HUMAN BIOLOGY

DNa, GeNes, aND BioteChNoloGy 423

A base triplet in an mRNA molecule is a codon. A given
combination of codons specifies the amino acid sequence of a
polypeptide chain.

sections 21.5, 21.6 Translation has
three stages. In initiation, a small ribosome
subunit and an initiator tRNA bind with an
mRNA transcript and move along it until
they reach an AUG start codon. The small
subunit binds with a large ribosome subunit.
In elongation, tRNAs deliver amino acids to the ribosome.
Their anticodons base-pair with mRNA codons. The amino
acids are joined (by peptide bonds) to form a new polypep-
tide chain.
In chain termination, an mRNA stop codon moves onto the
ribosome; then the polypeptide chain and mRNA detach from
the ribosome.

section 21.7 Recombinant DNA tech-
nology is the foundation for genetic
engineering. Restriction enzymes are used
to cut DNA molecules into fragments, which
are inserted into a cloning vector (such as
a plasmid) and then multiplied in rapidly
dividing cells.
A DNA clone is a foreign DNA sequence that has been
introduced and amplified in dividing cells. DNA sequences
also can be amplified in test tubes by the polymerase chain
reaction.
The genome of a species is all the DNA in a haploid set
of chromosomes. The human genome, including our species’
21,500 genes, consists of about 3.2 billion nucleotides. Some
tools of recombinant DNA technology can be used to identify
genes in the genome.

sections 21.8, 21.9 Automated

DNA sequencing can quickly determine

the sequence of nucleotides in segments

of DNA—and, accordingly, in genes. The

human genome has been sequenced, and

researchers are identifying genes responsible

for a variety of traits and genetic disorders.

sections 21.10, 21.11 Recombinant

DNA technology and genetic engineering

have enormous potential for research and

appli cations in medicine, agriculture, and

industry. Both also may pose ecological

and social risks.

review Questions

1. Why is DNA replication called “semiconservative”?


2. Name one kind of mutation that produces an altered
   protein. What determines whether the altered protein will
   have beneficial, neutral, or harmful effects?


3. How are the polypeptide chains of proteins that are
   specified by DNA assembled?


4. How does RNA differ from DNA?


5. Name the three classes of RNA and describe their functions.
   6. Distinguish between a codon and an anticodon.
   7. Describe the three steps of translation.
   8. What is a restriction enzyme?
   9. What is a “gene sequence”?

seLF-Quiz Answers in Appendix VI

1. Nucleotide bases, read at a time, serve as the “code
   words” of genes.


2. DNA contains genes that are transcribed into.
   a. proteins d. tRNAs
   b. mRNAs e. b, c, and d
   c. rRNAs


3. mRNA is produced by.
   a. replication c. transcription
   b. duplication d. translation


4. carries coded instructions for an amino acid
   sequence to the ribosome.
   a. DNA c. mRNA
   b. rRNA d. tRNA


5. tRNA.
   a. delivers amino acids to ribosomes
   b. picks up genetic messages from rRNA
   c. synthesizes mRNA
   d. all of the above


6. An anticodon pairs with the bases of.
   a. mRNA codons c. tRNA anticodons
   b. DNA codons d. amino acids


7. The loading of mRNA onto the small ribosomal subunit
   occurs during.
   a. initiation of transcription c. translation
   b. transcript processing d. chain elongation


8. Use the genetic code (Figure 21.8) to translate the mRNA
   sequence AUGcgcaccucaggaugagau. (Human reading
   frames start with AUG.) Which amino acid sequence is
   being specified?
   a. meth-arg-thr-ser-gly-stop-asp...
   b. meth-arg-thr-ser-gly...
   c. meth-arg-tyr-ser-gly-stop-asp...
   d. none of the above


9. Match the terms related to protein building.
   alters genetic a. initiation, elongation,
   instructions termination
   codon b. conversion of genetic
   transcription messages into
   translation polypeptide chains
   stages of c. base triplet for an amino
   transcription, acid
   translation d. RNA synthesis
   e. mutation


10. Rejoined cut DNA fragments from different organisms are
    best known as __.
    a. cloned genes
    b. mapped genes
    c. recombinant DNA
    d. conjugated DNA

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