Chapters 9 & 10. DNA Structure, Function, and Gene Expression
I. DNA Structure – discovered in 1953 by James Watson (U.S.) & Francis Crick (Britain)
Deoxyribonucleic
Acid is a double stranded molecule, each strand consisting of
repeating subunits called nucleotides - monomers of DNA that
contain nitrogenous bases [4 kinds in DNA]: Adenine
(A), Thymine (T), Cytosine (C), and Guanine (G). See Figure 9.4 p.
140
A
. . . T
T . . . A
C . . . G
G . . . C
The
sequence of N-bases of 2nd strand is dictated by
the 1st strand, such that Adenine pairs with Thymine and
Cytosine pairs with Guanine (this is complementary base pairing). To complete this model of DNA structure, imagine a flexible ladder where the rungs of the latter are
DNA
structure – DNA is a double stranded polymer of
(named
for their nitrogenous bases Adenine, Thymine, Cytosine,
Complementary base pairing explains a lot about molecular genetics.
II.
DNA Function – the following are functional aspects of DNA
1. DNA stores genetic information in the form of a molecular code
2. DNA self-replicates – DNA molecules are copied or cloned
3. DNA releases specific genetic instructions at appropriate times via protein synthesis
4. DNA is
capable of mutating (the ultimate source of all allelic variation!)
1. The
Genetic Code
ex. TTA
– codes for the amino acid Asparagine
GGA – codes for the amino acid Proline
AAA – codes for the amino acid Phenylalanine
AAG – also codes for the amino acid Phenylalanine
Hence, there is redundancy in the code. DNA doesn’t make the amino acids, it just specifies that the amino acid should be inserted here in the building of something huge, a large polymer of amino acids, a specific protein. This code is universal among all life. The coded information held by a segment of DNA needed to produce a protein constitutes a gene. A gene is a sequence of nucleotides that codes for the amino acid sequence of a protein. A DNA molecule of a single chromosome is contains hundreds of genes.
2. DNA Replication-produces 2 exact copies of DNA (see fig. 9.7, p. 143)
Requires several enzymes: especially DNA Polymerase (this enzyme is also important in genetic engineering)
A. DNA uncoils & the 2 sides pull apart (all under control of enzymes)
B. New sides are formed on each old half by complementary base pairing with free nucleotides (DNA Polymerase promotes the synthesis of the new side or strand.
=semiconservative replication: each new DNA molecule has 1 old and 1 new strand.
3. Protein synthesis (Ch. 10)- Amino acids are assembled and bound together to form proteins at ribosomes that occur in the cytoplasm (ribosomes may be bound to endoplasmic reticulum or not); however, the instructions (genes) for protein synthesis are in the nucleus. A simple solution - make a copy of the instructions (a transcript if you will) & send the copy out of the nucleus into the cytoplasm where ribosomes occur.
A. Transcription - produces a “copy” of the gene in the form of a complimentary strand of RNA for export to cytoplasm (see fig. 10.2 in Starr).
Under control of the enzyme RNA polymerase the two strands of DNA must buckle apart, exposing a segment of the template strand of DNA; Also under the control of RNA polymerase, complementary base pairing occurs between the DNA template strand and free RNA nucleotides (the monomers of RNA, i.e. adenine, uracil, guanine, & cytosine come one at a time to temporarily bond to the DNA template strand in complementary fashion). The result is a new strand of RNA nucleotides which detaches from the DNA template and and (after some modification ) leaves the nucleus as messenger RNA, or mRNA.
DNA
mRNA
template transcribed
strand
strand
│
│
├ T
A ┤
│
│
│
│
├ G
C┤
│
│
│
│
├ C
G ┤
│
│
│
│
├ A
U ┤
│
│
│
│
├ T
A ┤
│
│
│
│
note: RNA contains Uracil in place of Thymine, no Thymine is found in RNA, [simply substitute a “U” wherever a “T” would normally complimentary pair].
B. Translation – the assembly of a specific sequence of amino acids based on the codon sequence of mRNA; builds a particular protein; occurs at ribosomes in the cytoplasm or at ribosomes on endoplasmic reticulum. Ribosomes interact with mRNA, tRNA, and amino acids facilitating their alignment and the formation of peptide bonds between juxtaposed amino acids.
Know this term: codon - a triplet of N-bases on mRNA, complementary to the code of DNA.
Each codon codes for 1 Amino Acid (see fig. 10.4, p. 150)
mRNA “rests” within a ribosome and codons are “translated” by another type of RNA, transfer RNA, or tRNA
Translation is based on complimentary base pairing between the codon sequence of mRNA and an anticodon sequence of tRNA.
Know this term: anticodon – a triplet of N-bases found on tRNA; anticodons pair in complimentary fashion with specific codon sequences on mRNA and deliver specific amino acids (see fig. 10.7, p. 152-153).
Each tRNA with a particular anticodon sequence will carry only one type of amino acid. tRNA’s simply act as transports delivering the appropriate amino acids to the ribosome as dictated by the codon sequence on mRNA. The amino acids once delivered to the ribosome form peptide bonds and assemble into a particular type of protein.
The structure of DNA and the genetic code are extremely simple. But understanding the interaction among genes and the influence of the environment is incredibly complex. This idea was expressed in a National Geographic article published Oct. 1999 (see p. 55 to read analogy to piano keyboard if you have a copy).
4. DNA is capable of mutating. Mutations at the level of DNA involve changes in the normal sequence of N-Bases found in DNA. A genetic mutation may result in a different amino acid sequence in the protein produced. A change in the amino acid sequence may result in an altered function for the protein.
A. Mutations in DNA have given rise to alternate forms of genes (i.e., alleles)
ex. The gene for hemoglobin molecules is 1000’s of base pairs long and codes for 100’s of amino acids that comprise the protein hemoglobin. A portion of the gene (the template strand) is shown below: The arrows indicate the amino acid coded for in the particular sequence.
normal DNA mutated DNA
C T C C A C
GLUTAMATE VALINE
error is the result of a single base
change. The resulting hemoglobin with a valine in place of a
glutamate fails to support the shape of RBC
and sickle-shaped RBC may result. An advantage of this
mutation, however, is that the malarial parasite is less likely to survive
in one who
carries this mutation. It seems that a modest amount of
collapsed RBC deters the parasite yet is does little harm to the oxygen
carrying
capacity of one's blood stream. Thus, heterozygotes, i.e.
carriers of the sickle-cell allele, enjoy protection from malaria and the
unfortunate
potential to have children homozygous for the allele and who thus
develop sickle-cell anemia.
ex.
mutant mosquitoes withstand pesticides, mutant bacteria withstand
antibiotics. The mutants make altered proteins which provide for an
altered metabolism enabling them to live with the poison pesticide or
antibiotic. More about this in our coverage of microevolution
(chapter 12).