Chapter 17: From Gene to Protein

DNA makes RNA makes Protein

Summary of Transcription and Translation

A triplet code in DNA is transcribed into a codon sequence in mRNA

This pre-RNA is processed in the nucleus

The codon sequence is then translated into an amino acid sequence at the ribosome

DNA makes RNA makes Protein

If the strand of DNA triplets to be transcribed is: 5’ – AAA TAA CCG GAC – 3’

Then the strand of mRNA codons that forms is: 3’ – UUU AUU GGC CUG – 5’

The tRNA anticodon strand complementary to the mRNA strand is:

AAA UAA CCG GAC

Transcription

Transcription is the process by which DNA makes RNA

mRNA: messenger; carries messages directly from DNA to cytoplasm

tRNA: shaped like cloverleaf; carries amino acids to the mRNA at the ribosome

rRNA: ribosomal; makes up ribosome

3 Stages: initiation, elongation & termination

Initiation RNA polymerase recognizes and binds

to DNA at the promoter region (facilitates transcription of a gene)

A collection of proteins called transcription factors recognize a key area within the promoter called the TATA box and mediates the binding of RNA polymerase to DNA

Transcription factors + RNA polymerase + promoter = transcription initiation complex

3 Stages: initiation, elongation & termination

Now that RNA polymerase has attached to the promoter, transcription of the DNA template can begin

3 Stages: initiation, elongation & termination

Elongation RNA polymerase adds nucleotides to the

3’ end of a growing chain RNA polymerase pries the two strands of

DNA apart and attaches RNA nucleotides according to the base pairing rules: C with G, A with U

Stretch of DNA transcribed into mRNA molecule is called a transcription unit

3 Stages: initiation, elongation & termination

Each unit consists of triplets of bases called codons (AAU, CGA), which code for specific amino acids

3 Stages: initiation, elongation & termination

Termination Final stage of transcription Elongation continues for a short distance

after the RNA polymerase transcribes the termination sequence (AAUAAA)

mRNA is now cut free from the DNA template

RNA Processing

Pre-RNA that has just formed cannot be shipped to the ribosome until it is processed by a series of enzymes

A 5’ cap consisting of modified guanine nucleotide is added to the 5’ end and helps RNA bind to the ribosome

RNA Processing

A poly (A) tail, consisting of a string of adenine nucleotides, is added to the 3’ strand Protects RNA from degradation, helps

ribosomes attach to RNA, facilitates release of RNA into cytoplasm

Noncoding regions of mRNA called introns or intervening sequences are removed by snRNPs and spliceosomes Allows exons, or expressed

sequences, to only leave the nucleus

Translation

Translation is the process by which the codons of an mRNA sequence are changed into an amino acid sequence

Amino acids are present in a pool in the cytoplasm

They are carried by tRNA molecules to the codons of the mRNA strand at the ribosome according to base pairing rules

Translation

One end of the tRNA molecules holds the specific amino acid; the other end bears a nucleotide triplet called the anticodon

Energy for this process is provided by GTP

AUG is the start codon; UAA, UGA and UAG are the stop codons

Translation

Some tRNA molecules have anticodons that can recognize two or more different codons This happens because pairing rules for

the third base of a codon are not as strict as they are for the first two bases ; this is called wobble

Example: UCU, UCC, UCA and UCG all code for serine

Initiation, Elongation & Termination

Initation mRNA attaches to a subunit of the

ribosome The first codon is always AUG, which

codes for methionine

Elongation tRNA brings amino acids to the ribosome

and a polypeptide chain is formed

Initiation, Elongation & Termination

Termination The ribosome reaches one of three

termination or stop codons A release factor breaks the bond

between the tRNA and the last amino acid of the polypeptide chain

Polypeptide is then freed and mRNA is broken down

Gene Mutation

Mutations are changes in genetic material that occur spontaneously and randomly

Mutagenic agents include toxic chemicals and radiation

Genetic disorders or hereditary diseases occur when mutations have adverse effects in somatic cells

Gene Mutation

Mutations occurring in gametes can be passed on to offspring and change the gene pool of a population

Mutations are the raw material for natural selection

Point Mutation

Simples mutation caused by a base-pair substitution

Example: THE FAT CAT SAW THE DOG THE FAT CAT SAW THE HOG

Remember wobble If the final base in a sequence is

different, it may not call for a harmful mutation

Insertion or Deletion

Deletion = loss of a letter Insertion = addition of a letter

Both result in a frameshift

Example: Deletion THE FAT CAT SAW THE DOG THF ATC ATS AWT HED OG

Insertion or Deletion

Example: Insertion THE FAT CAT SAW THE DOG THE FTA TCA TSA WTH EDO G

A mutated polypeptide is formed or no polypeptide is formed

When point mutations or frameshifts change a codon within a gene into a stop codon, a missense or nonsense mutation occurs