From DNA to Proteins Chapter 15

Functions of DNA Heredity: passing on traits from parents to offspring  Replication Coding for our traits by containing the information to make proteins  Protein Synthesis Transcription Translation

Genes Genes are units of DNA that code to make a single polypeptide (protein) Found within specific location on the chromosomes (loci) Humans have >30,000 genes How do we make a protein from the information in a gene?

Same two steps produce all proteins: 1) Transcription: DNA (Gene) is transcribed to form messenger RNA (mRNA) Occurs in the nucleus 2) Translation:  mRNA is translated to form polypeptide chains, which fold to form proteins  Occurs in ribosomes which are in the cytoplasm Steps of Protein synthesis

Transcription and Translation

RNA vs. DNA DNARNA Number of strands TwoOne NucleotidesA T G CA U G C SugarDeoxyriboseRibose LocationNucleus onlyNucleus and Cytoplasm

Three Classes of RNAs Messenger RNA (mRNA)  Carries protein-building instruction Ribosomal RNA (rRNA)  Major component of ribosomes Transfer RNA (tRNA)  Delivers amino acids to ribosomes

A Nucleotide Subunit of RNA phosphate group sugar (ribose) uracil (base) Figure 14.2 Page 228

Transcription DNA  RNA Occurs in the nucleus Requires the enzyme RNA Polymerase Consists of 3 steps:  Initiation  Elongation  Termination

RNA Polymerases No primers needed to start complementary copy RNA is made in the 5´→ 3´ direction  DNA template strand is 3´→ 5´

Steps of Transcription: Initiation RNA Polymerase binds to Promoter  Promoter: A base sequence in the DNA that signals the start of a gene DNA is unwound i.e. hydrogen bonds are broken

Transcription: Initiation

Steps of Transctription: Elongation RNA ploymerase adds complementary RNA nucleotides to one strand of DNA – Template strand Forms Pre-mRNA

Transcription: Elongation

Steps of Transcription: Termination When mRNA synthesis is complete, RNA Polymerase falls off of DNA, RNA is released from DNA, and DNA rewinds

Transcription: Termination

Transcription vs. 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

Production of mRNAs in Eukaryotes Eukaryotic protein-coding genes are transcribed into precursor-mRNAs that are modified in the nucleus Introns are removed during pre-mRNA processing to produce the translatable mRNA Introns contribute to protein variability

Messenger RNA Prokaryotes  Coding region flanked by 5´ and 3´ untranslated regions Eukaryotes  Coding region flanked by 5´ and 3´ untranslated regions (as in prokaryotes)  Additional noncoding elements

Eukaryotic Pre-mRNA Precursor-mRNA (pre-mRNA)  Must be processed in nucleus to produce translatable mRNA 5´ cap  Reversed guanine-containing nucleotide  Site where ribosome attaches to mRNA Poly(A) tail  50 to 250 adenine nucleotides added to 3´ end  Protects mRNA from RNA-digesting enzymes

Eukaryotic Pre-mRNA Introns  Non-protein-coding sequences in the pre- mRNA  Must be removed before translation Exons  Amino acid coding sequences in pre-mRNA  Joined together sequentially in final mRNA

RNA Processing

mRNA Splicing Introns in pre-mRNAs removed Spliceosome  Pre-mRNA  Small ribonucleoprotein particles (snRNP) Small nuclear RNA (snRNA) + several proteins Bind to introns Loop introns out of the pre-mRNA, Clip the intron at each exon boundary Join adjacent exons together

mRNA Splicing

Why are Introns Present? Alternative splicing  Different versions of mRNA can be produced Exon shuffling  Generates new proteins

Alternative Splicing Exons joined in different combinations to produce different mRNAs from the same gene Different mRNA versions translated into different proteins with different functions More information can be stored in the DNA

Alternative mRNA Splicing α-tropomyosin in smooth and striated muscle

The next step: Translation “Translating” from nucleic acid (DNA/RNA) “language” (nucleotides) to protein “language” (amino acids) Occurs in the ribosome within the cytoplasm Requires tRNA – transfer RNA How does the mRNA (and DNA) code for proteins? The Genetic Code

Genetic Code Information  4 nucleotide bases in DNA or RNA sequences DNA: A,T,G,CRNA: A,U,G,C  20 different amino acids in polypeptides Code  One-letter words: only 4 combinations  Two-letter words: only 16 combinations  Three-letter words: 64 combinations

Genetic Code DNA  Three-letter code: triplet RNA  Three-letter code: codon

Genetic Code

Features of the Genetic Code Sense codons  61 codons specify amino acids  Most amino acids specified by several codons (degeneracy or redundancy)  Ex: CCU, CCC, CCA, CCG all specify proline Start codon or initiator codon  First amino acid recognized during translation  Specifies amino acid methionine

Features of the Genetic Code Stop codons or termination codons  End of a polypeptide-encoding mRNA sequence  UAA, UAG, UGA Commaless  Nucleic acid codes are sequential  No commas or spaces between codons  Start codon AUG establishes the reading frame

The Genetic Code

Genetic Code is Universal Same codons specify the same amino acids in all living organisms and viruses  Only a few minor exceptions Genetic code was established very early in the evolution of life and has remained unchanged

Translation Overview

Translation Purpose To “translate” from nucleic acid “language” to protein “language” RNA  protein What is needed for translation? mRNA transcript (processed) tRNAs Ribosomes

tRNAs Transfer RNAs (tRNA)  Bring specific amino acids to ribosome  Cloverleaf shape Bottom end of tRNA contains anticodon sequence that pairs with codon in mRNAs

tRNA Structure

Ribosomes Made of ribosomal RNA (rRNA) and proteins  Two subunits: large and small

Translation Stages Initiation  Ribosome assembled with mRNA molecule and initiator methionine-tRNA Elongation  Amino acids linked to tRNAs added one at a time to growing polypeptide chain Termination  New polypeptide released from ribosome  Ribosomal subunits separate from mRNA

Initiation Initiator tRNA (Met-tRNA) binds to small subunit

Initiation Complex binds to 5´ cap of mRNA, scans along mRNA to find AUG start codon

Initiation Large ribosomal subunit binds to complete initiation

Elongation tRNA matching the next codon enters A site carrying its amino acid A peptide bond forms between the first and second amino acids, which breaks the bond between the first amino acid and its tRNA Ribosome moves along mRNA to next codon  Empty tRNA moves from P site to E site, then released  Newly formed peptidyl-tRNA moves from A site to P site  A site empty again

Elongation

Termination Begins when A site reaches stop codon Release factor (RF) or termination factor binds to A site Polypeptide chain released from P site Remaining parts of complex separated

Termination

What Happens to the New Polypeptides? Some just enter the cytoplasm Many enter the endoplasmic reticulum and move through the cytomembrane system where they are modified

Gene Expression Summary: Transcription Translation mRNA rRNAtRNA Mature mRNA transcripts ribosomal subunits mature tRNA

Gene Mutations Changes in genetic material Base-pair mutations change DNA triplet  Results in change in mRNA codon  May lead to changes in the amino acid sequence of the encoded polypeptide

Gene Mutation Types Missense mutation Nonsense mutation Silent mutation Frameshift mutation

Missense Mutation Changes one sense codon to one that specifies a different amino acid

Sickle-Cell Anemia Caused by a single missense mutation

Nonsense Mutation Changes a sense codon to a stop codon

Silent Mutation Changes one sense codon to another sense codon that specifies the same amino acid

Frameshift Mutation Base-pair insertion or deletion alters the reading frame after the point of the mutation

Mutation Rates Each gene has a characteristic mutation rate Average rate for eukaryotes is between and per gene per generation Only mutations that arise in germ cells can be passed on to next generation

Mutagens Ionizing radiation (X rays) Nonionizing radiation (UV) Natural and synthetic chemicals