Biology Reference
In-Depth Information
sugar in DNA is deoxyribose, the sugar in RNA is ribose; (3) thymine in
DNA is replaced by uracil (U) in RNA.
There are 20 different amino acids, and each set of three bases in the
mRNA constitutes a codon, which specifies either a particular amino acid,
or a stop signal. This correspondence is called the genetic code. Thus, the
amino acid sequence of a protein can be deduced from the cDNA (or
mRNA) sequence. In Chapter 2, Avraham and Hasson provide a succinct
discussion of the organization of chromosomes and genes and the flow of
information from DNA to RNA to protein. Avraham and Hasson also
delineate how different types of mutations result in abnormal gene expres-
sion or gene products.
Less than 2% of DNA codes for proteins. Some of the noncoding
sequence seems to be important for control of gene expression, but a large
amount has no known function. Repetitive sequences are found dispersed
throughout the genome, with the most common being the
Alu
sequence,
which is about 300 bp in length and occurs about 500,000 times in the
human genome. Any DNA segment of 30,000 bp is likely to contain an
Alu
sequence, but whether these sequences serve any function is not
known.
Chromosome morphology is generally defined by the position of a
constricted region called the centromere, which separates the p (or short)
arm from the q (or long) arm. If the centromere is near the middle,
the chromosome is metacentric, and if it lies near the end (telomere), the
chromosome is acrocentric. The telomeres are specialized structures that
are thought to protect chromosome ends from degradation. Chromo-
somes can generally be distinguished from one another based on size
and position of the centromere, but they can also be differentiated in
terms of banding patterns that are obtained when the chromosomes
are stained with certain compounds. The resulting set of dark and light
bands is numbered so as to provide coordinates for specific regions of the
chromosome.
Abnormal chromosomes are probably responsible for more than half the
spontaneous abortions, and they are also often found in children with con-
genital malformations. Giersch and Morton (Chapter 3) provide a com-
prehensive review of the many types of chromosomal abnormalities that
occur, and particularly emphasize those in which the phenotype includes
hearing loss.
3. Genes in Populations
Allele and genotype frequencies in a population depend on factors such
as mating patterns, population size, mutation, migration, and selection.
