DNA & PROTEIN SYNTHESIS
DNA (deoxyribonucleic acid)—molecule in the shape of a double helix. Made up of long chains of
nucleotides consisting of a nitrogen base, a sugar molecule, and a phosphate group. The order that
these nucleotides occur in determines the structure of proteins during protein synthesis.
Francis Crick and James Watson won a Nobel Prize in 1962 for their discovery of the structure of the
DNA molecule in 1953.
Nucleotides are named for the nitrogen base that is contained in them. There are four nitrogen
bases:
1.
Adenine (A)
2.
Thymine (T)
3.
Guanine (G)
4.
Cytosine (C)
Base-pairing rule—when rungs in the ladder-like structure of DNA form, adenine always pairs with
thymine, and guanine always pairs with cytosine.
Replication—the process of the DNA molecule making a copy of itself. DNA replicates during the S
stage of the cell cycle so that each new cell will have a complete new copy of the genetic code after
mitosis occurs. It also replicates before the first meiotic division.
The steps DNA goes through during replication are:
1.
Chemical bonds between nitrogen bases break and the molecule unzips.
2.
Enzymes cause free nucleotides in the nucleus to pair with complementary bases on
original DNA strands.
3.
Other enzymes link the new nucleotides to form new DNA strands.
4.
Thus, each original strand serves as a template, or pattern, for each new strand.
5.
Each completed DNA molecule contains one old and one new strand.
6.
The process is powered by ATP and enzyme action.
Protein synthesis—process by which cells make protein. Occurs in the ribosomes.
RNA (ribonucleic acid)—reads and decodes the DNA molecule in the process of protein synthesis. It
is similar to DNA with the following exceptions:
1.
Consists of a single strand
2.
Contains the base uracil instead of thymine
3.
Ribose five carbon sugar
Types of RNA:
1.
mRNA (messenger RNA)—single stranded molecule that is active in the process of
transcription and translation.
2.
tRNA (transfer RNA)—single strand folded back on itself in hairpin fashion that is active
in the process of translation. Exists in 20 plus varieties. Each bonds to only one type of
amino acid.
3.
rRNA (ribosomal RNA)—a globular form of RNA which, along with proteins, makes up
the ribosomes.
Terminology associated with protein synthesis:
1.
Codon—three letter sequence of bases on the mRNA
molecule that codes or stands for a specific amino acid.
2.
Anticodon—three bases on the tRNA nucleotide that are
complementary to one of the mRNA codons.
Transcription—first stage in protein synthesis. It is the process by which mRNA copies the genetic
code from DNA. Steps in transcription:
1.
A segment of DNA that codes for a particular protein unzips. This occurs due to the
binding of RNA polymerase to regions of the DNA molecule called promoters. The
RNA polymerase then uses one of the strands of DNA as a template from which
nucleotides are assembled into a strand of mRNA.
2.
mRNA nucleotides in the nucleus bind with their complementary nucleotides on DNA
3.
Each three letter sequence of mRNA nucleotides is called a codon, and codes for a
specific amino acid.
4.
Other codons do not stand for any particular amino acid. They may be the initiation or
“start” codon (AUG) or the “stop” codons (UGA, UAA, UAG) which act as the period
at the end of a sentence.
Translation—second stage in protein synthesis. It is the process by which mRNA attaches to
ribosomes to be read by tRNA and carry out the formation of a polypeptide or a protein.
1.
DNA strands unzipped during transcription rejoin and mRNA moves into the cytoplasm
through a nuclear pore.
2.
mRNA attaches to a ribosome.
3.
tRNA nucleotides in the cytoplasm with the appropriate anticodon to match the mRNA
codon move into place in the ribosome, attaching to the mRNA.
4.
As the amino acids attached to the specific tRNA molecules come into close proximity
to one another in the ribosome, the ribosome initiates a peptide bond between
them.
5.
At the same time, the ribosome causes the bond to break that held the specific amino
acid to the tRNA anticodon. The ribosome then moves to the next mRNA codon,
where a tRNA nucleotide brings in the amino acid specified by that codon.
6.
The polypeptide chain continues to grow until the ribosome reaches a stop codon (UGA,
UAA, or UAG). The ribosome then releases the newly formed polypeptide and the
mRNA molecule, completing the process of translation
MUTATIONS
Mutations—permanent change in DNA or a chromosome. Most are harmful, beneficial ones
ultimately provide the variation upon which natural selection acts. If a mutation occurs in
reproductive cells (germ cells), and these cells take part in fertilization, the altered gene would
become part of the genetic makeup of the offspring.
Two types:
A. Germ cell mutations—occur in sex cells, may be passed on to offspring
B.
Somatic mutations—occur in body cells; cannot be passed on to offspring
Chromosome mutations—usually occur during cell division. May be changes in the structure of a
chromosome or loss of an entire chromosome. Types include:
A.
B.
C.
D.
E.
Deletion—piece of a chromosome breaks off
Inversion—piece breaks off of a chromosome and reattaches in reverse
orientation
Translocation—piece breaks off of a chromosome and attaches to a nonhomologous chromosome
Duplication—genes displayed twice on the same chromosome
Gene rearrangement—a gene may be moved to an entirely different location on
a chromosome
Gene mutations—may involve a single nitrogen base within a codon or larger segments of the
DNA molecule.
A.
Point mutation—change of a single nucleotide in a sequence from one kind of
base to another
B.
Insertion or deletion—rarely, errors in replication can cause the insertion or
deletion of one or more nucleotides in a sequence. An insertion or deletion
may cause a frameshift mutation.
Missense mutation—a codon is changed such that the new codon codes for a
different amino acid
Nonsense mutation—occurs when a codon that normally codes for an amino
acid is changed to a stop codon
C.
D.
Silent mutation—occurs when a mutation has no effect on a gene’s function.
E.
Mutations are often silent because the genetic code is redundant (each amino
acid has multiple codons).
Mutagens—environmental factors that may damage DNA, thus possibly causing a mutation (since
cancer is caused by a mutation, they are also called carcinogens)
May include:
A.
Radiation
B.
C.
Chemicals
Viruses