1. Chapter 3
The Double Helix

3. Genetic Material DNA is important because it allows us to:
Understand risk of transmitting genetic diseases
Identify criminals
Identify biological relationships
Track historical events
Identify victims of crimes

4. Genetic Material
The structure of DNA was discovered by Watson and Crick in 1953
It is shaped like a double helix

5. Genetic Material
The structure of DNA was discovered by Watson and Crick in 1953
It is shaped like a double helix
It is packaged into chromosomes in the nucleus of the cell and is responsible for 2 processes:
Replication (making a copy of itself)
Protein Synthesis (manufacture of proteins)

6. Nucleic Acids
Within the nucleus of eukaryotic cells are two types of nucleic acids:
DNA (deoxyribonucleic acid): actual genetic material
RNA (ribonucleic acid): the “helper” for DNA functions

8. Nucleic Acids
The building blocks of the nucleic acids are called nucleotides, which have 3 components:
Sugar molecule (deoxyribose or ribose)
Phosphate molecule
Base: adenine (A), guanine (G), thymine (T), cytosine (C), (and uracil (U) in RNA).
Bases are used in DNA replication and protein synthesis

9. Structure of DNA Nucleotides strung together are polynucleotide chains
DNA is in the form of a double helix, which is shaped like a twisted ladder
Alternating phosphates and sugars make the “backbone,” and the base pairs make up the rungs

10. Structure of DNA
These bases are found in pairs and held together by hydrogen bonds
The bases pair in specific ways
A  T
C  G
No other combinations!

12. DNA Replication
Vital process that always occurs just before protein synthesis and cell division
Occurs so a full set of genetic info can be passed on to daughter cells
It is like making an exact, Xeroxed copy

13. DNA Replication Replication is controlled by enzymes
1). Enzyme helicase breaks hydrogen bond holding bases together
2). Two DNA strands unwind, leaving base pairs exposed on each strand
3). The exposed bases attract free DNA molecules, which follow the rules of base pairing

14. DNA Replication 4). The paired bases thus form a new strand
5). This results in 2 double-stranded DNA molecules (…hence the name replication!)
6). The newly replicated DNA molecule coils into the chromosomal form that is found before cell division.
Look at page 41

16. Assignment
Start Self-Test 3.1
Lab 3.1
Lets extract some DNA

17. Protein Synthesis If proteins are made of subunits called amino acids
Strings of amino acids combine to form different proteins

18. Protein Synthesis Half the amino acids are produced in the body
Half must be taken in by food. These are essential amino acids

19. Protein Synthesis
Proteins are constructed outside the nucleus, in the cytoplasm
To form a protein, info contained in the nuclear DNA must be taken to the ribosome

20. Protein Synthesis
1). 1st step is inside nucleus, where DNA resides. The result is the formation of a strand of mRNA
2). 2nd step carries the message of the DNA sequence to the ribosome and “translates” it into a specific order of amino acids

21. Protein Synthesis 1st step is Transcription: 2nd step :
Begins like replication: 2 DNA strands begin to unwind, but only on a small portion (along a gene). The end result is the formation of a strand of mRNA
2nd step :
RNA comes into play. Exposed bases along DNA gene attract free RNA molecules and bond to form a new strand
Base pairing rules are the same, except there is no thymine (T). Instead, there is uracil (U), which bonds with adenine

22. Protein Synthesis 3rd step: Final product:
RNA strand forms quickly, then immediately breaks away from DNA. The new strand is called mRNA (messenger RNA), which leaves the nucleus and goes to the cytoplasm
Final product:
A single-stranded RNA molecule, complementary to the bases of the original DNA
Called mRNA, because it carries the message to the cytoplasm

24. Protein Synthesis Second step is called Translation:
Here, the message of DNA sequence will be “translated” into a specific order of amino acids
Involves the newly formed mRNA, tRNA, ribosomes, amino acids, and enzymes

25. Protein Synthesis 1st step: 2nd step:
After going into cytoplasm, mRNA goes to a ribosome, which reads the mRNA, 3 bases at a time. These “words” are called codons
2nd step:
Corresponding amino acids are brought into place along mRNA strand. This is done with tRNA, which carries the correct amino acids for each codon. 3 bases of tRNA are anticodons, which are complementary to mRNA codons

26. Protein Synthesis 3rd step: 4th step:
As each amino acid is brought into place by tRNA, it forms a chemical bond with transferred amino acid
4th step:
Eventually a ribosome reads a mRNA codon that has no anticodon. This is a stop codon, which terminates the chain
Final Product: amino acids that form protein

28. Protein Synthesis
Summary of transcription and translation on page 52

29. Mutations An inherited change in DNA is a mutation
3 main types are single-base substitution (point mutation): one base is accidentally replaced by another. Usually unnoticed.
Insertion: an extra base is added
Deletion: a base is deleted
Mutations can be beneficial, neutral or harmful/lethal

30. Assignment
Lab 3.2
Complete Self-Test 3.1/3.2
Do Self-Test 3.3