DNA and Replication

If you unwrap all the DNA you have in all your cells, you could reach the moon! 6000 times!!!

DNA… Deoxyribonucleic Acid (DNA) Primary Function = to store and transmit the genetic information that tells cells which proteins to make and when to make them.

The Hershey-Chase Experiment

made up of repeating subunits called NUCLEOTIDES DNA is... an ORGANIC compound made up of repeating subunits called NUCLEOTIDES composed of TWO long chains of nucleotides

DNA NUCLEOTIDE 3 PARTS: Sugar Molecule – deoxyribose Phosphate Group: phosphorous (P) & oxygen (O) Nitrogen Base: adenine (A), cytosine(C), guanine(G), and thymine(T)

Circle 1 nucleotide...

1. PURINES Nitrogen bases come in two types: Purines have two rings. And come in two types 1. Adenine 2. Guanine

Pyrimidines have ONE ring Nitrogen bases come in two types: 2. PYRIMIDINES Pyrimidines have ONE ring And come in two types: 2. Thymine 1. Cytosine

Nucleotides & DNA Nucleotides can join together in any order, any sequence of nitrogen bases is possible.

Complimentary Base Pairing Certain bases always pair together 1. Cytosine bonds with Guanine 2. Adenine bonds with Thymine Nitrogen bases form hydrogen bonds

chargaff’s rule Must be an equal number of Cs and Gs As and Ts 3 H bonds 2 H bonds

In 1953, JAMES WATSON & FRANCIS CRICK suggested a model for the structure of DNA. Two nucleotide strands wrap around each other to form a double spiral (spiral staircase) The Double Helix

Contention in Science World Rosalind Franklin used X-ray diffraction to reveal an X-shaped pattern in DNA, suggesting DNA contained 2 strands and was twisted like the coils of a spring.

Image of the Double Helix * **Sugar-phosphate backbone is held together by covalent bonds (strong) **Nitrogen bases connect by hydrogen bonds (weak) *

DNA Structure The nitrogen bases form the center rungs of DNA The outside, sugar-phosphate backbone, is made of alternating phosphates and sugars held together by covalent bonds

Antiparallel strands DNA strands run anti-parallel to each other One strand runs 3’5’ One strand runs 5’3’ This is important for DNA replication

DNA Replication The process of copying DNA. Overall: Two nucleotide chains separate and serve as TEMPLATE for a new nucleotide chain. Recall, when does DNA replication occur? Before what stages?

Steps to DNA Replication 1. DNA is split down the middle (between base pairs) by helicase, breaking the hydrogen bonds. This forms a replication fork Similar to a zipper 2. The nitrogen bases on each strand are used as a pattern for a new strand.

Steps to DNA Replication 3. Complementary bases are added to each individual strand by DNA polymerase (enzyme). 4. Each new cell can now get a complete copy of all the DNA. Semi-conservative: One of the original strand, one new

DNA primase enzyme adds a primer to each side of the open strand. DNA Replication Helicase enzymes untwist and unzip the DNA helix. (A replication fork is now formed.) DNA primase enzyme adds a primer to each side of the open strand. Primer: piece of RNA that starts replication

DNA polymerase adds the new nucleotides one at a time. DNA Replication DNA polymerase adds the new nucleotides one at a time. Lastly, DNA ligase enzyme “glues” nucleotides together on the lagging strand.

Antiparallel strands Because DNA is antiparallel, we call one strand the leading strand (5’ →3’) and the other the lagging strand (3’→5’). Leading Strand: copied continuously from a replication origin by in the same direction that the replication fork moves. Lagging Strand: copied discontinuously from a replication origin in the opposite direction that the replication fork moves. The small pieces that are created are referred to as Okazaki fragments.

DNA Replication Animation

Original strands of DNA ***You are left with 2 exactly identical molecules of DNA double helix.*** New New Original Original strands of DNA Original O N N O Semi-conservative Model

Telomeres The tips of chromosomes (the ends of the DNA molecule) Telomeres are difficult to copy and so they can be lost each time a chromosome is replicated. Telomerase (enzyme) adds short, repeated DNA sequences to telomeres to slightly lengthen the chromosome to make it less likely that important gene sequences will be lost.

Telomeres In humans, repeating sequence of TTAGGG At birth, roughly 11,000 bases long At old age, less than 4,000 bases long Declines faster in men than women

Shortened telomeres result in more chromosomal damage Shortened or damaged telomeres are found in many cancers Helps to explain increase in cancer prevalence as age increases Suggested to play role in human aging