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MolecuLan Gemettes
Gregor Mendel discovered how certain traits are
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inherited. Based on his work, scientists discovered
that genetics involves a variety of patterns of
inheritance. They also djscovered that traits were
nucteic
controlled by genes Located on the chromosomes.
The next steps were to determine the chernica[
large organic mo[ecule
deoxyribonucteic acid (DNA)-the nucteic
acid that stores the genetic information as
nature of the gene and how a gene operates to
express a parLicular trait. The answers are being
genes on a cnromosome
reptication-the process of copying
found jn a class of organic compounds known as
nucleic acids. Nucteic acids are very large organi'c
molecuLes
acid-a
that stores and carries qenetic information
in the ce[[
DNA
ribonucteic acid-the motecule that is
responsibte for assembting proteins based
on the genetic information contained in
that store information in the ce[t.
DNA
messenger RNA-the nucleic acid that
carries the genetic information from the
DNA
The nucteic acid that stores the genetic information as genes
on a chromosome is deoxyribonucteic acid, DNA. DNA is
a
[arge mo|'ecuLe that consjsts of two chains twisted around each
other. The buiLding bLock that makes up each chain is catled
a nucleotide. A DNA nucteotide has three parts: (1) a sugar
molecute catted deoxyribose; (2) a phosphate group; and (3)
nitrogen-containing base. There are four nitrogen-containing
bases in DNA: adenine (A), thymine (T), guanine (G), and
in the nucleus to the cytoplasm
transfer RNA-the nucleic ac'id that
transoorts amino acids to the ribosomes
where they wjU. be assembled into a
protein
ribosomal RNA-the nucleic acid that
a
cytosine (C).
In
1953. two scientists named James Watson and Francis
Crick proposed a modeL as to how these four nucleotides were
arranged to make up a DNA molecuLe. They suggested that DNA
is composed of two nucleotide chains that wrap around each
other to form a double spiral, Like a spiral staircase- This
structure is calted a doub[e helix, which is shown in the
fo[[owing i[[ustration.
combines with oroteins to form the
ribosomes
transcription-the process by which
genetic information is copied from DNA to
RNA
translation-the
process of assembLi ng
amino acids based on the information
in
mRNA
codon-a sequence of three
nitrogen-containing bases in mRNA
that snecifies a narlicular amino
acid
Phosphate
anticodon-a sequence of three
nitrogen-containing bases in IRNA
that pairs with the three-base
codon in mRNA
Sugar
Doubte Fletix
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Unit 4, Mo[ecuiar Genetics
BioLogy. 5V 0423-9
otice that the sugars and phosphate groups
the "backbone" of the mo[ecute. The four bases
the "steps" of the double heLix. The bases
's form pairs. These pairs
of bases are catted
Lementary base pairs. There are two
[ementary base pairs: A-T and G-C.
ach base pair is hetd together by a relativety weak
. These bonds are broken when a celt is about to
,go mitosis. Each chromosome must be dup[icated
e mitosis starts so that each cetl receives the
:r genetic information. The duplication of a
nosome means that the DNA must be copied.
)rocess of copying DNA is catled reptication.
leplication begins with the breaking of bonds
een the base pairs. This happens simultaneously
fferent sites atong a chromosome. The break'ing
ese bonds separates the two chains of the double
. Nucleotides in the nucteus are then joined to
rucleotides on each chain. This process is always
according to comptementary base pairing.
:fore, if A is present on a chain, then only T can
aired with it. Likewise, if C is present on a chain,
onty G can be paired lvith it. As a result of
cation, two DNA molecules are produced, each
lical to the origina[. The fotlowing diagram
trates the process of replication.
RruA
the synthesis of proteins. However, DNA is
found mainly in the nucleus of a cetl while protein
synthesis occurs in the cytopLasm. How then does a
motecule in the nucleus control a process that occurs
in the cytoplasm? The answer can be found in the
second type of nucleic acid-ribonucteic acid, RNA.
RNA is the motecute that is responsibte for assembling
proteins based on the genetic information contained
DNA directs
in
DNA.
Three major differences
in structure exist
between DNA and RNA. First, RNA contains the sugar
ribose, whereas DNA contains the sugar deoxyribose.
Second, RNA contains the base uracit (U) in p[ace
of thymine (T). Third, RNA exists in three different
types. Each type plays a different rote in protein
synthesis.
type of RNA is called inessenger RNA (mRNA).
Messenger RNA carries the genetic information from
the DNA in the nucleus to the cytoplasm. A second
type of RNA is calLed transfer RNA (IRNA), which
transports amino acids to the ribosomes where they
witl be assembted into a protein. A third type of RNA
is caLled ribosomal RNA (rRNA), which combines with
proteins to form the rjbosomes.
Att RNA molecutes are made from DNA in a process
calted transcription. During transcription, the two
strands of a DNA molecute separate just as they do in
reptication. However, enzymes invotved in transcription
wit[ direct the synthesjs of RNA from one of the DNA
strands. Base pairings are a[so folLowed in transcription,
just as they are in replication. However, in transcription,
uracit (U) pairs with adenine (A). The fo[towing figure
illustrates the process of transcription.
One
Reptication
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Unit 4, Molecular Genetics
Biotogy, 5V 0423-9
compLementary
R'ofreim
to and pairs with its corresponding
mRNA codon.
furnatBeesEs
For exampte, assume
A sequence of nitrogen-containing bases in DNA wjl.t
direct the order in which amino acids are assembled
into a proter'n. This process begins with transcription.
The mRNA that is made by transcription moves from
that
mRNA contains ihe
codon UUU. A IRNA with the anticodon AAA woutd
pair with this mRNA codon. A tRNA with the
anticodon AAA is specific for the amino acjd
phenylaLanine. Assume that the next mRNA codon is
GGG. A IRNA with the anticodon CCC would pair with
the nucteus to the cytop[asm where it attaches to
ribosomes. Here is where the amino acids wit[ be
assembted. The process of assembling amino acids
based on the information in mRNA is calted
translation.
Recatl that IRNA transfers amino acids floating
freeLy in the cytoplasm to the ribosomes. The genetic
information in mRNA is decoded in a series of three
nitrogen-containing bases. Each combinatjon of three
such bases in mRNA is called a codon. Each codon
codes for a specific amino acid.
this mRNA codon. A tRNA with the anticodon CCC
is specific for the amino acid glycine. This glycine
molecule is then joined to the phenylatanine
molecule. Therefore, the two mRNA codons
have specified the amino acid sequence:
phe ny[aLa ni ne-g Lyci ne.
Transtation continues untiI atl the amino acids
have been assembted into a polypeptide. The
polypeptide is released from the ribosome. It then
may combine wjth other poLypeptides and fotd to form
a functionat protein. The fotlowing diagram i[lustrates
A three-base combination is also found on IRNA.
The sequence of three nitrogen-containing bases on
the nrocess of transtation-
IRNA is catled an anticodon. The IRNA anticodon is
Transfer RNA
Amino".,Oa,
$
y3
^4
n€g
>h
Potypeptide chain
Peptide bond
^_P-
Anticodon
_1 ,
Ribosome
[-cnc
4uG--'NLACACAU
cccnu
Messenger RflA
Traris[atios-r
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Unit 4, Mglecular Genetics
Biology, SV 0423-s
DATE
rdgw &
rken the circle bY the best answer.
Which represents a correct base pair found in
a DNA molecule?
5. In translation, what anticodon on IRNA pairs
with the mRNA codon AUC?
@ G-A
@ C-T
O C-G
@
@
@ T-U
@
DNA
replication jnvolves
@ on the ribosomes.
@ according to a sequence of nitrogen-
RNA.
containing bases.
O
7.
RNA
depending on codon-anticodon pairings.
@ a[[ of the above
in place of G.
@ Ir a large molecule
UUU
order
@ substitution of the nitrogen-containing
Un[ike DNA,
I
6' Amino acids are assembled into a specific
the
separation of the two strands that make
up the doubte hetix.
base T
TAG
O UAG
@ synthesis of proteins.
@ formation of messenger
O
AUC
Describe what
is meant by a double helix.
made uP of
nuc[eotides.
@ contains the sugar ribose.
O
is found mainly in the nucleus of a ce[[-
@ contains the nitrogen-containing
base
thymine.
In transcription,
niU is synthesized from DNA-
@
@ DNA is synthesized from RNA.
@ a protein is made on the ribosomes.
.n exact copy of a DNA molecule is made
@
'
in the
8. What is the ro[e of ribosomes in protein
synthesis?
nucleus.
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Unit 4, Molecular Genetics
Biotoqy, 5V 0423-9
DATE
NAME
KFruse
&
ffiruA amd RepL€eatfceT
X" Why is the structure of a DNA motecule sometimes referred to as having the shape of a spiral
stai rcase?
2. What nitrogen-containing
3.
base pairs are found
in a DNA motecule?
Explain why DNA repLication must occur before mitosjs starts.
Exan'rine
4.
FiLL'in
the folftowimg
the letter for
iLLn"astnat'iorn
wfiich shows a short piece of a ENA snolecute.
each missi ng nitrogen-containing base.
that form after the piece of
molecules compare to the original molecule?
5" Draw the
DNA molecules
Urit 4,
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o
Hanourt
Adfu€!fft. frliffits
DNA shown above replicates. How do these
Molecutar 6enetics
AiC.gtL 5V 0423-s
reserved.
1l
-
Li:ar-s9lEQ.Qdfefs@afe@!'-+'1w*a"'"-''"''.
DATE
ME
Hruee
&
RNA amd TnaffiserlPttom
[ist of words to complete the passage below concerning the
you wi[[ not
ructure and function of RNA. There is one word on the list that
e the fotlowing
adenine
double helix
phosphate
thymine
comp[ementarY
guanine
ribose
transcription
cytop[asm
messenger RNA
ribosomes
transfer
cytosine
nuc|-eus
ribosomal
use.
RNA
uraci I
RNA
deoxyribose
DNA
'{A is different from
in
(1)
severaI ways' Whereas DNA contains the sugar
. In addition, RNA'is made using the
\A contains the sugar (2)
, whil-e
itrogen-containing base (3)
DNA contains
the
base
. Both RNA and DNA, however, contain (5)
+)
rouDs. unLike DNA, RNA exists
in three different wpes.
to the (B)
the (7)
arries the genetic information from
rhere
One type, known as (6)
the proteins are synthesized' A second type of RNA' calLed (9)
These structures are made
ransports the amino acids to the (10)
of proteins'
A[[ three tyPes of RNA are made from
he third type of RNA is caLted (11)
. Just as the DNA does in reptication, the two
)NA in a process call"ed (12)
separate. Then DNA makes a strand of RNA that is
;trands of the (13)
(14)
to the
r15'\
wiLL
base sequence of
the DNA. Therefore. the nitrogen-containing
pair with a cytosine base
in
DNA,
base
while (16)
wilt pair with a thymine base in DNA'
What is the only word that you did not use from the list above
to complete this passage?
(17 )
Unit 4. Motecular Genetics
www.
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BioLogy, SV 0423-9
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