dna replication

• The leading strand is continuous, while the lagging strand consists of short fragments called Okazaki fragments.
• Okazaki fragments are joined together by ligases to form a complete lagging strand.
• DNA replication is semi-conservative, resulting in two new DNA molecules that each contain one original and one newly synthesized strand.
• an enzyme called helicase starts the process by unzipping the double helix to separate the strands of DNA at numerous places called origins of replication.
• an enzyme called primase makes a small piece of RNA called primer complementary to the single stranded DNA.
• a primer marks the starting point for the construction of the new strand of DNA.
• a group of enzymes called DNA polymerases bind to the primer and pairs up the free-floating nucleotides with the existing DNA strand in the 5’ to 3’ direction
• Each existing strand is a template for a new strand, which makes DNA semi-conservative
• Replication proceeds from hundreds of origins of replication, so the process takes just a few hours
• A

  Adenine
• T


  Thymine
• C


  cytosine
• G

  guanine
• U

  Uracil
• The Central Dogma of Biology

  DNA transcripts into RNA translates to protein
• Transcription converts a DNA message into an intermediate molecule, called RNA
• Translation interprets an RNA message into protein.
• RNA acts as an intermediate link between DNA in the nucleus and protein synthesis in the cytoplasm.
• Like DNA, RNA is a chain of nucleotides (composed of ribose, phosphate group, and a nitrogen-containing base)
• Like DNA, RNA is a chain of nucleotides (composed of ribose, phosphate group, and a nitrogen-containing base)
• RNA has the base uracil in place of thymine. (U = A)
• RNA is a single strand of nucleotides (DNA is double-stranded)
• 1. RNA polymerase recognizes the transcription start site (promoter) of a gene, splits open the double stranded DNA and transcribes just the template strand of DNA, stringing together a complementary strand of RNA nucleotides.
  2. The new mRNA strand is processed (mRNA processing)

  The mRNA transcript moves to the cytoplasm for translation.
• Methylated 5’ Cap: Helps the mRNA strand bind to a ribosome and prevents the strand from being broken down too fast.
• Poly (A) tail: Helps the mRNA molecule exit the nucleus. (A also adds stability to the mRNA)
• RNA splicing: Noncoding sections called introns are removed from the mRNA.

  Introns: noncoding sections of DNA

  Exons: coding sections of DNA
• After mRNA is transcribed, all introns are cut out and exons are joined together before mRNA leaves the nucleus. This mechanism allows various combination of exons, resulting in different proteins.
• The role of introns? They may protect DNA against harmful mutations. If large regions of DNA are noncoding “junk,” then mutations occurring in those regions will have no effect. They may regulate gene expression.
• DNA is more stable due to its hydrogen bonds
• Location of transcrption

  Nucleus
• Transcription and Replication Are Similar

  • Both processes occur within the nucleus.
  • Both are catalyzed by large, complex enzymes (Polymerases).
  • Both involve unwinding of the DNA double helix.
  • Both involve complementary base pairings to the DNA strand