1. DNA: The Molecule of Life

2. The Importance of DNA
Every cell in your body contains a full set of DNA (deoxyribonucleic acid)
Only molecule capable of replicating itself
DNA codes for a series of chemical messages and regulates the production of protein
Contained in 46 chromosomes in the human body
Made of base pairs (an estimated 3 billion) in humans
genes on the chromosomes
DNA ensures the continuity of life
New combinations of genes and mutations to the DNA may cause for biodiversity and variation

3. Chromosomes
Long thin threads of genetic material, protected by a cell nucleus
Roughly half nucleic acids and half protein
Proteins are made of different sequences of amino acids, of which there are about 20
By changing a single amino acid, a different protein can be created
Genes = segments of DNA that code for specific proteins
WHY ARE PROTEINS SO IMPORTANT??

4. James Watson and Francis Crick
Rosalind Franklin used a technique called x-ray defraction to help determine the structure of the DNA molecule
Other scientists gathered evidence that nucleotide base pairs were in definite proportion to one another
With this data, Watson and Crick suggested that the nitrogen bases where arranged in pairs, and the pairs were held together by a sugar-phosphate compound, which has been dubbed “backbone”
This created a 3-D model of what DNA looked like

5. Structure of DNA
Because of many contributions by research scientists, DNA structure has been well defined
DNA is made of a double helix, or spiraling ladder, where nucleotide base pairs act like rungs on the ladder
The nucleotide pairs are attracted to one another because of a hydrogen bond
The four nitrogen base pairs are adenine, thymine, cytosine and guanine
Guanine always pairs and bonds with cytosine (G-C)
Thymine always pairs and bonds with adenine (A-T)

6. Nucleotide The basic structure of nucleic acid is a nucleotide
Nucleotides are arranged in long chains, which eventually form chromosomes
They are made of a sugar phosphate structural support, attached to one of four nitrogen base units

7. Deoxyribose sugar + phosphate group + nitrogenous base
Nucleotides:
Deoxyribose sugar + phosphate group + nitrogenous base
Purine pair with Pyrimidine
A, G T, C

8. Practice Problems
A sample of DNA contains A and C nucleotides in the following proportions
A = 34% C = 16%
What are the proportions of G and T nucleotides in this sample?
2. Use Chargaff’s rule to complete the following table
3. Draw a linear stretch of a double stranded DNA molecule about 20 base pairs long, with a nucleotide composition that corresponds to the nucleotide composition of the table above.
Nucleotide
Proportion (%) A 24 C G T

9. Replication of DNA Only molecule capable of replicating itself
During replication the hydrogen bonds separate the base pairs
The parent strand acts as a template, and new base pairs, which are complimentary to the parents code, join the parent strand
Each newly created strand is half the parents DNA and half the new DNA
This causes the DNA to be semi-conservative

10. Semiconservative Replication
During replication, the new DNA strands are “half old, half new”
Each parent strand is a mold or template
An enzyme, known as polymerase, fuses the new bases to the old template
Free nucleotides are derived from the food you eat

11. Replication- Overview
DNA helicase
Unzips double helix (breaks the hydrogen base pair bonds)
Single strands serve as TEMPLATE to build the complimentary strands
DNA polymerase III
Builds new strands from templates ( 5’ to 3’)
LEADING STRAND vs LAGGING STRAND
Leading strand – continuous
Lagging strand – formed in Okazaki fragments
Start with RNA primer
DNA Polymerase I replaces primer with appropriate DNA nucleotides
DNA ligase joins fragments
RESULT:
Formation of 2 identical DNA molecules
Both with 2 strands

12. Errors in Replication (mutations)
Genetic mistakes are very infrequent
1 mistake (addition, deletion or substitution) per million would mean 3000 errors in one translation, which would cause enormous mutations
DNA polymerases acts as quality control by proofreading newly synthesized strands – they backtrack to incorrect nucleotide, cut it out and replace it with the correct base
Mutations can be triggered by environmental factors, such as drugs, exposure to radiation

13. PRACTICE PROBLEMS
DNA Template Strand
GCC CTA TAA CGC

14. PRACTICE PROBLEMS
5’ TACTTTGGCCCAGAG 3’
DRAW THE COMPLIMENTARY STRAND

15. Replication - worksheet
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