Download BIOL 241 Nucleic Acids and Gene Expression I. Genes (Overview) A

BIOL 241
Nucleic Acids and Gene Expression
I. Genes (Overview)
A. Regions of DNA (on chromosomes) within nucleus
B. A gene codes for a specific mRNA
C. mRNA exits nucleus---associates with ribosomes
D. mRNA supplies code for a specific protein
1. sequence of mRNA bases coded by DNA (gene)
2. 10 structure of protein coded by mRNA base sequences
E. Gene expression
1. transcription---RNA synthesis
2. translation---protein synthesis
II. Nucleic Acids (DNA and RNA)
A. Building blocks (subunits) are nucleotides
5-Carbon sugar (deoxyribose) bonded to:
- phosphate group
-nitrogenous (organic) base
B. All DNA built from combinations of four nucleotides
C. Nucleotides differ in organic bases
1. adenine and guanine-----> purines
- double-rings (N,C)
2. cytosine and thymine------> pyrimidines
- single ring
D. Nucleotides bonded by phosphate groups
1. phosphates (5’) bind to sugars (3’) ---> dehydration synthesis
2. sugars and phosphates form “backbone”
3. bases project from backbone (forming side chains)
4. H-bonds can form between bases (on other chains)
a. double stranded
b. double helix
E. Law of complementary base-pairing
1. adenine to thymine
2. guanine to cytosine
3. sequence of bases in one chain determines the sequence of bases in the
other chain
4. paired chains run in opposite directions
F. Chromatin
1. nuclear DNA + histone proteins
2. is the strands of material that make up the chromosomes
3. histones
a. have (+) charge
b. form (+) spools around which (-) charged DNA are wound
c. = nucleosomes
d. histones may act as gene repressors
Functioning cells use information in DNA to build proteins.
Depends on the synthesis of RNA.
G. RNA structure
1. similar to DNA: 3 differences
a. sugar is ribose
b. uracil instead of thymine
c. single-stranded
H. Classes of RNA
1. mRNA- carries coded info to ribosomes
2. rRNA- structural molecule (forms part of ribosome) DNA is in nucleolus
3. tRNA- brings amino acids to the ribosomes
III. Transcription
Only a few genes are usually active at any one time in a cell
If gene active----> coded info in DNA must be copied into an mRNA molecule
Transcription (DNA-directed RNA synthesis)
A. Occurs in nucleus
B. RNA polymerase (enzyme)
1. “unzips” DNA double helix
2. breaks H-bonds
3. Separates DNA strands only at regions to be transcribed
4. Builds RNA strand
C. Formed RNA molecule detaches
D. More copies synthesized or DNA double helix is reformed
E. Precursor (pre)mRNA
1. inactive, must be modified in nucleus (enzymatically)
2. introns (noncoding DNA) removed
3. exons spliced together
4. active mRNA molecule moves to cytoplasm
IV. Translation
A. Genetic code is translated from the the molecular “language” of mRNA to the
molecular “language” of a protein.
B. Proteins “carry out instructions” in the genetic code.
C. A protein is a string of amino acids
D. 10 structure of protein coded by mRNA base sequences
V. Codon: essential unit of translation of mRNA
A. Group of 3 adjacent nucleotides
1. specifies one amino acid
2. codons “tell” the ribosome which amino acid to add in sequence
B. Each amino acid (20) coded for by at least one codon
See Figure 3.35
1. all but two amino acids can have more than one codon
- usually differ in the third base
2. 3 codons are STOP codons
3. mRNA = series of codons translated into chains of amino acids
C. Change in a single nucleotide (mRNA) can “scramble” entire message
- small changes in DNA or mRNA can cause dramatic changes in proteins
VI. tRNA
A. “Taxicabs” that bring free (unattached) amino acids in cytoplasm to chain
B. TWO key regions
1. amino acid attachment site,---> specific for ONE amino acid
2. anticodon
a. one per tRNA
b. three adjacent nucleotides
c. complementary to codon on mRNA
d. example: GGA codon on mRNA codes for glycine (Fig 3.35)
- attracts tRNA with anticodon CCU (which will always bring glycine)
VII. Polypeptide Synthesis
A. Occurs on ribosome “Workbench”
B. Ribosome has a large and a small subunit
1. come together in cytoplasm at protein-synthesis time
2. mRNA binds to small subunit via its leader sequence
3. small subunit binds to large
4. large subunit has enzymes for amino acid linkage
C. Chaperonins
1. prevents premature folding of protein
2. may “escort” protein to destination
VIII. Post-translational modification
A. Proteins synthesized on RER ribosomes processed for export
1. “Injection” of signal sequence (first 15-30 aa’s, many nonpolar)
2. Leader (signal) sequence clipped (protein “stuck in RER” cisterna)
3. Many proteins glycosylated
B. Golgi complex is site of “final processing”
1. receives transport vesicles from RER
2. carbohydrates added, altered, and/or otherwise elaborated upon
3. major sorting center of cell (by function and destination)
4. packaging and shipping
- Golgi vesicles ---> secretory vesicles