Download Lecture 16

Gene Expression
Gene: contains the recipe for a protein
1. is a specific region of DNA on a chromosome
2. codes for a specific mRNA
mRNA: exits the nucleus and associates with ribosomes
1. carries the code for a specific protein to the cytoplasm
2. primary structure of a protein is encoded by the mRNA base
sequences
3. sequence of mRNA bases is encoded by DNA (genes)
Two processes involved:
1. transcription - mRNA synthesis (copy the code)
2. translation - protein synthesis (translate the code)
DNA Structure
A. Rosalind Franklin
X-ray crystallography
DNA exists as a helix
B. Watson and Crick
1. DNA is a double helix
2. two complementary strands
Nucleic Acids
A. Building blocks are nucleotides
1. 5-carbon sugar (ribose or deoxyribose)
2. phosphate group
3. nitrogenous/organic base (varies)
B. DNA is a polymer made using 4 different nucleotides
- the four nucleotides differ only in their bases
D. Nucleotides are bonded via their phosphate groups
1. phosphate binds to a sugar of another nucleotide
2. sugars and phosphates form a “backbone”
3. bases project from backbone (forming side chains)
4. H bonds form between bases of opposing side chains
a. DNA is double stranded
b. forms a double helix
Double helix:
E. Law of complementary base-pairing (Chargaff’s rules)
1. adenine to thymine
2. guanine to cytosine
The sequence of bases in one chain (strand) determines the sequence
of bases in the other chain.
F. RNA structure (3 major differences with DNA)
1. sugar is ribose
2. uracil instead of thymine
3. single-stranded
G. Classes of RNA
1. mRNA: carries coded info to the ribosomes
G. Classes of RNA
1. mRNA: carries coded info to the ribosomes
2. rRNA: structural molecule of ribosomes
G. Classes of RNA
1. mRNA: carries coded info to the ribosomes
2. rRNA: structural molecule of ribosomes
3. tRNA: brings amino acids to ribosomes for protein synthesis
All three types of RNA necessary for protein synthesis.
DNA Replication
A. Copying of DNA
done before mitosis (S phase)
DNA Replication
B. Process
1. unzip the double helix
2. each strand used as a template
3. complementary strands constructed
4. new DNA molecules identical to the original
5. enzyme (DNA polymerase) also “proofreads”
DNA Replication
B. Process
1. unzip the double helix
2. each strand used as a template
3. complementary strands constructed
4. new DNA molecules identical to the original
5. enzyme (DNA polymerase) also “proofreads”
Transcription
Only a few genes usually active at any one time in a cell.
If a gene is active, then coded info in gene must be copied onto an
mRNA molecule.
Transcription = DNA-directed RNA synthesis.
A. Transcription occurs in the nucleus
B. RNA polymerase
1. unzips DNA double helix
2. only at area to be transcribed
3. H bonds between strands are broken
4. mRNA is built using one DNA strand as template
C. Formed mRNA detaches from template DNA (gene)
D. Can make many copies quickly if needed
E. Post-transcriptional modifications
1. removal of introns
2. exons spliced together