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Introduction to Central Dogma: DNA Replication, Transcription and Translation What is Central Dogma? • It is the flow of genetic information from DNA to Proteins Transcription DNA mRNA Replication DNA Translation Proteins What is DNA? • It is the fascinating molecule that contains the Code of Life. What Is the Code of Life? • It is the Genetic Code, which is the set of “messages” that are “translated” by ribosomes into proteins that define YOU Structure of Nucleic Acids • Recall that DNA and RNA belong to the class of biomolecules called Nucleic Acids, which are made up of nucleotide monomers Structure of Nucleotides The nucleotide subunits consist of: 3. A Phosphate Group 1. A Nitrogen Base 2. A Sugar (either Deoxyribose or Ribose) Nitrogen Bases Nitrogen Bases Can Be: Adenine A Guanine G Cytosine C Thymine (only DNA) T Uracil (only RNA) U Chargaff’s Rule of Base Pairing Erwin Chargaff analyzed DNA from different organisms and found that A=T & G=C According to Chargaff, in DNA: A always bonds with T G always bonds with C The Sugar Group The Sugar Group Can Be Either: Deoxyribose (in DNA) orRibose (in RNA) Image by: Riedell Purines & Pyrimidines Purines consist of Adenine and Guanine bases; they are double-ring structures Pyrimidines consist of Thymine and Guanine bases; they are single-ring structures A Purine ALWAYS bonds with a Pyrimidine….A with T and G with C Nitrogen Base Difference DNA & RNA DNA has no URACIL RNA has no THYMINE Bonding in DNA • The nitrogen bases are held together by weak hydrogen Covalent bonds Phosphodiester Bonds • The sugar and phosphate groups are held together by strong covalent phosphodiester bonds DNA Replication (occurs in the nucleus) Semi-Conservative: • Strand separation, followed by copying of each strand. • Each separated strand acts as a template for the synthesis of a new complementary strand. DNA Replication (occurs in the nucleus) Step 1: • Unwinding of the double helix Helicase enzyme unwinds the double helix & creates a replication fork DNA Replication (occurs in the nucleus) Step 2: • One DNA strand is used as a template to create the new copy DNA DNA A G C A A – – – – – T C G T T etc DNA Transcription (occurs in the nucleus) • Transcription produces genetic messages in the form of mRNA mRNA DNA Template DNA Transcription (occurs in the nucleus) • STEP 1: As in replication, the DNA mRNA double helix unzips • STEP 2: RNA nucleotides line up along one strand of DNA, following the base-pairing rules DNA Template DNA Transcription (occurs in the nucleus) • STEP 3: DNA is transcribed into RNA DNA G T A G G T mRNA C A U G C A mRNA DNA Template DNA Transcription (occurs in the nucleus) • Single-stranded mRNA peels away from DNA and prepares to move into the cytoplasm DNA Nucleus mRNA Cytoplasm DNA Transcription (occurs in the nucleus) • Eukaryotic RNA is processed before leaving the nucleus The non-coding segments, called introns, are spliced out A cap & tail are added to the ends Exon Intron Exon Intron Exon DNA Cap RNA transcript with cap and tail Transcription Addition of cap and tail Introns removed Tail Exons spliced together mRNA Coding sequence NUCLEUS CYTOPLASM DNA Translation (occurs in the cytoplasm) • mRNA leaves the nucleus and moves into the cytoplasm where it will be “translated” into a polypeptide (a fancy word for a protein) TRANSCRIPTION NUCLEUS TRANSLATION PROTEIN CYTOPLASM DNA Translation (occurs in the cytoplasm) • • The words that will be translated are triplets of mRNA bases called codons The codons in a gene determine the amino acids in the polypeptide sequence DNA molecule Gene 3 Gene 1 Gene 2 DNA strand TRANSCRIPTION RNA Codon TRANSLATION Polypeptide Amino acid Codon Codon Codon DNA Translation (occurs in the cytoplasm) • • In the cytoplasm, a ribosome attaches to the mRNA A tRNA pairs with each codon, adding an amino acid to the growing polypeptide mRNA Genetic Code Chart Second Base C U UUU UUC UUA UUG C CUU CUC CUA CUG A AUU AUC ile AUA AUG met (start) ACU ACC ACA ACG G GUU GUC GUA GUG GCU GCC GCA GCG phe leu leu val UCU UCC UCA UCG CCU CCC CCA CCG A ser UAU UAC UAA UAG pro CAU CAC CAA CAG thr AAU AAC AAA AAG ala GAU GAC GAA GAG G tyr stop stop his gln asn lys asp glu UGU UGC UGA UGG CGU CGC CGA CGG AGU AGC AGA AGG GGU GGC GGA GGG cys stop trp arg ser arg gly U C A G U C A G U C A G U C A G Third Base First Base U Table 14.2 Types of RNA Type of RNA Functions in Messenger RNA (mRNA) Nucleus, migrates to ribosomes in cytoplasm Transfer RNA (tRNA) Cytoplasm Provides linkage between mRNA and amino acids; transfers amino acids to ribosomes Ribosomal RNA (rRNA) Cytoplasm Structural component of ribosomes Function Carries DNA sequence information to ribosomes Mutations • Mutations are changes in the DNA base sequence Are caused by errors in DNA replication Are caused by environmental factors, such as exposure to radiation and chemicals or temperature changes • Mutations most likely cause altered proteins to be produced • 2 Kinds of Mutations: Gene Mutations & Chromosomal Mutations Gene Mutations • Point mutations – changes of a single DNA nucleotide – can cause sickle-cell anemia and many other disorders Normal hemoglobin DNA mRNA Mutant hemoglobin DNA mRNA Normal hemoglobin Sickle-cell hemoglobin Glu Val Gene Mutations • Types of Point mutations Substitutions Insertions Deletions Gene Mutations NORMAL GENE mRNA Protein Met Lys Phe Gly Ala Lys Phe Ser Ala BASE SUBSTITUTION Met Missing BASE DELETION Met Lys Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Leu Ala His Chromosomal Mutations Deletion Homologous chromosomes Duplication Inversion Reciprocal translocation Non-homologous chromosomes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chromosomal Mutations • Human karyotype showing addition of 1 chromosome on autosome 21 – Down Syndrome Chromosome painting