1. Molecular Biology of the Gene
Chapter
BIOL1000
Dr. Mohamad Termos

2. Objectives Following this lecture, you should be able to describe:
DNA molecules
Duplication
Mutations
Mitosis

3. DNA Molecules
DNA molecules are large polymers made of nucleotides. Each nucleotide is made up of a deoxyribose sugar, a phosphate group, and nitrogenous base. These nucleotides stack up using sugar-phosphate bonds to make one side of the double helix

4. DNA Molecules
DNA nucleotides can have any one of four bases attached to the sugar (deoxyribose). The four possible bases are thymine (T), cytosine (C), adenine (A), and guanine (G). The single-ring bases are called pyrimidines, whereas the larger, double-ring bases are called purines.

5. DNA Molecules
DNA is double stranded. The two strands of DNA are linked together by hydrogen bonds. Adenine always pairs with thymine, and guanine always pairs with cytosine. This is called base pairing. Once base-paired, the two strands coil up like a rope ladder, giving us the familiar double-helix structure of DNA

6. The DNA Genome
The sequence of nucleotides are what make up our genes. DNA contains many different kinds of genes. Each gene contains the info needed to synthesize one kind of protein. A single molecule of DNA has many, many different genes on it. The human genome contains ~100,000 genes. The genome is contained in every single one of your cells

7. Biology, 9th ed,Sylvia Mader
Chapter 13
Structure of DNA
Nucleotides:
Purine bases (double ring)
Adenine (A)
Guanine (G)
Pyrimidine bases (single ring)
Thymine (T)
Cytosine (C)
DNA Structure & Function

8. Biology, 9th ed,Sylvia Mader
Chapter 13
Chargaff’s Rules
The amounts of A, T, G, and C in DNA:
Varies between individuals of a species
Varies more from species to species
There are equal amounts of:
A & T
G & C
Human chromosome estimated to contain, on average, 140 million base pairs
DNA Structure & Function

9. Complementary base pairing
Biology, 9th ed,Sylvia Mader
Chapter 13
Complementary base pairing
Purine is always bonded to a pyrimidine: A bonded to T, G bonded to C
DNA Structure & Function

10. Biology, 9th ed,Sylvia Mader
Chapter 13
Watson and Crick Model
Watson and Crick, 1953
Constructed a model of DNA
Double-helix model
- similar to a twisted ladder
Sugar-phosphate backbones make up the sides
- Bases make up the rungs and are bonded by hydrogen bonds
DNA Structure & Function

11. Biology, 9th ed,Sylvia Mader
Chapter 13
DNA Replication
Process of copying a DNA molecule to result in two DNA molecules -Semi-conservative - a strand of the original DNA is present in each resulting copy of DNA -replication is necessary before a cell can divide
DNA Structure & Function

12. Biology, 9th ed,Sylvia Mader
Chapter 13
DNA Replication
Steps in replication: 1- Unwinding, 2- Complementary Base Pairing, 3- Joining
DNA Structure & Function

13. Semiconservative Replication
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5′ 3′ G C G C G A T A
region of parental
DNA double helix T A C G
DNA
polymerase
enzyme A T A T G G T C G C G C G C A A C A T C G C
region of
replication:
new nucleotides
are pairing
with those of
parental strands T G A A T T A T T A G C T C A T A G A T A T A A G A G G T
region of
completed
replication T A C C C A G C 3′ G n e w o l d A s t r a n d s t r a n d 5′
daughter DNA double helix
old
strand
new
strand
daughter DNA double helix

14. Mutations
You might also be wondering: what happens if something goes wrong? What if the wrong RNA base pairs with the DNA during transcription? Well, those “wrongs” are called mutations. Mutations are errors in the genetic code (DNA) that can cause errors in the final amino acid sequence. Sometimes, these errors, even just a change in one amino acid, can cause huge problems. For example, sickle-cell anemia is a disease caused by a change in one amino acid.

15. Mutations
Mutation can cause change in just one amino acid (like base substitution in sickle-cell anemia) or many amino acids, in the case of a base deletion. Sometimes, it can create a prematurely short protein, if a codon that normally codes for an amino acid gets changed into a stop codon instead.
However, mutations can be a good thing. What if the change in protein structure helped the protein work better? We might want to keep this change! If we do, then this is one way we are able to select for better survival factors. These kinds of mutations work towards evolutionary change.