DNA vs. RNA DNA: deoxyribonucleic acid (double stranded) RNA: ribonucleic acid (single stranded) Both found in most bacterial and eukaryotic cells RNA

Transcription

1 DNA Replication

2 DNA vs. RNA DNA: deoxyribonucleic acid (double stranded) RNA: ribonucleic acid (single stranded) Both found in most bacterial and eukaryotic cells RNA molecule can assume different structures - results in different types of RNA with each having a particular function (some important in DNA replication) 3 key differences: 1. Sugar component of RNA is ribose not deoxyribose 2. RNA does not have nucleotide thymine (T). It is replaced with nucleotide uracil (U) 3. RNA single stranded

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4 Genes and the Genome Genes are functional subunits of DNA Direct production of one or more polypeptides (protein molecules) Genome of an organism: sum of all DNA carried in each cell of the organism - includes non-coding regions as well Genes not spaced regularly along chromosomes ex) Chromosome 4: base pairs long, 800 genes Chromosome 19: base pairs long, 1500 genes No relationship between number of genes and size of genome ex) human genome: 3 billion bp s, genes amoeba: 650 billion bp s, fewer than 7000 genes

5 DNA Replication Occurs during S phase of interphase in mitosis DNA must copy itself and be equally divided between daughter cells - must be exact copy of parent - human cell replicates in a few hours, error rate of one per one billion nucleotide pairs

6 DNA Replication Semiconservative replication: separating two parent strands and using them to synthesize two new strands Hydrogen bonds break, DNA helix unzips Each single strand acts as a template to build the complementary strand Errors then repaired, result is TWO identical DNA molecules - one for each daughter cell

7 Initiation & Separation Replication starts at a specific nucleotide sequence - replication origin - can have many replication origins simultaneously DNA helicase bind to DNA at replication origin - unwinds segment of helix by breaking hydrogen bonds - proteins bind to separated strands to prevent reformation Opening of DNA creates Y-shaped replication fork Separated strands now template strands with exposed unpaired bases - one strand runs in 3 to 5 direction, other in 5 to 3 direction (in relation to replication fork)

8 Replication occurs in both directions and bubbles grow until they meet.

9 DNA Replication

10 Building Complementary Strands Synthesis begins of two new DNA strands on template strands - complementary base pairing DNA polymerase III: adds free nucleotides one at a time that are complementary to the template - elongation RNA primer: short piece of RNA attached to template strand - gives DNA polymerase III a starting point Nucleotides added in only ONE DIRECTION - 5 to 3 Leading strand: synthesized continuously in 5 to 3 direction TOWARD replication fork - free 5 end of nucleotides bind to free 3 hydroxyl end on template

11 Building Complementary Strands Lagging strand: synthesized away from replication fork, in short fragments later joined together - Okazaki fragments Synthesized in 5 to 3 direction as well - able to do so since it runs in opposite direction of leading strand RNA primers needed in multiple locations - recall: lagging strand synthesized in fragments - then primers cut out and replaced with DNA nucleotides by DNA polymerase I Nicks left in between fragments - DNA ligase links sugar-phosphate backbone of fragments

12 Building the Lagging Strand

13 Review DNA Replication fork

14 DNA Repair DNA polymerase III and I used as checkers throughout synthesis of complementary strands Mistake occurs?? DNA polymerases backtrack! - cut out incorrect nucleotide, continue adding correctly Prevents mistake from being copied in future replications

15 Termination Replication fork progresses throughout helix - only short region of DNA unravelled in single stranded form at a given time Newly formed strands completed: rewind automatically into helix structure Replication proceeds until new strands complete, DNA separates from one another TERMINATION.