Biology Reference
In-Depth Information
nitrogenous bases, and a phosphoric acid component. DNA consists of two com-
plementary strands in a helix form. Pairing of the nitrogenous bases adenine (A)
with thymine (T) and cytosine (C) with guanine (G) on the two complementary
strands occurs by hydrogen bonding. A pairs with T by two hydrogen bonds, and
C pairs with G by three hydrogen bonds. DNA has chemically distinct 5
′
and 3
′
ends, and the two strands are antiparallel, with one strand running in the 5
′
to
3
′
direction and the other strand running in the 3
′
to 5
′
direction. The antipar-
allel orientation of the two strands creates a special problem when the DNA is
duplicated or replicated during mitosis or meiosis.
Genetic information in protein-coding genes is determined by the sequence
of nitrogenous bases (A, T, G, C) in one of the strands, with a three-base (triplet)
codon designating an amino acid. The genetic code is degenerate, meaning that
more than one codon specifies most amino acids. The genetic information is
expressed when DNA is transcribed into pre-messenger RNA (pre-mRNA) that is
processed into mRNA and then translated into polypeptides. Most insect genes
have intervening noncoding sequences (introns) that must be removed from the
primary RNA molecule before translation into the protein can occur.
Efficient and accurate replication of DNA must occur at each cell division, or the
cell or organism may not survive. DNA replication is semiconservative, i.e., one of
the nucleotide strands of each new DNA molecule is new and the other nucleo-
tide strand is old in each “cell generation.” The new DNA strand is complementary
to the parental (or template) strand. DNA replication occurs in one direction only,
from the 5
′
to the 3
′
end of the strand, and thus replication takes place differently
on the two antiparallel strands. Replication on the “leading strand” can occur in the
5
′
to 3
′
direction in a continuous manner. DNA replication on the other strand, the
“lagging strand,” occurs in short segments (Okazaki fragments) because the DNA
runs in the 3
′
to 5
′
direction. Subsequently, the Okazaki fragments must be ligated
together. Replication of DNA in chromosomes begins at multiple sites called origins
of replication along the chromosome, and it involves many enzymes and proteins.
Although DNA replication is usually highly accurate, errors in DNA replication, or
mutations, can result from duplications, deletions, inversions, and translocations of
nucleotides, all of which may affect the functioning of the resultant polypeptide.
New combinations of genes can occur through recombination during meiosis.
1.2 DNA is the Hereditary Material: A Brief History
Gregor Johann Mendel founded modern genetics in 1866 by publishing his
studies on inheritance in garden peas. He confirmed that hereditary traits were
transmitted from generation to generation, and he proposed the principles
