Chapter 3 DNAandMolecularGenetics

Fig

DNA History: Structure of the Hereditary Material Experiments in the 1950s showed that DNA is the hereditary materialExperiments in the 1950s showed that DNA is the hereditary material Scientists raced to determine the structure of DNAScientists raced to determine the structure of DNA Watson and Crick proposed that DNA is a double helix Watson and Crick proposed that DNA is a double helix

Structure of DNA Nucleotides Each nucleotide consists ofEach nucleotide consists of –Deoxyribose (5-carbon sugar) –Phosphate group –A nitrogen-containing base Four basesFour bases –Adenine, Guanine, Thymine, Cytosine

Composition of DNA Chargaff showed:Chargaff showed: –Amount of adenine relative to guanine differs among species –Amount of adenine always equals amount of thymine and amount of guanine always equals amount of cytosine A=T and G=C

Watson-Crick Model DNA consists of two nucleotide strandsDNA consists of two nucleotide strands Strands run in opposite directionsStrands run in opposite directions Strands are held together by H-bonds between basesStrands are held together by H-bonds between bases A binds with T and C with GA binds with T and C with G Molecule is a double helixMolecule is a double helix

Watson-Crick Model DNA RNA

Rosalind Franklin’s Work Was an expert in x-ray crystallographyWas an expert in x-ray crystallography Used this technique to examine DNA fibersUsed this technique to examine DNA fibers Concluded that DNA was some sort of helixConcluded that DNA was some sort of helix

DNA Replication new old Each parent strand remains intactEach parent strand remains intact Every DNA molecule is half “old” and half “new”Every DNA molecule is half “old” and half “new”

Base Pairing during Replication Each old strand serves as the template for complementary new strand

Enzymes in Replication Enzymes unwind the two strandsEnzymes unwind the two strands DNA polymerase attaches complementary nucleotidesDNA polymerase attaches complementary nucleotides DNA ligase fills in gapsDNA ligase fills in gaps Enzymes wind two strands togetherEnzymes wind two strands together

DNA Repair Mistakes can occur during replicationMistakes can occur during replication DNA polymerase can read correct sequence from complementary strand and, together with DNA ligase, can repair mistakes in incorrect strandDNA polymerase can read correct sequence from complementary strand and, together with DNA ligase, can repair mistakes in incorrect strand

Protein Synthesis

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 Steps from DNA to Proteins

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

Base Pairing during Transcription A new RNA strand can be put together on a DNA region according to base- pairing rulesA new RNA strand can be put together on a DNA region according to base- pairing rules As in DNA, C pairs with GAs in DNA, C pairs with G Uracil (U) pairs with adenine (A)Uracil (U) pairs with adenine (A)

Transcription & DNA Replication Like DNA replicationLike DNA replication –Nucleotides added in 5’ to 3’ direction Unlike DNA replicationUnlike DNA replication –Only small stretch is template –RNA polymerase catalyzes nucleotide addition –Product is a single strand of RNA

Base Pairing Compared

Promoter A base sequence in the DNA that signals the start of a geneA base sequence in the DNA that signals the start of a gene For transcription to occur, RNA polymerase must first bind to a promoterFor transcription to occur, RNA polymerase must first bind to a promoter promoter region

Gene Transcription DNA template winding up DNA template unwinding newly forming RNA transcript DNA template at selected transcription site

Adding Nucleotides 3´ 5´ direction of transcription 5´ 3´ growing RNA transcript

Transcript Modification

Genetic Code Set of 64 base tripletsSet of 64 base triplets Codons Codons –Nucleotide bases read in blocks of three 61 specify amino acids61 specify amino acids 3 stop translation3 stop translation

Code Is Redundant Twenty kinds of amino acids are specified by 61 codonsTwenty kinds of amino acids are specified by 61 codons Most amino acids can be specified by more than one codonMost amino acids can be specified by more than one codon Six codons specify leucineSix codons specify leucine –UUA, UUG, CUU, CUC, CUA, CUG

Near-Universal Genetic Code

Three Stages of Translation Initiation InitiationElongationTermination

Initiation Initiator tRNA binds to small ribosomal subunitInitiator tRNA binds to small ribosomal subunit Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codonSmall subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon Large ribosomal subunit joins complexLarge ribosomal subunit joins complex

Fig

Binding Sites on Large Subunit binding site for mRNA P (first binding site for tRNA) A (second binding site for tRNA)

Elongation mRNA passes through ribosomal subunitsmRNA passes through ribosomal subunits tRNAs deliver amino acids to the ribosomal binding site in the order specified by the mRNAtRNAs 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 growsPeptide bonds form between the amino acids and the polypeptide chain grows

Elongation

Termination A STOP codon moves into the area where the chain is being built. It is the signal to release the mRNA transcript from the ribosome. The new polypeptide chain is released from the ribosome. It is free to join the pool of proteins in the cytoplasm or to enter rough ER of the endomembrane system. The two ribosomal subunits now separate also.

Fig

Polysome A cluster of many ribosomes translating one mRNA transcriptA cluster of many ribosomes translating one mRNA transcript Transcript threads through the multiple ribosomes like the thread of bead necklaceTranscript threads through the multiple ribosomes like the thread of bead necklace Allows rapid synthesis of proteinsAllows rapid synthesis of proteins

Polysome

Fig