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UNIT 1 INFORMATION METHODS OF A CELL What do you know about DNA? • Building blocks are called? – nucleotides • The shape is ? – Double helix • The three primary components of a nucleotide are? – Sugar, phosphate, nitrogen base • Where are each of these found? – The sugar and phosphate are in the backbone; the nitrogen base is on the rungs between DNA strands • The sugar is? – deoxyribose DNA RNA DNA Backbone Structure • Alternate phosphate and sugar (deoxyribose), phosphate ester bonds DNA Backbone Structure • Alternate phosphate and sugar (deoxyribose), phosphate ester bonds DNA RNA DNA RNA DNA Primary Structure Summary • Backbone (sugar+phophate with phosphate ester bonds and base side chains) DNA RNA What are the base pairing rules for DNA? DNA Double Helix • Base pairing by unique hydrogen bonds • C - G and A - T pairs DNA Double Helix • Base pairing by unique hydrogen bonds • C - G and A - T pairs • Each strand of the double helix is oriented in the opposite direction 5 end 3 end P P P P P • What is this arrangement called? P P • “Antiparallel “ P Figure 10.5B 3 end 5 end What process is used in the cell to make new strands of DNA? DNA Replication DNA What do you remember? • What does semi conservative replication mean? – The resulting DNA includes one original strand and one new strand – One original strand is “conserved” or saved. Untwisting and replication of DNA • each strand is a template for a new strand helicase DNA polymerase Figure 10.4B What is the job of the helicase enzyme? – To unzip the DNA What is the job of the DNA polymerases? – To build new DNA strands by attaching nucleotides to the parent strand What is the job of ligase? – To attach fragments of DNA together DNA Replication • Complementary base pairs form new strands. • DNA polymerase works in only one direction • Telomere sequences are lost with each replication. • Cancer, aging DNA polymerase molecule 5 end Daughter strand synthesized continuously Parental DNA 5 3 Daughter strand synthesized in pieces 3 5 P 5 3 3 5 P telomeres DNA ligase Overall direction of replication Figure 10.5C So what is a telomere anyway? • A telomere is a repeating DNA sequence (for example, TTAGGG) at the end of the body's chromosomes. • The telomere can reach a length of 15,000 base pairs. • Telomeres function by preventing chromosomes from losing base pair sequences at their ends. They also stop chromosomes from fusing to each other. • Each time a cell divides, some of the telomere is lost (usually 25-200 base pairs per division). • When the telomere becomes too short, the chromosome reaches a "critical length" and can no longer replicate. • This means that a cell becomes "old" and dies by a process called apoptosis. So what do telomeres have to do with Cancer? • Its all about the enzyme, telomerase. • Telomerase causes the continued addition of DNA ends (telomeres) so cells with active production of telomerase continue to grow and divide. Body cells no longer produce telemerase. • What kind of cells keep growing and dividing? • Fetal cells, tumor cells including cancer cells How can entire chromosomes be replicated during S phase of the cell cycle? • DNA replication begins at many specific sites Origin of replication Parental strand Daughter strand Bubble Two daughter DNA molecules What are these sites called? Replication Bubbles Figure 10.5A DNA DNA RNA polymerase • In transcription, DNA helix unzips – RNA nucleotides line up along one strand of DNA, following the base-pairing rules – single-stranded messenger RNA peels away and DNA strands rejoin – What are the 3 steps of transcription? Figure 10.9B DNA of gene Promoter DNA Initiation Elongation Terminator DNA Area shown in Figure 10.9A Termination Growing RNA Completed RNA RNA polymerase DNA RNA Transcription • In nucleus, DNA guides the synthesis of mRNA RNA Transcription • RNA polymerase and elongation reaction RNA Transcription • RNA polymerase and elongation reaction Eukaryotic RNA is processed before leaving the nucleus • Noncoding segments, introns, are spliced out • A cap and a 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 Figure 10.10 RNA vs. DNA • RNA contains Uracil, not Thymine • Sugar is Ribose • Usually single stranded Phosphate group Sugar (ribose) Nitrogenous base (A, G, C, or U) Types of RNA • mRNA contains codons which code for amino acids. DNA Types of RNA • rRNA - Ribosome - contains enzymes and keeps everything together Types of RNA • tRNA - Transfer RNA carries amino acid and read codons on m-RNA through its own anticodons. Types of RNA • tRNA - Transfer RNA carries amino acid and read codons on m-RNA through its own anticodons. Protein Synthesis Protein Synthesis Protein Synthesis Protein Synthesis Protein Synthesis Check • The following section of DNA is used to build a strand of mRNA • 3’ – GAA- CCC- TTT- 5’ • What is the corresponding mRNA sequesnce? • What are the anticodons on the tRNA in the next step? Modified from www.elmhurst.edu/~chm/onlcourse/CHM103/Rx22DNA.ppt • Summary of transcription and translation TRANSCRIPTION DNA mRNA RNA polymerase Stage 1 mRNA is transcribed from a DNA template. Amino acid TRANSLATION Enzyme Stage 2 Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. tRNA Initiator tRNA mRNA Figure 10.15 Anticodon Large ribosomal subunit Start Codon Small ribosomal subunit Stage 3 Initiation of polypeptide synthesis The mRNA, the first tRNA, and the ribosomal subunits come together. New peptide bond forming Growing polypeptide Codons Stage 4 Elongation A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time. mRNA Polypeptide Stop Codon Figure 10.15 (continued) Stage 5 Termination The ribosome recognizes a stop codon. The polypeptide is terminated and released.