco)

Chemical mutagens that cause cancer are called carcinogens

Exposure to mutagens increases over an organism's lifetime

So, what is a gene?So, what is a gene?

A discrete unit of inheritance that affects phenotype

A specific segment of a chromosome

A specific nucleotide sequence on a DNA molecule

A DNA sequence that codes for a specific polypeptide product

Let's use...Let's use...

Gene = a region of DNA that can be expressed to produce a final functional product that is either a polypeptide or an RNA molecule

MutationsMutations

Silent mutations do not change what the gene expresses

Frameshift mutations can change what, if anything, the gene expresses

Exit SlipExit Slip

How are transcription and translation similar? How are they different?