Molecular Genetics ❮ 121this means that only one of the strands can be produced in a continuous fashion. This con-
tinuous strand is known as the leading strand.The other strand is affectionately known as
thelagging strand.You will notice that in the third step of the process in Figure 11.4, the
lagging strand consists of tiny pieces called Okazaki fragments,which are later connected
by an enzyme called DNA ligase to produce the completed double-stranded daughter DNA
molecule. This is the semi-conservative model of DNA replication.
Unlike our work, DNA replication is not a perfect process—mistakes are made.
A series of proofreading enzymes function to make sure that the DNA is properly replicated
each time. During the first run-through, it is estimated that a nucleotide mismatch is made
during replication in one out of every 10,000 basepairs. The proofreaders must do a pretty
good job since a mismatch error in replication occurs in only one out of every billion
nucleotides replicated. DNA polymerase proofreads the newly added base right after it is
added on to make sure that it is the correct match. Repair is easy—the polymerase simply
removes the incorrect nucleotide, and adds the proper one in its place. This process is
known as mismatch repair.Another repair mechanism is excision repair,in which a section
of DNA containing an error is cut out and the gap is filled in by DNA polymerase. There
are other proteins that assist in the repair process, but their identities are not of major
importance. Just be aware that DNA repair exists and is a very efficient process.
Here is a short list of mutation types that you should know:- Frameshift mutations.Deletion or addition of DNA nucleotides that does not add or
remove a multiple of three nucleotides. mRNA is produced on a DNA template and is
read in bunches of three called codons,which tell the protein synthesis machinery
which amino acid to add to the growing protein chain. If the mRNA reads: THE FAT
CAT ATE HER HAT, and the F is removed because of an error somewhere, the frame
has now shiftedto read THE ATC ATA THE ERH AT... (gibberish). This kind
of mutation usually produces a nonfunctional protein unless it occurs late in protein
production. - Missense mutation.Substitution of the wrong nucleotides into the DNA sequence.
These substitutions still result in the addition of amino acids to the growing protein
chain during translation, but they can sometimes lead to the addition of incorrect
amino acids to the chain. It could cause no problem at all, or it could cause a big prob-
lem as in sickle cell anemia, in which a single amino acid error caused by a substitution
mutation leads to a disease that wreaks havoc on the body as a whole. - Nonsense mutation.Substitution of the wrong nucleotides into the DNA sequence.
These substitutions lead to premature stoppage of protein synthesis by the early place-
ment of a stop codon,which tells the protein synthesis machinery to grind to a halt.
The stop codons are UAA, UAG, and UGA. This type of mutation usually leads to a
nonfunctional protein. - Thymine dimers.Result of too much exposure to UV (ultraviolet) light. Thymine
nucleotides located adjacent to one another on the DNA strand bind together when
this exposure occurs. This can negatively affect replication of DNA and help cause fur-
ther mutations.
Transcription of DNA
Up until this point, we have just been discussing DNA replication,which is simply the pro-
duction of more DNA. In the rest of the chapter, we discuss transcription, translation, and
other processes involving DNA. While DNA is the hereditary material responsible for
the passage of traits from generation to generation, DNA does not directly produce theKEY IDEANY teacher:
“Know the basic
principles. They’ll
ask you about this
process.”
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