C h a p t e r 5
Genetic Control of Cell Function and Inheritance
95
Chromosomes
Most genetic information of a cell is organized, stored,
and retrieved in discrete bundles of DNA called
chro-
mosomes.
Although the chromosomes are visible only
in dividing cells, they retain their integrity between cell
divisions. The chromosomes are arranged in pairs: one
member of the pair is inherited from the mother, the
other from the father. The maternal and paternal chro-
mosomes of a pair are called
homologous chromosomes
(homologs). Humans have 23 pairs of chromosomes (46
total). Of the 23 pairs, 22 are called
autosomes
. These
have the same appearance in all individuals, males
and females, and each has been given a numeric des-
ignation for classification purposes (Fig. 5-6). The sex
chromosomes, which make up the 23rd pair of chro-
mosomes, determine the sex of a person. All males have
an X and Y chromosome (i.e., an X chromosome from
the mother and a Y chromosome from the father); all
females have two X chromosomes (i.e., one from each
parent). The much smaller Y chromosome contains the
male-specific region
(MSY) that determines sex. This
region comprises more than 90% of the length of the
Y chromosome.
Only one X chromosome in the female is active in
controlling the expression of genetic traits; however,
both X chromosomes are activated during gametogen-
esis. In the female, the active X chromosome is invisible,
but the inactive X chromosome can be visualized with
appropriate nuclear staining. This inactive chromatin
mass is called a
Barr body
. The genetic sex of a child
can be determined by microscopic study of cell or tissue
samples for the presence of a Barr body. For example,
the cells of a normal female have one Barr body and
therefore a total of two X chromosomes. A normal male
has no Barr bodies. Males with Klinefelter syndrome
(one Y, an inactive X, and an active X chromosome)
exhibit one Barr body.
FIGURE 5-6.
Karyotype of a normal
male (xy). (From National Cancer
Institute Visuals. No. AV-9700-4394.)
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Genetic information is stored in a stable
macromolecule called deoxyribonucleic acid
(DNA). Genes transmit information contained in
the DNA molecule as a triplet code consisting of
an arrangement of the nitrogenous bases of the
four nucleotides (i.e., adenine, guanine, thymine
[or uracil in RNA], and cytosine).
■■
Gene mutations represent accidental errors in
duplication, rearrangement, or deletion of parts of
the genetic code. Fortunately, most mutations are
corrected by DNA repair mechanisms in the cell.
■■
The production of proteins requires both
DNA and a second type of macromolecule
called ribonucleic acid (RNA).The process is
accomplished by (1) the transcription of the DNA
code onto messenger RNA, (2) the synthesis
of proteins by ribosomal RNA, and (3) the
translation of messenger RNA code by transfer
RNA, which delivers the amino acids needed for
protein synthesis to ribosomal RNA of ribosomes
located in the cytoplasm.
■■
The degree to which a gene or particular group
of genes is active is called gene expression.
Gene expression involves a set of complex
interrelationships among different levels
of control including RNA transcription and
posttranslational processing.
■■
Posttranslational processing involves the proper
folding of the newly synthesized polypeptide chain
into its unique three-dimensional conformation.
Posttranslational processing may also involve the
combination of polypeptide chains from the same
or an adjacent chromosome, the binding of small
cofactors, or enzyme modification.
