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The blueprint of life; from DNA to Protein Biology 261 Medgar Evers College Prof. Santos • DNA, the substance of inheritance – Is the most celebrated molecule of our time • Hereditary information – Is encoded in the chemical language of DNA and reproduced in all the cells of your body • It is the DNA program – That directs the development of many different types of traits G C A T T A 1 nm C G C 3.4 nm G A T G C T A T A A T T A G C 0.34 nm A Figure 16.7a, c T (a) Key features of DNA structure (c) Space-filling model 5 end O OH Hydrogen bond P –O 3 end OH O O A T O O O CH2 P –O O H2C O –O P O O G O C O O CH2 P O O– O P H2C O O C O G O O O CH2 P –O O– O O O– O P H2C O O A O T O CH2 OH 3 end O O– P O Figure 16.7b (b) Partial chemical structure O 5 end H N N N N Sugar O H H CH3 N N N O Sugar Thymine (T) Adenine (A) H O N N Sugar N H N N N N N Figure 16.8 H H Guanine (G) H O Sugar Cytosine (C) DNA The Components and Structure of DNA DNA is made up of nucleotides. A nucleotide is a monomer of nucleic acids made up of: • Deoxyribose – 5-carbon Sugar • Phosphate Group • Nitrogenous Base There are four kinds of bases in in DNA: adenine guanine cytosine thymine Chargaff’s rule Chargaff's Rules Erwin Chargaff discovered that: • The percentages of guanine [G] and cytosine [C] bases are almost equal in any sample of DNA. – The percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA. DNA Double Helix • There are 2 hydrogen bonds between adenine and thymine and three hydrogen bonds between cytosine and guanine. DNA to RNA to Protein connection Protein synthesis 3 steps 1- transcription 2- RNA processing 3- translation transcription • 3 steps 1- initiation 2- elongation 3- termination Transcription • Copying the genetic code directly from DNA. • We make a single strand of messenger RNA. • We begin initiation by unwinding the double stranded DNA and copying only one of the strands. The strand that is copied is called the nonsense strand. It serves as a template for the production of messenger RNA. • Transcription begins when an enzyme called RNA polymerase binds to a special region of the DNA called promoter sequence. Unlike DNA polymerase, RNA polymerase doesn’t need a primer. • RNA polymerase brings free floating RNA nucleotides to the nonsense strand. • The immediate product of this transcription is a resultant initial RNA transcript, which contains a sequence of nucleotides that is identical to the that of the sense strand. The exception to this is that uracil is used for nucleotide sequencing of RNA molecules rather than thymine. Role of transcription factors • Transcription factors are proteins that aid in the process of transcription. They can be involved in promoting or blocking (repressing) the action of RNA Polymerase. • Guanine and cytosine pair up. • But, there is no thymine in RNA. Another base called Uracil pairs up with adenine. • The messenger RNA strand will continue to elongate until it reaches a termination point. RNA processing • The pre messenger RNA made must be modified before it can leave the nucleus and direct the production of proteins. • 3 steps 1- removal of introns 2- addition of cap at 5 end 3- poly A tail at 3 end • The coding regions are called exons and the non-coding regions are called introns. • The introns are removed by an enzymeRNA complex known as the spliceosome. • A tail of adenine bases is added to the 3 prime end and a modified guanine nucleotide is added to the 5 prime end. • Once the messenger RNA has been processed, it is ready to leave the nucleus and bind to a ribosome. • The mature messenger RNA carries the message from DNA in the forms of codons. • A codon is a group of 3 bases that correspond to one of the 20 amino acids. • There are 64 possible codons and only 20 amino acids. There is redundancy with some of the amino acids! • The initial codon is AUG or methionine and there are three stop codons, UAA, UGA and UAG. CUU CUC C CUA CUG CCU CCC Leu CCA CCG Pro AUU AUC A AUA AUG ACU ACC ACA ACG Thr GUU G GUC GUA GUG lle Met or start GCU GCC Val GCA GCG Ala G U UGU Cys UGC C UGA Stop A UGG Trp G U CAU CGU His CAC CGC C Arg CAA CGA A Gln CAG CGG G U AAU AGU Asn AAC AGC Ser C A AAA AGA Lys Arg G AAG AGG U GAU GGU C GAC Asp GGC Gly GAA GGA A Glu GAG GGG G Third mRNA base (3 end) First mRNA base (5 end) Figure 17.5 Second mRNA base U C A UAU UUU UCU Tyr Phe UAC UUC UCC U UUA UCA Ser UAA Stop UAG Stop UUG Leu UCG Translation • The messenger RNA attaches to the ribosome and the message in the form of codons is “translated” and the appropriate amino acid is put in place. • A molecule of RNA called transfer RNA brings along the amino acid. It resembles a four leaf clover. • On the top is the amino acid and on the bottom is a sequence known as the anti codon. 3 A C C A 5 C G G C C G U G U A A U A U U C UA C A C AG * G * G U G U * C C * * U C * * G AG C G C U A * G A * A C U * A G A Amino acid attachment site Anticodon C U C G A G A G * * G A G G Hydrogen bonds • The anticodon pairs up with the codon. This allows the amino acids to put placed in the correct sequence or order.