(c)1998 Brooks/Cole Publishing Company/ITP 1-800-590-9951
Last Updated August 1998
From DNA to Proteins (Chapter 14)
Beyond Byssus
A.The marine mussel manufactures the ultimate underwater
adhesive, a protein called byssus.
B.DNA is like a book of instructions in each cell.
1.The instructions are written in the alphabet of A, T, G,
and C. But merely knowing the letters does not tell us
how the genes work.
2.DNA consists of two strands of nucleotides twisted
together in a double helix.
a.All DNA is composed of nucleotide subunits
utilizing the same four bases but the base sequence
differs from species to species.
b.In replication, the two strands unwind to serve as
templates for assembly of new complementary
strands.
3.Each gene is a linear stretch of DNA nucleotides that
codes for the assembly of amino acids into a polypeptide
chain.
4.The path from genes to proteins has two steps:
a.In transcription, molecules of RNA are produced
on the DNA templates in the nucleus.
b.In translation, RNA molecules shipped from the
nucleus to the cytoplasm are used as templates for
polypeptide assembly.
5.The overall plan is expressed thus:
transcription translation
DNA ----------} RNA ---------} proteins
I.Focus on Science: Discovering the Connection Between Genes and
Proteins
II.Transcription of DNA into RNA
A.The Three Classes of RNA
1.Messenger RNA (mRNA) carries the "blueprint" for
protein assembly to the ribosome.
2.Ribosomal RNA (rRNA) combines with proteins to form
ribosomes upon which polypeptides are assembled.
3.Transfer RNA (tRNA) brings the correct amino acid to
the ribosome and pairs up with an mRNA code for that
amino acid.
B.How RNA Is Assembled
1.RNA differs from DNA in two ways:
a.RNA uses ribose sugar, not deoxyribose.
b.RNA bases are A, G, C, and URACIL (U).
2.Transcription differs from replication in three ways:
a.Only one region of one DNA strand is used as a
template.
b.RNA polymerase is used instead of DNA
polymerase.
c.RNA is single stranded; DNA is double.
3.Transcription begins when RNA polymerase binds to a
promoter region (a base sequence at the start of a gene)
and then moves along to the end of a gene; an RNA
transcript is the result.
C.Finishing Touches on the mRNA Transcripts
1.Newly formed mRNA is an unfinished molecule, not yet
ready for use.
2.mRNA transcripts are modified before leaving the
nucleus.
a.The 5' end is capped with a special nucleotide that
may serve as a "start" signal for translation.
b.A "poly-A tail" of about 100-200 molecules of
adenylic acid is added to the 3' end.
c.Noncoding portions (introns) are snipped out, and
actual coding regions (exons) are spliced together
to produce the mature transcript.
III.Deciphering the mRNA Transcripts
A.The Genetic Code
1.Both DNA and its RNA transcript are linear sequences of
nucleotides carrying the hereditary code.
2.Every three bases (a triplet) specifies an amino acid to be
included into a growing polypeptide chain; the complete
set of triplets of is called the genetic code.
a.Each base triplet in RNA is called a codon.
b.The genetic code consists of sixty-one triplets that
specify amino acids and three that serve to stop
protein synthesis.
c.AUG (specifies methionine) is the "start" codon.
d.With few exceptions, the genetic code is universal
for all forms of life.
B.Roles of tRNA and rRNA
1.Each kind of tRNA has an anticodon that is
complementary to an mRNA codon; each tRNA also
carries one specific amino acid.
2.After the mRNA arrives in the cytoplasm, an anticodon
on a tRNA bonds to the codon on the mRNA, and thus a
correct amino acid is brought into place.
3.The first two bases of the anticodon must pair up with the
codon by the usual rules of base pairing (A with U and G
with C), but there is some latitude in the pairing of the
third base (called the wobble effect).
4.A ribosome has two subunits (each composed of rRNA
and proteins) that perform together only during translation
IV.Stages of Translation
A.In initiation, a complex forms in this sequence: initiator tRNA +
small ribosomal subunit + mRNA + large ribosomal subunit.
B.In elongation, a start codon on mRNA defines the reading
frame; a series of tRNAs deliver amino acids in sequence by
codon-anticodon matching; a peptide bond joins each amino
acid to the next in sequence.
C.In termination, a stop codon is reached and the polypeptide
chain is released into the cytoplasm or enters the cytomembrane
system for further processing.
D.The three steps just outlined can be repeated many times on the
same mRNA because several ribosomes may be moving along
the mRNA at the same time (polysome).
V.How Mutations Affect Protein Synthesis
A.A gene mutation is a change in one to several bases in the
nucleotide sequence of DNA, which can result in a change in the
protein synthesized.
B.Common Types of Gene Mutations
1.Mutations can result from base-pair substitutions,
insertions ("frameshift mutation") deletions.
2.They can also result when DNA regions (called
transposable elements) move form one location to another
in the same DNA molecule of a different one.
C.Causes of Gene Mutations
1.Mutations are rare, chance events but each gene has a
characteristic mutation rate.
2.Mutations can be caused by mutagens such as ultraviolet
radiation, ionizing radiation (gamma and X-rays) and
chemicals such as alkylating agents, which act as
carcinogens.
D.The Proof is in the Protein
1.If a mutation arises in a somatic cell, it will affect only the
owner of that cell and will not be passed on to offspring.
2.If however, the mutation arises in a gamete, it may be
passed on and thus enter the evolutionary arena.
3.Either kind of mutation may prove to be harmful,
beneficial, or neutral in its effects.
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