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chapter 5

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BLG 144
Ron Babin

Genes, DNA, RNA  Nucleic acids carry the genetic code that determines the order of amino acids in proteins  Genetic material stores information, can be replicated, and undergoes mutations  Differs from proteins as it has phosphorus and NO sulphur DNA Deoxyribonucleic Acid  Nucleotides are smaller units of long chains of nucleic acids. Each nucleotide has o A pentose sugar (deoxyribose in DNA, ribose in RNA) o A phosphate group o An organic base which fall into 2 groups,  Purines (double rings of C and N - bigger)  Adenine or Guanine  Pyrimidines (single ring of C and N - smaller)  Thymine or Cytosine  Base pairing by weak hydrogen bonds  Adenine-Thymine 2 H- bonds  Cytosine-Guanine 3 H- bonds  Chains are directional according to the attachment between sugars and phosphate group  They are antiparallel which is essential for gene coding and replication  DNA molecule has 2 separate chains of nucleotides hold together by base pairing / DNA normally twist into a helix (coil) / forms a double helix Ribonucleic Acid (RNA)  Ribose instead of deoxyribose  Single chain (shorter than DNA - lower molecular mass)  Base difference: Uracil instead of Thymine. Adenine, Guanine and Cytosine are the same o Ribosomal RNA (rRNA)  Located in the cytoplasm - ER  Reads mRNA code and assembles amino acids in their correct sequence to make a functional protein (translation) o Messenger RNA (mRNA)  Commutes between nucleus and cytoplasm  Copies the code for a single protein from DNA (transcription)  Carries the code to ribosomes in the cytoplasm o Transfer RNA (tRNA)  In the cytoplasm  Transfer amino acids from the cytoplasm to the ribosomes The Genetic Code  DNA codes for assembly of amino acids / forms a polypeptide chain (proteins - enzymes)  The code is read in a sequence of three bases called o Triplets on DNA e.g. CAC TCA o Codons on mRNA e.g. GUG AGU o Anticodons on tRNA e.g. CAC UCA o (must be complementary to the codon of mRNA)  Each triplet codes for one amino acid / single amino acid may have up to 6 different triplets for it due to the redundancy of the code / code is degenerate. Some amino acids are coded by more than one codon  Same triplet code will give the same amino acid in virtually all organisms, universal code  We have 64 possible combinations of the 4 bases in triplets, 43  No base of one triplet contributes to part of the code next to it, non-overlapping  Few triplets code for START and STOP sequences for polypeptide chain formation  eg START AUG and STOP UAA UAG UGA DNA Replication (Semi-Conservative Replication)  Happens during Interphase 'S'  Separate the strands, a little at a time to form a replication fork  Events: o Unwinding / Enzyme DNA helicase separates 2 strands of DNA by breaking hydrogen bonds o Semi-conservative replication / each strand acts as a template for the formation of a new strand o Free DNA molecules join up to exposed bases by complementary base pairing  Adenine with Thymine (A=T 2 -H bonding)  Cytosine with Guanine (CΞG 3 -H bonding) o For the new 5' to 3' strand the enzyme DNA polymerase catalyses the joining of the separate nucleotides o "All in one go" → completed new strand o For the 3' to 5' strand DNA polymerase produce
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