DNA Structure and Replication And 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 between DNA and RNA DNARNA DeoxyriboseRibose Purines: Adenine and Guanine Pyrimidines: Cytosine and Thymine Uracil in lieu of Thymine Double StrandedUsually 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

Meselsohn and Stahl Experiment

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 REPLICATION 1.DNA replication begins at an Origin of Replication 2.Helicase untwists the DNA at the replication fork forming a replication bubble 3.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 Helicase Topoisomerase Primase DNA Polymerase Ligase

Structure and Function of RNA RNA Can perform both coding, regulation, and enzymatic function 4 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