From DNA to Proteins

Same two steps produce all proteins:1) DNA is transcribed to form RNA

– Occurs in the nucleus– RNA moves into cytoplasm

2) RNA is translated to form polypeptide chains, which fold to form proteins– occurs at the ribosome

Steps from DNA to Proteins

Three Main Classes of RNAs

• Messenger RNA

– Carries protein-building instruction

• Ribosomal RNA

– Major component of ribosomes

• Transfer RNA

– Delivers amino acids to ribosomes

Differences Between DNA and RNA

1. Sugars – DNA = deoxyribose, RNA = ribose

2. DNA = double strand, RNA = single strand

3. DNA has Thymine as one base, RNA has Uracil instead

A Nucleotide Subunit of RNA

phosphate group

sugar (ribose)

uracil (base)

Figure 14.2Page 228

Base Pairing during Transcription

DNA

DNA DNA

RNAG C A T

C G T A

G C A U

C G T A

base pairing in DNA replication base pairing in transcription

Transcription & DNA Replication

• Like DNA replication– Nucleotides added in 5’ to 3’ direction

• Unlike DNA replication– Only small stretch is template

– RNA polymerase catalyzes nucleotide addition

– Product is a single strand of RNA

• A base sequence in the DNA that signals the start of a gene (TATA)

• For transcription to occur, RNA polymerase must first bind to a promoter (in eukaryotes this requires many transcription factors)

• Once RNA polymerase is bound it will unwind the DNA and nucleotides can be added

Gene Transcription

transcribed DNA winds up again

DNA to be transcribed unwinds

mRNAtranscript

RNA polymerase

Figure 14.4cPage 229

Adding Nucleotides

growing RNA transcript5’

3’5’

3’

direction of transcription

Figure 14.4dPage 229

Once the termination site is reached, the RNA molecule will be released. In eukaryotic cells, the RNA polymerase will actually pass the termination point before the RNA molecule is released.

Transcript Modificationunit of transcription in a DNA strand

exon intron

mature mRNA transcript

poly-A tail

5’

5’ 3’

3’

snipped out

snipped out

exon exonintron

cap

transcription into pre-mRNA

3’ 5’

Figure 14.5Page 229

RNA processing-

1. 5' cap with a modified guanine nucleotide is added.

2. At the 3' end 30-200 adenine nucleotides are added (poly-A-tail).

-These modifications prevent the mRNA from being degraded and signal the ribosome where to attach.

3.There are noncoding regions (introns) that are removed in eukaryotic cells. The remaining regions (exons) are joined together. A particle called a spliceosome removes the introns. Spliceosomes are composed of smaller particles called snRNP (made of proteins and snRNA).

-The average immature RNA is 8000 nucleotides long but the mature mRNA is 1200 nucleotides long.

Genetic Code

• Set of 64 base triplets

• Codon = 3 bases

• 61 specify amino acids. There is "redundancy" in the code or more than one codon codes for the same amino acid. One that codes for methionine also codes for “start”.

• 3 stop codons

Figure 14.7Page 230

tRNA Structure

codon in mRNA

anticodon

amino acid OH

amino-acidattachment site

Figure 14.8Page 231

Ribosomestunnel

small ribosomal subunit large ribosomal subunit intact ribosome

Figure 14.9b,cPage 231

Three Stages of Translation

Initiation

Elongation

Termination

Initiation

• Initiator tRNA binds to small ribosomal subunit

• Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon

• Large ribosomal subunit joins complex

Fig. 14.10a-cPage 232

Elongation

• mRNA passes through ribosomal subunits

• tRNAs deliver amino acids to the ribosomal binding site in the order specified by the mRNA

• Peptide bonds form between the amino acids and the polypeptide chain grows

Elongation

Fig. 14.10e-gPage 233

Termination

• Stop codon into place

• No tRNA with anticodon

• Release factors bind to the ribosome

• mRNA and polypeptide are released

new polypeptide chain

mRNA

Fig. 14.10j-kPage 233

What Happens to the New Polypeptides?

• Some just enter the cytoplasm

• Many enter the endoplasmic reticulum lumen and move through the cytomembrane system where they are modified

Overview

Transcription

Translation

mRNA rRNA tRNA

Mature mRNA transcripts

ribosomal subunits

mature tRNA

http://www.fed.cuhk.edu.hk/~johnson/teaching/genetics/animations/transcription.htm

http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.swf

http://www.youtube.com/watch?v=WsofH466lqk

Gene Mutations

Base-Pair Substitutions (point mutation)

Insertions (frameshift)

Deletions (frameshift)

Base-Pair Substitution

original base triplet in a DNA strand

During replication, proofreading enzymes make a substitution

a base substitution within the triplet (red)

original, unmutated sequence

a gene mutation

possible outcomes:

or

Figure 14.11Page 234

•Results in 1 wrong amino acid•Protein may still function

Frameshift Mutations

• Insertion

– Extra base added into gene region

• Deletion

– Base removed from gene region

• Both shift the reading frame

• Result in many wrong amino acids

Frameshift Mutation

mRNA parental DNA amino acids

altered mRNA

DNA withbase insertion

altered amino-acid sequence

arginine glycine tyrosine tryptophan asparagine

arginine glycine leucine glutamateleucine

Figure 14.12Page 234

Transposons

• DNA segments that move spontaneously about the genome

• When they insert into a gene region, they usually inactivate that gene

Mutation Rates

• Each gene has a characteristic mutation rate

• Average rate for eukaryotes is between 10-4 and 10-6 per gene per generation

• Only mutations that arise in germ cells can be passed on to next generation

Mutagens – things that can lead to mutations

• Ionizing radiation (X rays)

• Nonionizing radiation (UV)

• Natural and synthetic chemicals