Genes as DNA: How Genes Encode Proteins

Chapter 5

Central Points

Genes made of DNA that encodes proteins

Transcription: DNA copied into mRNA

Translation: information transferred to protein

Mutations: changes in DNA

Changes in DNA produce changes in proteins

5.1 How Do Genes Control Traits?

Individuals carry two copies of each gene

One from each parent

Different forms are alleles

Genes contain information to produce proteins

Proteins contribute to the observable traits or phenotype

What Is a Protein?

Provide structure

Be enzymes

Be chemical messengers

Act as receptors

Be carrier molecules

Protein Subunits: Amino Acids

20 found in the body

amino acids have different chemical groups

All contain both a carboxyl group and an amino group

Billions of combinations possible

Chemical Structure of Amino Acids

Essential Amino Acids

Fig. 5-3, p. 86

Amino acids

How Does DNA Carry Information?

DNA carries four nucleotides: A, T, G, and C • Three nucleotide codon in messenger RNA

(mRNA) specifies one amino acid

Order of DNA bases determine the order of amino acids but not all DNA codes for proteins

Gene to Protein

Transcription: DNA mRNA

Translation: mRNA Protein

Animation: Overview of transcription

5.2 What Happens in Transcription?

First step of information transfer

Information in DNA sequence gene is copied into sequence of bases in mRNA

p. 88

RNA polymerase DNA to be transcribed Initiation

1

Promoter Terminator

Elongation2

mRNA transcriptTermination

3

mRNA

4

RNA polymerase

Completed pre-mRNA

Transcription

RNA polymerase binds to promoter, DNA is template to produce mRNA

mRNA is a complementary copy of DNA

Bases pair, except T, is replaced by U

End of the gene, marked termination sequence

mRNA processed before leaving nucleus

Animation: From DNA to proteins (gene transcription)

5.3 What Happens in Translation?

Second step, processed mRNA to the ribosome

Protein produced from information on mRNA

Each mRNA codon codes for an AA

Transfer RNA (tRNA) acts as an adaptor

Transfer RNA

Recognizes and binds to one amino acid

Recognizes the mRNA codon for that amino acid

At one end binds a specific amino acid

Other end has a 3 nucleotide anticodon that pairs with mRNA codon for specific amino acid

p. 88

Ribosome

mRNA

tRNA

Growing protein

Translation (1)

Synthesis of protein from mRNA

Occurs within ribosomes

AUG (start codon) encodes for methionine

Second AA is in position, an enzyme forms a peptide bond between the two AA

tRNA for the first AA is released

 

Translation (2)

Ribosome to next codon and repeats adding AA to growing AA chain

Stop codons (UAA, UAG, and UGA) do not code for AA and ribosome detaches from mRNA

AA chain released, folds into a 3-D protein

p. 89

DNA

TRANSCRIPTION

tRNA

mRNAtRNA

rRNANucleus

Cytoplasm

mRNA

Ribosomes

TRANSLATION

Protein

Animation: The 4 steps of translation

Genetic Code for Amino Acids

5.4 Turning Genes On and Off

Only 5–10% genes active

Gene regulation turns genes on and off

Promoter controls expression

Also, cells receive signals

Enhancers increase protein production

Controlling Gene Expression

5.5 Mutations

Changes in DNA

Produce:• Nonfunctional protein• Partially functional protein• No protein

Affect the timing and level of gene expression

Some no change

Mutagens

Increase chance of mutation

Mistakes during DNA replication

By-product of normal cell functions

Include:• Environmental factors• Radiation • Chemicals

Animation: Mutations and translation

Animation: From DNA to proteins (base substitution)

5.6 Cause of Genetic Disorders

Change in DNA alters mRNA

Single nucleotide change can alter codon and possibly amino acid

Change in amino acid sequence causes changes in • 3-D structure of protein • Defective protein folding• Protein function

Genetic Code

Disorders from Altered 3-D Shape

Cystic fibrosis

Form of Alzheimer disease

Mad cow disease

Cruzfelt-Jacob disease (CJD)

Sickle Cell Anemia

Mutation in the hemoglobin gene

Hemoglobin (HbA) is composed of two proteins: • Alpha globin • Beta globin

Single nucleotide point mutation alters one of 146 AA, affects the beta globin

Causes hemoglobin molecules to stick together

Hemoglobin Molecule

Red Blood Cells

Point Mutation in Sickle Cell Anemia

Fig. 5-9, p. 93

Valine Histidine Leucine Threonine Proline Glutamic acid

Glutamate

Valine Histidine Leucine Threonine Proline Valine Glutamate

5.7 Other Single-Gene Defects

Cystic fibrosis (CF)• Misfolded protein• Protein destroyed

Huntington disease (HD)• Trinucleotide repeats• Multiple CAG repeats