DNA Transcription and Translation: The Central Dogma

The DNA inherited by an organism leads to specific traits by directing the synthesis of proteins

DNA directs protein synthesis in two stages, called transcription and translation

Protein Synthesis

DNA RNA protein

Transcription: RNA is made, under the direction of DNA◦messenger RNA (mRNA)

Translation: protein is made◦under the direction of mRNA ◦Occurs on ribosomes

Transcription and Translation

Transcription and Translation

Transcription:DNA-directs synthesis of RNA

RNA synthesis◦RNA polymerase pries DNA strands apart & hooks together the RNA nucleotides

◦Uses the same base-pair rules as DNA, except that RNA has uracil for thymine

Transcription

RNA is single stranded, not double stranded like DNA

RNA is short, only 1 gene long, where DNA is very long and contains many genes

RNA uses the sugar ribose instead of deoxyribose in DNA

RNA uses the base uracil (U) instead of thymine (T) in DNA.

So, in RNA, A U and C G

RNA

Table 17.1

Translation: RNA directs synthesis of a protein (= a series of amino acids)

Translation: mRNA

◦Ribosomes◦rRNA◦tRNA ◦Codons (genetic code from DNA to mRNA)

Translation

Genetic information is encoded as a sequence of base triplets=codons

The Genetic Code

DNAmolecule

Gene 1

Gene 2

Gene 3

DNA strand(template)

TRANSCRIPTION

mRNA

Protein

TRANSLATION

Amino acid

A C C A A A C C G A G T

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

Trp Phe Gly Ser

Codon

3 5

35

Codons: 3 base code for a specific amino acid

There are 20 amino acids; most have more than 1 codon

One codon is used as a START signal: it is at the start of every protein

Universal: in all living organisms

The Genetic Code

The Genetic Code

Consists of a single RNA strand

Each tRNA has an “anticodon” on one end

tRNA brings the amino acids to the ribosomes

The “anticodon” in the tRNA matches the codon on the mRNA

Transfer RNA

Transfer RNA (tRNA)

A summary of transcription and translation in a eukaryotic cell

Figure 17.26

TRANSCRIPTION

RNA is transcribedfrom a DNA template.

DNA

RNApolymerase

RNAtranscript

RNA PROCESSING

In eukaryotes, theRNA transcript (pre-mRNA) is spliced andmodified to producemRNA, which movesfrom the nucleus to thecytoplasm.

Exon

Poly-A

RNA transcript(pre-mRNA)

Intron

NUCLEUSCap

FORMATION OFINITIATION COMPLEX

After leaving thenucleus, mRNA attachesto the ribosome.

CYTOPLASM

mRNA

Poly-A

Growingpolypeptide

Ribosomalsubunits

Cap

Aminoacyl-tRNAsynthetase

Aminoacid

tRNAAMINO ACID ACTIVATION

Each amino acidattaches to its proper tRNAwith the help of a specificenzyme and ATP.

Activatedamino acid

TRANSLATION

A succession of tRNAsadd their amino acids tothe polypeptide chainas the mRNA is movedthrough the ribosomeone codon at a time.(When completed, thepolypeptide is releasedfrom the ribosome.)

Anticodon

A C C

A A AU G G U U U A U G

U A CE A

Ribosome

1

Poly-A

5

5

3

Codon

2

3 4

5

A mismatching of base pairs, at a rate of 1 per 10,000 bases.

DNA polymerase proofreads/repairs accidental mismatched pairs.

Chances of a mutation occurring at any gene is 1 in 100,000

Since the human genome is so large, mutations add up. Each of us inherited 3-4 mutations!

Mutations

Since we have about 6 billion bases of DNA in each cell, every cell in your body contains several mutations.

Most mutations are neutral: have no effect.

Only mutations in cells that become sperm or eggs—are passed on to future generations.

Mutations in other body cells only cause trouble when they cause cancer or related diseases.

Mutation Causes and Rate

Point mutations = only one base pair is involved.

Point mutations can significantly affect protein structure and function

Point mutations are caused by damage to the DNA from radiation, chemicals or occur spontaneously

Point mutations

Physical agents include high-energy radiation like X-rays and ultraviolet light

Chemical mutagens fall into several categories.

Most carcinogens are mutagenic and most mutagens are carcinogenic.

Mutagens

Tumor viruses cause cancer in various animals, including humans

About 15% of human cancers are caused by viral infections that disrupt normal control of cell division

All tumor viruses transform cells into cancer cells because the viral nucleic acid joins host cell DNA

Viral Mutagens

The change of a single nucleotide in the DNA’s template strand leads to the production of an abnormal protein

Sickle Cell Anemia is a point mutation

Point Mutation

Sickle Cell Anemia

Point mutations within a gene can be divided into two general categories◦ Base-pair substitutions◦ Base-pair insertions or deletions

Types of Point Mutations

Insertion and Deletion Mutations