Genes as DNA How Genes Encode Proteins Chapter

Genes as DNA: How Genes Encode Proteins Chapter 5

Central Points § Genes made of DNA that encodes proteins § Transcription: DNA copied into m. RNA § 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

Amino acids Fig. 5 -3, p. 86

How Does DNA Carry Information? § DNA carries four nucleotides: A, T, G, and C • Three nucleotide codon in messenger RNA (m. RNA) 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 m. RNA § Translation: m. RNA 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 m. RNA

RNA polymerase DNA to be transcribed Initiation 1 Promoter Terminator Elongation 2 m. RNA transcript Termination 3 m. RNA 4 RNA polymerase Completed pre-m. RNA p. 88

Transcription § RNA polymerase binds to promoter, DNA is template to produce m. RNA § m. RNA is a complementary copy of DNA § Bases pair, except T, is replaced by U § End of the gene, marked termination sequence § m. RNA processed before leaving nucleus

Animation: From DNA to proteins (gene transcription)

5. 3 What Happens in Translation? § Second step, processed m. RNA to the ribosome § Protein produced from information on m. RNA § Each m. RNA codon codes for an AA § Transfer RNA (t. RNA) acts as an adaptor

Transfer RNA § Recognizes and binds to one amino acid § Recognizes the m. RNA codon for that amino acid § At one end binds a specific amino acid § Other end has a 3 nucleotide anticodon that pairs with m. RNA codon for specific amino acid

Ribosome m. RNA t. RNA Growing protein p. 88

Translation (1) § Synthesis of protein from m. RNA § Occurs within ribosomes § AUG (start codon) encodes for methionine § Second AA is in position, an enzyme forms a peptide bond between the two AA § t. RNA 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 m. RNA § AA chain released, folds into a 3 -D protein

TRANSCRIPTION DNA t. RNA m. RNA r. RNA t. RNA Nucleus Cytoplasm m. RNA Ribosomes TRANSLATION Protein p. 89

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 m. RNA § 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 (Hb. A) 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

Valine Histidine Leucine Threonine Proline Glutamic Glutamate acid Valine Histidine Leucine Threonine Proline Valine Glutamate Fig. 5 -9, p. 93

5. 7 Other Single-Gene Defects § Cystic fibrosis (CF) • Misfolded protein • Protein destroyed § Huntington disease (HD) • Trinucleotide repeats • Multiple CAG repeats