Biology Reference
In-Depth Information
Box 1.1 Key Points About DNA and RNA
DNA
l
DNA is a double helix with
antiparallel strands
consisting of
four types of nucleotides
.
l
Complementary base pairing
is essential to its structure and to maintaining an accurate code during
replication.
l
Adenine pairs with thymine (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen
bonds).
l
Replication is semiconservative
, a mechanism that helps to maintain the genetic information without
errors. Daughter molecules contain one old and one new strand.
l
DNA replication occurs only from the 5
′
to the 3
′
direction and requires an RNA primer.
l
DNA replication begins at multiple origins of replication on the chromosome.
l
Replication is continuous on the leading strand of DNA but is discontinuous on the lagging
strand.
l
The
genetic code consists of a triplet of bases
and is
degenerate
; 64 codons code for only 20 amino
acids.
l
Protein-coding genes contain introns and exons in most eukaryotes
; the introns must be
removed from pre-mRNA before the genetic information can be translated into proteins in
eukaryotes.
l
DNA is organized in
chromosomes with telomeres at their ends
, an organization that helps to
preserve the ends of the chromosome.
l
DNA can be mutated or damaged and repaired by multiple repair mechanisms.
l
Mutations affect the phenotype of the insect in multiple ways.
l
DNA can be modified during meiosis by crossing over
and can result in new combinations of genes in
the gametes.
RNA
l
RNA contains uracil
rather than thymine
and ribose
sugar instead of deoxyribose.
l
Pre-mRNA is processed in the nucleus
before the mRNA can move to the cytoplasm for
translation.
l
Some RNAs are used directly
(ribosomal, transfer, and a variety of small RNAs) without being
translated into proteins.
Figure 1.2
Structure of sugars found in nucleic acids; 2
′
-deoxyribose is found in DNA and ribose is
found in RNA.
