Biomedical Engineering Reference
In-Depth Information
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X Y
FIGURE 3.12
This karyotype of a normal human male shows the 22 pairs of autosomal chromosomes in
descending order based on size, as well as the X and Y sex chromosomes.
chromosomes, XX for females and XY for males (Figure 3.12). If the DNA from all 46 chro-
mosomes in a human somatic cell—that is, any cell that does not become an egg or sperm
cell—was stretched out end to end, it would be about 2 nm wide and 2 m long. Each
chromosome contains thousands of individual genes that are the units of information about
heritable traits. Each gene has a particular location in a specific chromosome and contains
the code for producing one of the three forms of RNA (ribosomal RNA, messenger RNA,
and transfer RNA). The Human Genome Project was begun in 1990 and had as its goal to
first identify the location of at least 3,000 specific human genes and then to determine the
sequence of nucleotides (about 3 billion!) in a complete set of haploid human
chromosomes (one chromosome from each of the 23 pairs). See Chapter 13 for more infor-
mation about the Human Genome Project.
DNA replication occurs during cell division (Figure 3.13). During this semiconservative
process, enzymes unzip the double helix, deliver complementary bases to the nucleotides,
and bind the delivered nucleotides into the developing complementary strands. Following
replication, each strand of DNA is duplicated so two double helices now exist, each consist-
ing of one strand of the original DNA and one new strand. In this way, each daughter
cell gets the same hereditary information that was contained in the original dividing cell.
During replication, some enzymes check for accuracy, while others repair pairing mistakes
so the error rate is reduced to approximately one per billion.
Since DNA remains in the nucleus, where it is protected from the action of the cell's
enzymes, and proteins are made on ribosomes outside of the nucleus, a method (transcrip-
tion) exists for transferring information from the DNA to the cytoplasm. During tran-
scription (Figure 3.14), the sequence of nucleotides in a gene that codes for a protein is
transferred to messenger RNA (mRNA) through complementary base pairing of the nucle-
otide sequence in the gene. For example, a DNA sequence of TACGCTCCGATA would
become AUGCGAGGCUAU in the mRNA. The process is somewhat more complicated,
since the transcript produced directly from the DNA contains sequences of nucleotides,
called introns, that are removed before the final mRNA is produced. The mRNA also
has a tail, called a poly-A tail, of about 100-200 adenine nucleotides attached to one end.
A cap with a nucleotide that has a methyl group and phosphate groups bonded to it
is attached at the other end of the mRNA. Transcription differs from replication in that
(1) only a certain stretch of DNA acts as the template and not the whole strand, (2) different
enzymes are used, and (3) only a single strand is produced.
