Gene Expression and Gene Regulation

The Link between Genes and Proteins

•At the beginning of the 20th century, Garrod proposed: – Genetic disorders such as alkaptonuria result from

biochemical alterations or “inborn errors of metabolism”•Using a yeast, Beadle and Tatum showed that mutations:– Can produce a loss of enzyme activity– Can produce a mutant phenotype– Genes-->Proteins--> Phenotype Nobel Prize 1958

The Genetic Code

• Information transferred from DNA to mRNA is encoded in a set of three nucleotides (codons)

• Codons – Encode the information for a specific amino acid in a protein. 3

nucleotides in 1 codon

• Of 64 possible codons, 61 code for amino acids, and 3 are stop codons

• The codon AUG is a start codon and specifies methionine

The Genetic Code

9.4 The Flow of Genetic Information

Transfer of information from the linear sequence of nucleotides in DNA to the linear sequence of amino acids in a protein occurs in two steps:

– Transcription

– Translation

The Flow of Genetic Information (contd.)

• Transcription – Transfer of genetic information from the base

sequence of DNA to the base sequence of RNA, mediated by RNA synthesis

• Translation – Conversion of information encoded in the nucleotide

sequence of an mRNA molecule into the linear sequence of amino acids in a protein

What is the central The Central Dogma?

DNA

Transcription

pre-mRNACell

mRNA processingCytoplasm

Nucleus

mRNA

Translation

Polypeptide

Interaction of Components

• Transcription and translation require the interaction of – Ribosomes, – mRNA, – tRNA, – Amino acids, – Enzymes, and – Energy sources

Interaction of Components (contd.)

• Ribosomes are the sites on which protein synthesis occurs

• tRNA molecules are adapters that recognize amino acids and the nucleotide sequence in mRNA. – Each tRNA has one anticodon (the match to a

codon on a mRNA) and can only carry one type of amino acid

Transcription

• Begins when DNA unwinds and one strand is used as template to make a pre-mRNA molecule

• Has three stages: – Initiation – Elongation – Termination

ANIMATION: Transcription—A molecular view

Transcription

Initiation and Termination

• Promoter region – The region of a gene on a DNA molecule to which

RNA polymerase binds and initiates transcription

• Terminator Region– The nucleotide sequence at the end of a gene that

signals the end of transcription

Transcription of a GeneGene region

5’ Promoter region

RNA polymerase, the enzyme that catalyzes transcription

(a) RNA polymerase binds to a promoter in the DNA, along with regulatory proteins (initiation). The binding positions the polymerase near a gene in the DNA.

Only one strand of DNA provides a template for transcription of mRNA.

Transcription of a GeneNewly forming RNA transcript

DNA template winding up

DNA template unwinding

(b) The polymerase begins to move along the DNA and unwind it. As it does, it links RNA nucleotides into a strand of RNA in the order specified by the base sequence of the DNA (elongation).

The DNA double helix rewinds after the polymerase passes. The structure of the “opened” DNA molecule at the transcription site is called a transcription bubble, after its appearance.

Transcription of a GeneTranscription site

Growing RNA transcript G

(c) What happened in the gene region? RNA polymerase catalyzed the covalent bonding of many nucleotides to one another to form an RNA strand. The base sequence of the new RNA strand is complementary to the base sequence of its DNA template––a copy of the gene.

AA

G

Pre-mRNA processing

• Transcription produces large mRNA precursor molecules called pre-mRNA

• Pre-mRNA is processed in the nucleus to produce mature mRNA

Processing and Splicing mRNA

• Cap– A modified base (guanine nucleotide) attached to

the 5’ end of eukaryotic mRNA molecules

• Poly-A tail– A series of A nucleotides added to the 3’end of

mRNA molecules

Processing and Splicing mRNA

• Introns– DNA sequences present in some genes that are

transcribed, but are removed during processing and therefore are not present in mature mRNA

• Exons– DNA sequences that are transcribed, joined with other

exons during mRNA processing, and translated into the amino acid sequence of a protein

Processing and Splicing of mRNAUnit of transcription in DNA strand

Exon Intron Exon Intron Exon

Transcription into pre-mRNA

Cap Poly-A tail

Snipped out Snipped out

Mature mRNA transcript

Alternative Splicing

Smooth-muscle mRNA

Exons 1–12pre-mRNA

Striated-muscle mRNA

Mutations in Splicing Sites

• Splicing defects cause several human genetic disorders

• One hemoglobin disorder, b-thalassemia, is due to mutations at the exon/intron region and lower splicing efficiency