Lecture 5: Genetic Control PDF

GENETIC CONTROL 1 5.1 Structure and replication of DNA Learning outcomes: To describe the structure of nucleotides To describe the structure of RNA and DNA and explain the importance of base pairing and the different hydrogen bonding between bases To describe the semi-conservati...

GENETIC CONTROL 1 5.1 Structure and replication of DNA Learning outcomes: To describe the structure of nucleotides To describe the structure of RNA and DNA and explain the importance of base pairing and the different hydrogen bonding between bases To describe the semi-conservative replication of DNA 2 3  Nucleic acids Monomers: nucleotides Functions:- storage and transmission of genetic information, structural or catalytic roles There are 2 types of nucleic acids found in living organism 1. Deoxyribonucleic acid (DNA) 2. Ribonucleic acid (RNA) 4  NUCLEOTIDES Consist of 3 parts: 1. A pentose sugar (ribose/deoxyribose) 2. Nitrogenous base 3. Phosphate group sugar + nitrogenous base nucleoside Nucleotides of DNA also known as deoxyribonucleoside monophosphates Sugar phosphate backbone is linked by phosphodiester bonds (phosphate atom esterified to 2 oxygen atoms) 5 DNA RNA 6 7 DNA STRUCTURE Double helix 1 full turn= 3.4 nm 10 base pairs/turn Base pair~0.34 nm 2 strands are anti-parallel A=T; G≡C, hydrogen bonds, complementary base pairing Radius= 1 nm Sugar phosphate backbone linked by phosphodiester bonds 8 X-ray diffraction photo of DNA - Rosalind Franklin 9  Relative amount of bases A and T will always be the same as well as G and C (Chargaff’s rules) E.g. if a sample of DNA has 10% of base A, it must have 10% of base T. Challenges: Which 1/> of the following equations accurately reflects DNA base pairing??? (a) A+G = 1.0 (b) A+T= 1.0 (c) GA =T C+T G+C C 10 11  STRUCTURE OF RNA Similar to DNA except uracil replaces thymine & pentose sugar ribose replaces deoxyribose Single polynucleotide chain There are 3 types of RNA: 1. Messenger RNA (mRNA) 2. Transfer RNA (tRNA) 3. Ribosomal RNA (rRNA) 12  TYPES OF RNA 1. mRNA is formed in the nucleus. -single chain twisted into helix (length & base sequence vary) -short life and involve in protein synthesis 2. tRNA is single chain folded into clover leaf shape -structure similar always 3 bases of anticodon -protein synthesis 3. rRNA is made in nucleolus and froms over half mass of ribosomes 13  DNA REPLICATION Base pairing enables existing DNA strands to serve as templates for new complementary strands Basic principle: 1. When a cell copies a DNA molecule, 2 strands unwind/untwist and each serve as template for ordering nucleotides into a new complementary strand. 2. Enzymes link the nucleotides to form new strand 3. Produces 2 DNA strands, exact replica of ‘parent’ semi-conservative model 14 15 5.2 Protein synthesis Learning outcomes: To state that a polypeptide is coded for by a gene and that a gene is a sequence of nucleotides that forms part of a DNA molecule To describe how the information in DNA is used during transcription and translation to construct polypeptides, including the role of messenger RNA (mRNA), transfer RNA (tRNA) and ribosomes 16 17 1st expressed by Francis Crick Detailed transfer of sequential information Once the genetic information flow into protein, it cannot flow back to nucleic acids DNADNA (DNA replication) DNA mRNA (transcription) mRNA protein (translation) 18  GENETIC CODE Triplet code genetic instructions for a polypeptide chains that are written in the DNA as 3-nucleotide words DNA stores information as sequences of bases The genetic code cannot be duplet as only code for 16 amino acids (42) Degenerate code amino acids are code by more than 1 codons. E.g. AGA & AGG code for Arginine So that mutation which substitute 1 DNA base will not have effect on protein produced 19 Non-overlapping code each DNA base only contributes to code for amino acid E.g. CGTAGA will give 2 amino acids and not 4 amino acids Universal code the DNA base triplets code for the same amino acids in all organisms (same STOP and START signals) 20 GENETIC CODE 21  TRANSCRIPTION & TRANSLATION Transcription synthesis of RNA (mRNA) under the direction of DNA DNA provides a template for the assembling a unique sequence of RNA nucleotides mRNA serves as genetic message from DNA to protein-synthesizing machinery of the cell Both nucleic acids have same language (complementary) 22 Translation actual synthesis of a polypeptide Occur under the direction of mRNA Change in language (nucleotide amino acid) The cell must translate the base sequence of mRNA into amino acid sequence of a polypeptide 23  Stages of protein synthesis Stage 1: transcription - DNA strands unwind and hydrogen bonds broken (catalyzed by RNA polymerase) - Non-coding strand act as template for copying of mRNA - Free RNA nucleotides attach to the template DNA (catalyzed by RNA polymerase) - Each sequence of 3 bases on mRNA codon 24  Stage 2: mRNA carries information to ribosomes - Completed mRNA leaves nucleus via nuclear pore and enters cytoplasm - Move to ribosome Stage 3: amino acid activation - Each tRNA with different sequence of bases (anticodon) becomes attached to a particular amino acid 25  Stage 4: translation - Each codon of mRNA bases attract a tRNA with the complementary anticodon due to specific base- pairing - Each tRNA carries a particular amino acid - Peptide bonds form between amino acids polypeptide - Sequence of amino acids in polypeptide primary structure - tRNA detaches and leaves ribosome - mRNA has short life-span, broken down after several times (avoid synthesis of excess protein) 26  Stage 5: functional protein produced - Completed polypeptide is processed by rough endoplasmic reticulum and Golgi apparatus to produce final functional protein (e.g. enzyme) - Enzymes control cell activities - Thus, by controlling protein synthesis, DNA controls the structure, function and development of organisms 27 28  Transcripti Translatio Replicatio on n n mRNA copied from DNA √ x x Occurs at ribosomes x √ x DNA copied from DNA x x √ Catalyzed by RNA √ x x polymerase Catalyzed by DNA x x √ polymerase Involves joining of tRNA x √ x Involves producing codons √ x x Assembles amino acids into x √ x polypeptide Occurs in nucleus √ x √ Involves joining of codon x √ x and anticodon Occurs in cytoplasm x √ x All of the DNA unzips x x √ 29 Concept check 1. Summarize the structural differences between DNA and RNA. 2. Give 2 similarities between transcription and DNA replication. 3. The gene for 1 of the polypeptide chains in hemoglobin consists of 438 DNA bases. How many amino acids are in the polypeptide? 4. Mutations can change DNA bases causing a different amino acid to be coded for. If a mutation changes 1 base, suggest: (a) why a non-overlapping code is better than a overlapping one? (b) why a degenerate code is better than a non-degenerate code? 30