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Chapter 14

BIO316 Chapter 14 - Molecular Genetics

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University of Toronto Mississauga
Don Mc Kenzie

CHAPTER 14: MOLECULAR GENETICS  Genes are composed of DNA which contain information coded in the sequence of its base pairs, providing the cell with a blueprint for protein synthesis  DNA has the ability to self-replicate and is the basis of heredity DNA A. Structure o The nucleotide is the basic unit of DNA  Made of a deoxyribsoe bonded to a P group and nitrogenous base o Two types of bases: purines (double-ringed) and pyrimidines (single-ringed) o Purines in DNA are adenine (A) and guanine (G) o Pyrmidines in DNA are cytosine (C) and thymine (T) o Nucleotides bond together to form polynucleotides  The 3’ hydroxyl group of the sugar on one nucleotide is joined to the 5’ hydroxyl group of the adjacent sugar by a phosphodiester bond o Is a double-stranded helix with a sugar-phosphate backbone  T always forms two hydrogen bonds with A  G always forms three hydrogen bonds with C  This base-pairing forms ‘rungs’ on the interior of the double helix that link the two polynucleotide chains together o The strands are positioned antiparallel to each other  One strand has a 5’  3’ polarity  The complementary strand has a 3’  5’ B. DNA replication (eukaryotic) 1. Semiconservative Replication  The helix unwinds and each strand acts as a template for complementary base-pairing in the synthesis of two new daughter helices  The daughter helix contains the parent helix and a new synthesized helix o DNA replication is semiconservative 2. Origin of Replication  Begins at specific sites along the DNA called origin or replication o Proceeds in both directions simultaneously and forms a replication fork  Rate of about 50 nucleotide additions per second (in mammals) 3. Unwinding and Initiation  DNA helix unwound by helicase  Single-strand binding (SSB) proteins prevents the strands from recoiling  DNA gyrase (topoisomerase) introduces negative supercoils to the DNA molecule to enhance the helicase action  A primer (made of RNA) is necessary for the initiation process o Primase synthesizes the primer  The primer binds to a segment of DNA complementary and serves as the site for nucleotide addition  The first nucleotide binds to the 3’ end of the primer chain 4. Synthesis  Proceeds in the 5’  3’ direction  Catalyzed by DNA polymerases  As the helix unwinds, free nucleotides are aligned opposite the parent strand and form phosphodiester linkages o The bases form H-bonds with their components  One daughter strand is the leading strand that is continuously synthesized by DNA polymerase in the 5’  3’ direction  The other strand is the lagging strand synthesized discontinuously in the 5’  3’ direction o Forms a series of short segments called Okazaki fragments o The overall growth of this strand occurs in the 3’  5’ direction o Each fragment is started with a primer and these RNA primers will be removed and replaced with DNA o The fragments are linked covalently by DNA ligase RNA  Similar to DNA except that its sugar is ribose, contains uracil (U) instead of thymine, and it is usually single-stranded  Can be found in both the nucleus and the cytoplasm  There are several types A. Messenger RNA (mRNA) o Carries the complement of a DNA sequence and transports it from the nucleus to the ribosomes where protein synthesis occurs o Monocistronic – one mRNA strand codes for one polypeptide B. Transfer RNA (tRNA) o Found in the cytoplasm o Aids in the translation of mRNA’s nucleotide code into a sequence of amino acids o Brings amino acids to the ribosomes during protein synthesis o There is at least one type of tRNA for each amino acid  Therefore, 40 know types of tRNA C. Ribosomal RNA (rRNA) o Structural component and is the most abundant of all types of RNA o rRNA is synthesized in the nucleolus D. Heterogeneous nuclear RNA (hnRNA) o Large ribonucleoprotein complex that is the precursor of mRNA Protein Synthesis A. Transcription o Information coded in the base sequence of DNA is transcribed into a strand of mRNA (similar process to DNA replication) o DNA helix unwinds at the point of transcription, and synthesis occurs in the 5’  3’ direction, using only one DNA strand (the antisense strand) as a template o mRNA is synthesized by the enzyme RNA polymerase  binds to sites on the DNA called promoters to being RNA synthesis o Synthesis continues until the polymerase encounters a termination sequence o The strand is then processed and leaves the nucleus through nuclear pores B. Post-Transcriptional RNA Processing o Most eukaryotic DNA does not code for proteins  There are non-coding sequences found between the coding sequences o A typical gene consists of several coding sequences, exons, interrupted by noncoding sequences, introns. o The RNA initially transcribed is a precursor molecule, hnRNA, which contains both introns and exons.  When processed, the introns are cleaved and removed  The exons are spliced to form a mRNA molecule coding for a single polypeptide C. The Genetic Code o The DNA language (A, T, C, and G) must be translated into the language of the proteins (20 amino acids)  Done through the triplet code (i.e. codons) o The sequence of three consecutive bases codes for a particular amino acid  Universal for almost all organisms o Because there are 64 different codons possible based on the triplet code and there are only 20 amino acids, most amino acids have more than one codon specifying them  Known as degeneracy or redundancy of the genetic code D. Translation o Process whereby mRNA codons are translated mRNA codons are translated into sequence of amino acids o Occurs in the cytoplasm and involves tRNA, ribosomes, mRNA, amino acids, enzymes, and other proteins  tRNA o Brings amino acids to the ribosomes in the correct sequence for polypeptide synthesis o Recognizes both the amino acid and the mRNA codon o One end contains a three-nucleotide sequence (anti-codon) which is complementary to one of the mRNA codons o Other end is the site of amino acid attachment and consists of a CCA sequence for all tRNA o Each amino acid has its own aminoacyl-tRNA synthetase which binds to both the amino acid and its corresponding tRNA  Forms an aminoacyl-tRNA complex  Ribosomes o Composed of two subunits: a large and one small that bind together only during protein synthesis o Have three binding sites: one for mRNA, and two for tRNA  P site (peptidyl-tRNA binding site)  A site (aminoacyl-tRNA complex binding site) o The P site binds to the tRNA attached to the growing polypeptide chain, while the A site binds to the incoming aminoacyl-tRNA complex  Polypeptide Synthesis (Fig. 14.9) o Initiation  Begins when the small ribosomal subunit binds to the mRNA near its 5’ end in the presence of initiation factors  Ribosomes scan the mRNA until it binds to a start codon (AUG)  The methionine-tRNA base pairs w/ the start codon  The large ribosomal unit then binds to the small one, creating a complete ribosome i. The met-tRNA complex sitting in the P site o Elongation  Hydrogen bonds form between the mRNA codon in the A site and its complementary anticodon on the incoming aminoacyl-tRNA complex  Peptidyl transferase catalyzes the formation of a peptide bond b/w the amino acid attached to the tRNA in the A site and the met attached to the tRNA in the P site  This cycle is completed by translocation i. The ribosome advances 3 nucleotides along the mRNA in the 5’  3’ direction  The tRNA is the P site is expelled and the peptidyl-tRNA from the A site moves into the P site
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