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 14.2 Page 228

Base Pairing during Transcription DNA RNAG C A T C G T A G C A U C G T A base pairing in DNA replicationbase 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 mRNA transcript RNA polymerase Figure 14.4c Page 229

Adding Nucleotides growing RNA transcript 5’ 3’ 5’ 3’ direction of transcription Figure 14.4d Page 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 Modification unit of transcription in a DNA strand exonintron mature mRNA transcript poly-A tail 5’ 3’ snipped out exon intron cap transcription into pre-mRNA 3’5’ Figure 14.5 Page 229

RNA processing- 1. 5' cap with a modified guanine nucleotide is added. 2. At the 3' end 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 14.7 Page 230

tRNA Structure codon in mRNA anticodon amino acid OH amino-acid attachment site Figure 14.8 Page 231

Ribosomes tunnel small ribosomal subunitlarge ribosomal subunitintact ribosome Figure 14.9b,c Page 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 a-c Page 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 e-g Page 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 j-k Page 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 rRNAtRNA Mature mRNA transcripts ribosomal subunits mature tRNA

genetics/animations/transcription.htm at/molgenetics/translation.swf

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 Page 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 with base insertion altered amino- acid sequence arginineglycinetyrosinetryptophanasparagine arginineglycineleucineglutamateleucine Figure Page 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 and 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