DNA and replication ppt

• DNA


  Deoxyribonucleic acid
• DNA location


  • Nucleus of eukaryotic cells
  • Cytoplasm of prokaryotic cells
• DNA function


  • Important information carrying molecule
  • Carries/stores genetic information (codes for proteins of organisms)
• Chromosomes


  Made of DNA that wraps around histone proteins
• DNA in non-dividing cell


  DNA and histones not tightly coiled into chromosomes
• DNA in dividing cell


  DNA and histones tightly coiled up and visible when stained
• Prokaryotic DNA


  • Shorter
  • Circular
  • Not associated with protein (histones)
  • Can be in plasmids
• Mitochondria & chloroplast DNA


  • Shorter
  • Circular
  • Not associated with proteins with smaller ribosomes
• DNA molecule structure

  • Double helix (twisted ladder) structure
  • Made of 2 polynucleotide strands held together by many hydrogen bonds
• Nucleotides


  • Monomers which are bonded together to make up a polynucleotide chain
  • Basic structure: phosphate group, deoxyribose sugar, nitrogen-containing organic base
• Nitrogen-containing bases


  • Adenine (A)
  • Guanine (G)
  • Thymine (T)
  • Cytosine (C)
• Nucleotide bonding to form a polynucleotide strand

  Condensation reaction between phosphate group of one nucleotide and deoxyribose of adjacent nucleotide to form phosphodiester bond
• Polynucleotide strand


  • Formed by nucleotides bonding together
  • Sugar-phosphate backbone formed
• Complementary base pairing


  A=T, G=C
• DNA structure related to function

  • Double stranded - stable molecule, and the sequence of bases remains unchanged. semi-conservative replication can occur and each strand acts as a template
  • DNA is a Helix/coiled - compact so the many long molecules can fit into the nucleus
  • DNA molecules are very long - stores lots of genetic information (many genes on 1 chromosome)
  • It has specific complementary base pairing - accurate replication can occur making identical DNA for daughter cells
• DNA structure related to function

  • Many hydrogen bonds holding the two strands together - stable molecule
  • Each Individual hydrogen bond is weak - strands can be broken apart (by helicase) for DNA replication and transcription
  • One strand carries specific base sequence - contains codes for amino acid sequence in a polypeptide (a gene)
• Stages of semi-conservative DNA replication

  1. Parent DNA molecule (strand x & strand y)

  2. Strands separated by breaking hydrogen bonds (x and y now act as template strands)

  3. New strands made (x1 & y1) by new nucleotides following specific complementary base pairing rules (y1 pairs to x and x1 pairs to y)

  4. Two new DNA molecules made identical to each other and identical to parent DNA
• Semi-conservative DNA replication ensures genetic continuity in each new generation of cells produced
• DNA replication

  1. DNA 'unzips' due to DNA helicase, unwinding DNA by breaking hydrogen bonds between specific base pairs in the parent DNA molecule

  2. New nucleotides attracted to exposed bases on both template strands and new hydrogen bonds form (specific complementary base pairing)

  3. DNA polymerase joins sugar-phosphate backbones of new strands by forming new phosphodiester bonds between adjacent nucleotides

  4. Each new DNA daughter molecule has one old (parent) strand and one newly synthesised strand of DNA (semi-conservative rep)
• DNA helicase


  Unwinds DNA by breaking hydrogen bonds between specific base pairs in parent DNA molecule
• DNA polymerase


  • Carries out specific base pairing
  • Joins sugar-phosphate backbones together in new strands
• The two DNA strands run in opposite directions (anti-parallel)
• DNA polymerase can only move along a polynucleotide strand in one direction
• DNA polymerase only joins nucleotides in one orientation
• Evidence for semi-conservative DNA replication

  • Meselson and Stahl experiment - bacteria cultured in 15N medium, DNA then extracted and centrifuged, then bacteria is transferred to 14N medium and DNA extracted and allowed to undergo one cycle of cell division (binary fission) DNA then extracted and centrifuged, bacteria allowed to undergo another cycle of cell division in 14N medium, DNA then extracted and centrifuged
• Eukaryotic DNA

  • Linear DNA
  • Longer
  • Associated with histone proteins
• One nucleotide

  
• How are nucleotides bonded together to form a polynucleotide strand/chain?

  • A condensation reaction occurs between the phosphate group of one nucleotide and the deoxyribose of the adjacent nucleotide to form a phosphodiester bond
  • A sugar phosphate backbone is formed
• Specific complimentary base pairing rules

  • The two polynucleotide strands of the DNA molecule are held together by many hydrogen bonds between the specific complimentary base pairs:
  • Adenine=Thymine
  • Guanine=Cytosine
• DNA structure to function 


  • Double strand
  • Stable molecule
  • Sequence of bases remains unchanged
  • Semi conservative replication can occur - each strand acts as a template
• DNA structure to function

  • DNA molecules they are very long
  • So that lots of genetic information can be stored (many genes on 1 Chromosome)
• What is the purpose of the name of semi-conservative replication?

  One DNA molecule replicates to give 2 new ones. Each new DNA molecule keeps one original (parent) strand and has one newly synthesised strand
• Why is it important that the 2 new daughter DNA molecules are identical to each other and identical to the original parent DNA molecule?

  So each of your cells have the same DNA- DNA ensures genetic continuity in each new generation of cells produced
• DNA polymerase move along a polynucleotide in one direction because the two strands are in opposite direction to each other and the shape of its active site is only complimentary to the shape of the correctly aligned nucleotide
• DNA polymerase


  • The shape of its active site is only complementary to the shape of the correctly aligned nucleotide
• DNA polymerase moves 5' to 3' on the template strand
• DNA replication is a semi-conservative process
• DNA is made of 2 polynucleotide strands/chains
• Compare the differences in structure between eukaryotic and prokaryotic DNA

  • Eukaryotic
  • Linear DNA
  • Longer
  • Associated with Justine proteins
  • Prokaryotic
  • Circular DNA
  • Shorter
  • Not associated with proteins
• Mitochondria and chloroplasts also contain their own DNA molecules

  • They share a similar structure of DNA
  • DNA is shorter, circular, and not associated with proteins
  • Ribosomes are smaller