dna structure and replication and a brief introduction to rna
TRANSCRIPT
DNA Structure and ReplicationAnd a brief introduction to RNA
DNA and RNA molecules have structural similarities and differences that define
function. 1. Nucleotides are made of three parts– 5 carbon sugar (Ribose or Deoxyribose)– Phosphate– Nitrogenous base
3. Both DNA and RNA exhibit specific nucleotide base pairing that is conserved through evolution:
• Purines: Adenine and Guanine (double ring)• Pyrimidines: Thymine, Cytosine and Uracil (single
Ring)• Adenine pairs with thymine or uracil (A-T or A-U)• Cytosine pairs with guanine (C-G).
Nitrogenous Bases
• Purines:
• Pyrimidines:
How Nucleotides Attach Together
1. Nucleotides connect via phosphodiester bonds
2. The Resulting molecule is linear with a directionality
3. Opposite strands are Antiparallel
What causes the Twisting in DNA?• Hydrogen Bonding along the Backbone of DNA
causes the double helix• Similar to hydrogen bonding along the peptide
backbone of Protein forming the alpha helix and beta sheet (secondary protein structure)
Structural differences betweenDNA and RNA
DNA RNA
Deoxyribose Ribose
Purines: Adenine and GuaninePyrimidines: Cytosine and Thymine
Uracil in lieu of Thymine
Double Stranded Usually single stranded
Double strands are antiparallel in directionality
RNA vs. DNA
Possible Models of DNA replication
• Meselsohn and Stahl tested the three proposed models of DNA Replication
DNA replication ensures continuity of hereditary information.
• Replication is a semiconservative process; that is, one strand serves as the template for a new, complementary strand.
DNA Replication
• You must know the names and roles of the following enzymes in this process:– Helicase– DNA polymerase– Primase– Ligase – Topoisomerase
• Replication occurs bidirectionally, and differs in the production of the leading and lagging strands.
Helicase uncoils the DNA exposing the nitrogenous bases in the middle.
DNA Polymerase and Primase• DNA Polymerase elongates DNA by attaching
a nucleic acid to an existing short polynucleotide that the enzyme Primase made.
• Can only move in the 5’-3’ Direction along DNA.
Topoisomerase • Topoisomerase makes cuts in the DNA to lessen
the tension of the super coiled DNA (Super coiling is caused by the enzyme Helicase)
You do not need to memorize these steps
Ligase
• Ligase attaches Okazaki fragments of lagging strand together.
DNA REPLICATION1. DNA replication begins at an Origin of Replication2. Helicase untwists the DNA at the replication fork forming a
replication bubble3. A new complementary strand of DNA is synthesized at each
replication fork. 4. When Replication bubbles meet the enzyme ligase attaches the
strands of DNA together.
DNA Replication at the Replication Fork• DNA Polymerase elongates DNA in
the 5’ to 3’ Direction• DNA polymerase cannot initiate
DNA synthesis on an empty strand so Primase makes an RNA primer for DNA Polymerase to elongate.
• The leading strand grows continuously towards the replication fork.
• The lagging strand grows in fragments called Okazaki Fragments that must be attached together by Ligase.
DNA Replication at the Replication Fork
DNA Replication
HelicaseTopoisomerasePrimaseDNA PolymeraseLigase
Structure and Function of RNA• RNA Can perform both coding, regulation, and
enzymatic function4 Types of RNA (That we know of):– Messenger RNA (mRNA): Temporary transcript that carries
information from The DNA to the ribosome for Protein synthesis.
– Transfer RNA (tRNA): Decode the mRNA into a specific amino acid to make a linear peptide sequence during Protein synthesis.
– Ribosomal RNA (rRNA): The functional and catalytic parts of the Ribosome
– Small interfering RNA (RNAi): Cause specific messenger RNAs to be degraded in the cytoplasm (a mechanism for combating Viral infection and regulating Protein expression)
RNA Can fold and Base Pair with itself
• Transfer RNA