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4GENIE
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The knowledge of gene activity arose from the experiments of several investigators. Garrod reasoned the
cause for inborn errors of metabolism. Beadle and Tatum,
working with red bread mold, suggested the one gene—
one enzyme hypothesis. Pauling and Itano refined this
to the one gene—one polypeptide hypothesis.
RNA differs from DNA in several ways: (1) The
pentose sugar is ribose, not deoxyribose; (2) the base
uracil replaces thymine; and (3) RNA is single stranded.
According to the central dogma of molecular biology,
DNA is the template for its own replication and also for
RNA formation. The sequence of bases in DNA specifies
the proper sequence of amino acids in a polypeptide. The
genetic code is a triplet code, and each codon (code word)
carried by mRNA consists of three bases. The code is almost universal among organisms.
Polypeptide (protein) synthesis requires transcription and translation. Transcription, the synthesis of an
EIVES WORK_
RNA molecule off a DNA template, begins when RNA
polymerase attaches to a promoter. Elongation of an RNA
molecule occurs through the process of complementary
base pairing until there is a DNA stop sequence. Messenger
RNA (mRNA), which now carries codons, is processed
before it leaves the nucleus; in particular, introns are removed by RNA splicing. In humans, 95% of proteincoding genes are introns, which increases the proportion
of non-protein-coding DNA sequences, a finding that is
of interest to investigators.
Translation, the making of a polypeptide in the
cytoplasm, requires several types of RNA. Ribosomal
RNA (rRNA) and various proteins make up a ribosome
where a polypeptide is formed. As a ribosome moves
down an mRNA strand, the codons pair with the anticodons of transfer RNA (tRNA), which bring amino acids
to the ribosomes. Because of this process, the amino acids
are joined according to the sequence of bases in DNA.
Study the text section by section as you answer the questions that follow.
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• Each gene specifies the amino acid sequence of one polypeptide of a protein, a molecule that is essential to the
structure and function of a cell.
Indicate whether these statements are true (T) or false (F).
a.
Garrod was the first to suggest an association between genes and proteins.
b.
Beadle and Tatum induced mutations in asexual haploid mold spores.
c.
Beadle and Tatum proposed the one gene–one enzyme hypothesis.
Pauling and Itano showed that a mutation leads to a change in the structure of protein in hemoglobin.
d.
Pauling and Itano proposed the one gene–one enzyme hypothesis.
2. The work of these investigators made it possible to conclude that
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• The expression of genes leading to a protein product involves two steps, called transcription and translation.
• Three different types of RNA molecules are involved in transcription and translation.
3. Because genes (DNA) reside in the a.
the b-
,
of the cell and polypeptide (protein) synthesis occurs in
they must have a go-between. The most likely molecule to fill this role is
C.
4. Indicate whether these statements about differences between DNA and RNA are true (T) or false (F).
a.
DNA is double stranded; RNA is single stranded.
b.
DNA is a polymer; RNA is a building block of that polymer.
c.
DNA occurs in three forms; RNA occurs in only one form.
d.
The sugar of DNA is ribose, which is absent in RNA.
e.
Uracil, in RNA, replaces the base thymine, found in DNA.
5. Complete this table to describe the function of the various types of RNA involved in protein synthesis.
RNA
Function
messenger RNA (mRNA)
a.
ribosomal RNA (rRNA)
b.
transfer RNA (tRNA)
C.
6. Label the following diagram, which pertains to the central concept of genetics.
1, mi ,r
0 cc,
xc.
.
;
1
a.
J
cc
J J GI .
ii
mRNA
i
K
I i
I
II
I
I
I
codon 1 codon 2 codon 3
0
0
0
I
I
I
polypeptide —N—C—C—N—C—C—N—C—C-
I
, R,
.
I
I
i
arginine
1
R,
"
tyrosine
i
R,
"
tryptophan
Ill
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• The genetic code is a triplet code; each code word, called a codon, consists of three nucleotide bases, and
stands for a particular amino acid of a polypeptide.
7. Study the following diagram, which lists the mRNA codons.
First
Base
u
C
A
G
Second
Base
Third
Base
U
C
A
G
UUU
phenylalanine
UUC
phenylalanine
UUA
leucine
UUG
leucine
CUU
leucine
CUC
leucine
CUA
leucine
CUG
leucine
AUU
isoleucine
AUC
isoleucine
AUA
isoleucine
AUG (start)
methionine
GUU
valine
GUC
valine
GUA
saline
GUG
valine
UCU
serine
UCC
serine
UCA
serine
UCG
serine
CCU
proline
CCC
proline
CCA
proline
CCG
proline
ACU
threonine
ACC
threonine
ACA
threonine
ACG
threonine
GCU
alanine
GCC
alanine
GCA
alanine
GCG
alanine
UAU
tyrosine
UAC
tyrosine
UAA
stop
UAG
stop
CAU
histidine
CAC
histidine
CAA
glutamine
CAG
glutamine
AAU
asparaqine
AAC
asparagine
AAA
lysine
AAG
lysine
GAU
aspartic acid
GAC
aspartic acid
GAA
glutamic acid
GAG
qlutamic acid _
UGU
cysteine
UGC
cysteine
UGA
stop
UGG
tryptophan
CGU
arginine
CGC
arginine
CGA
arginine
CGG
arginine
AGU
serine
AGC
swine
AGA
arginine
AGG
arginine
GGU
glycine
GGC
glycine
GGA
glycine
GGG
qlycine
u
c
A
G
u
c
A
G
u
C
A
G
U
c
A
G
a. What does it mean to say that the genetic code is a triplet code?
b. What are the mRNA codons for leucine?
c. What does it mean to say that the genetic code is degenerate? _
d. What does it mean to say that the genetic code is unambiguous?
• During transcription, a segment of the DNA strand serves as a template for the production of an RNA
molecule.
8. During transcription, an mRNA molecule is formed that has a sequence of bases a
of one DNA strand. The bases pair in this manner: A in DNA pairs with b
c.
'
to a portion
'
,
and G pairs with
(and vice versa) in the mRNA being formed. If the sequence of bases in DNA is CGA AGC
TCT, then the sequence in mRNA is d
Why is there a space between every three bases? e
118
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9. a. Which are spliced out when primary RNA is processed—exons or introns?
b. What tells a ribosome where to attach—the cap at the 5' end or the poly-A tail at the 3' end?
c. How are introns removed from mRNA?
d. Which one, a spliceosome or a ribozyme, is an intron and also an enzyme?
e. What are two possible functions of introns?
• During translation, the amino acids of a specific polypeptide are joined in the sequence directed by a type of
RNA called messenger RNA (m RNA).
• Transfer RNA (tRNA) molecules transfer amino acids to the ribosomes.
• Ribosomal RNA (rRNA) is produced from a DNA template in the nucleolus.
10. Features of tRNA structure include: At one end, an a.
attaches, and at the other end there is a(n)
complementary to a codon in mRNA.
11. Features of rRNA structure include: Each ribosome is composed of a a.
subunit and a
b.
subunit. Ribosomes have a binding site for C.
and three d '
at a
time. Several ribosomes moving down the same mRNA plus the mRNA make up a e.
12. Label the following diagram using the alphabetized list of terms.
amino acid
A site
codon
E site
large ribosomal subunit
mRNA
P site
small ribosomal subunit
h.
9.
f.
e.
13. Use the diagram in question 12 to help you answer questions with regard to initiation.
During initiation, two ribosomal subunits come together to form a a.
. An initiator tRNA
occupies the P site of the ribosome. A b.
will soon occupy the A site.
b.
c.
14. What are the three steps of translation? a.
119
15. Use the following diagram to help you answer the questions with regard to elongation.
During elongation, the tRNA at the a-
b.
site passes its peptide to the tRNA-amino acid at the
site. Then C.
occurs, and the spent tRNA leaves from the d.
site. The tRNA bearing the peptide moves to the P site, and the A site is now empty. The next tRNA-amino
acid arrives, and this tRNA pairs with its e.
. This sequence of events reoccurs until the
polypeptide is complete.
16. Use the following diagram to help you answer questions with regard to termination.
.to
3'
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During termination, the ribosomal a last tRNA before the stop codon UGA.
120
separate, and the b.
is released from the
17. Complete this diagram by identifying each of the components depicted.
TRANSCRIPTION -c
1. DNA' in nucleus1
..„7,—,..t r-r-,
,,,, t,_
b.
ir
/.."._
-. . ,„-.
serves as a template
,----'' '\ introns
d.
„
...."'"'
e.
,
---'
----
mature
,...-
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---::.a&-z--- 1 ,....................--_..,::„
.--
...--'
A
L a.
,
f.
...."4 ' — I.
.
-
3. mRNA moves into
cytoplasm and
5'
becomes associated
4..
with ribosomes.
-.. .
9.
,
/-*
4. tRNAs with
anticodons carry
amino acids
to mRNA.
a
h
6. Polypeptide synthesis
takes place one amino
acid at a time.
cccUGGUUU
5'
.
0 GG ACC 1 4t
k.
1
i rn
L-T---/
""rairl441,..__
j.
5. Anticodon—codon
complementary
base pairing occurs.
If you identified all correctly, you have found
your way out.
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Review key terms by completing this crossword puzzle, using the following alphabetized list of terms:
anticodon
codon
exon
intron
messenger RNA
mutagen
ribosomal RNA
ribozyme
RNA polymerase
transcription
transfer RNA
translation
triplet code
Across
2 Type of RNA found in ribosomes that coordinates
the coupling of anticodons with codons during
polypeptide synthesis (two words)
Genetic code (mRNA, tRNA) in which sets of three
bases call for specific amino acids in the formation
of polypeptides (two words)
Noncoding segment of DNA that is transcribed but
removed before mRNA leaves the nucleus
Type of RNA formed from a DNA template and
bearing coded information that directs the amino
acid sequence of a polypeptide (two words)
11 Enzyme that speeds the formation of RNA from a
DNA template (two words)
12 Process whereby the sequence of codons in mRNA
determines the sequence of amino acids in a polypeptide
I
Down
1 Type of RNA that transfers a particular amino acid
to a ribosome during protein synthesis; at one end,
it binds to the amino acid, and at the other end, it
has an anticodon that binds to an mRNA codon (two
words)
3 Three nucleotides on a tRNA molecule that are attracted to a complementary codon on mRNA
5 In a gene, the portion of DNA base sequence that
is expressed resulting in polypepetide formation
6 Three nucleotides of DNA or mRNA; it stands for
a particular amino acid
8 Process whereby a DNA strand is a template for
the formation of mRNA
9 Agent, such as radiation or a chemical, that brings
about a mutation in DNA
10 Enzyme that carries out mRNA processing