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Your Name __________________________
Biology 160 Lab Module 8
DNA Replication, Transcription, and Translation
Learning Outcomes
Upon successful completion of this lab, you should be able to demonstrate:
1. An understanding of the complementary and antiparallel nature of the double-stranded
DNA molecule.
2. An understanding of the products of transcription.
3. An ability to translate a messenger RNA to produce a polypeptide (protein).
Introduction
DNA Macrostructure
The genetic instructions that are used to build and maintain an organism are arranged in an
orderly manner in strands of DNA. Within the nucleus are threads of chromatin composed of
DNA and associated proteins (see Figure below). Along the chromatin threads, the DNA is
wrapped around a set of histone proteins. A nucleosome is a single, wrapped DNA-histone
complex. Multiple nucleosomes along the entire molecule of DNA appear like a beaded
necklace, in which the string is the DNA and the beads are the associated histones. When a cell
is in the process of division, the chromatin condenses into chromosomes, so that the DNA can
be safely transported to the “daughter cells.” The chromosome is composed of DNA and
proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000
genes distributed on 46 chromosomes.
Figure. DNA
Macrostructure
Strands of DNA are
wrapped around
supporting histones.
These proteins are
increasingly bundled
and condensed into
chromatin, which is
packed tightly into
chromosomes when
the cell is ready to
divide.
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DNA Replication
In order for an organism to grow, develop, and maintain its health, cells must reproduce
themselves by dividing to produce two new daughter cells, each with the full complement of
DNA as found in the original cell. DNA replication is the copying of DNA that occurs before cell
division can take place (see ‘DNA replication’ Figure below). A DNA molecule is made of two
strands that “complement” each other in the sense that the molecules that compose the
strands fit together and bind to each other, creating a double-stranded molecule that looks
much like a long, twisted ladder. Each side rail of the DNA ladder is composed of alternating
sugar and phosphate groups (see ‘Molecular structure of DNA’ Figure below). The two sides of
the ladder are not identical, but are complementary. These two backbones are bonded to each
other across pairs of protruding bases, each bonded pair forming one “rung,” or cross member.
The four DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Because of their
shape and charge, the two bases that compose a pair always bond together: A always binds
with T, and C always binds with G. The particular sequence of bases along the DNA molecule
determines the genetic code. Therefore, if the two complementary strands of DNA were pulled
apart, you could infer the order of the bases in one strand from the bases in the other,
complementary strand. For example, if one strand has a region with the sequence AGTGCCT,
then the sequence of the complementary strand would be TCACGGA.
Figure. Molecular Structure of DNA
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Figure. DNA Replication. The copying of DNA.
From DNA to RNA: Transcription
There are several different types of RNA, each having different functions in the cell. The
structure of RNA is similar to DNA with a few small exceptions. For one thing, unlike DNA, most
types of RNA, including mRNA, are single-stranded and contain no complementary strand.
Second, the ribose sugar in RNA contains an additional oxygen atom compared with DNA.
Finally, instead of the base thymine (T), RNA contains the base uracil (U). This means that A will
always pair up with U during the protein synthesis process. Gene expression begins with the
process called transcription, which is the synthesis of a strand of mRNA that is complementary
to the gene of interest. RNA polymerase is the enzyme that carries out transcription. The
process is called transcription because the mRNA is like a transcript, or copy, of the gene’s DNA
code. Transcription begins in a fashion somewhat like DNA replication, in that a region of DNA
unwinds and the two strands separate, however, only that small portion of the DNA will be split
apart. The triplets within the gene on this section of the DNA molecule are used as the template
to transcribe the complementary strand of RNA (see Figure below). A codon is a three-base
sequence of mRNA, so-called because they directly encode amino acids.
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Figure. Transcription: from DNA to mRNA
In the first of the two stages of making protein from DNA, a gene on the DNA molecule is
transcribed into a complementary mRNA molecule.
From RNA to Protein: Translation
Like translating a book from one language into another, the codons on a strand of mRNA must
be translated into the amino acid alphabet of proteins. Translation is the process of
synthesizing a chain of amino acids called a polypeptide (see Figure below). Translation
requires two major aids: first, a “translator,” the molecule that will conduct the translation, and
second, a substrate on which the mRNA strand is translated into a new protein, like the
translator’s “desk.” Both of these requirements are fulfilled by other types of RNA. The
substrate on which translation takes place is the ribosome. Ribosomal RNA (rRNA) is a type of
RNA that, together with proteins, composes the structure of the ribosome. Ribosomes exist in
the cytoplasm as two distinct components, a small and a large subunit. When an mRNA
molecule is ready to be translated, the two subunits come together and attach to the mRNA.
The ribosome provides a substrate for translation, bringing together and aligning the mRNA
molecule with the molecular “translators” that must decipher its code. The other major
requirement for protein synthesis is the translator molecules that physically “read” the mRNA
codons. Transfer RNA (tRNA) is a type of RNA that ferries the appropriate corresponding amino
acids to the ribosome, and attaches each new amino acid to the last, building the polypeptide
chain one-by-one. Thus tRNA transfers specific amino acids from the cytoplasm to a growing
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polypeptide. The tRNA molecules must be able to recognize the codons on mRNA and match
them with the correct amino acid. The tRNA is modified for this function. On one end of its
structure is a binding site for a specific amino acid. On the other end is a base sequence that
matches the codon specifying its particular amino acid. This sequence of three bases on the
tRNA molecule is called an anticodon. For example, a tRNA responsible for shuttling the amino
acid glycine contains a binding site for glycine on one end. On the other end it contains an
anticodon that complements the glycine codon (GGA is a codon for glycine, and so the tRNAs
anticodon would read CCU). Equipped with its particular cargo and matching anticodon, a tRNA
molecule can read its recognized mRNA codon and bring the corresponding amino acid to the
growing chain.
Figure. Translation: from RNA to protein.
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The Genetic Code
The cellular process of transcription generates messenger RNA (mRNA), a mobile molecular
copy of one or more genes with an alphabet of A, C, G, and uracil (U). Translation of the mRNA
template converts nucleotide-based genetic information into a protein product. Protein
sequences consist of 20 commonly occurring amino acids; therefore, it can be said that the
protein alphabet consists of 20 letters. Each amino acid is defined by a three-nucleotide
sequence called the triplet codon. The relationship between a nucleotide codon and its
corresponding amino acid is called the genetic code. Given the different numbers of “letters” in
the mRNA and protein “alphabets,” combinations of nucleotides corresponded to single amino
acids. Using a three-nucleotide code means that there are a total of 64 (4 × 4 × 4) possible
combinations; therefore, a given amino acid is encoded by more than one nucleotide triplet –
the figure below summarizes this genetic code.
Figure. The Genetic Code for translating each nucleotide triplet, or codon, in mRNA into an
amino acid or a termination signal in a nascent protein.
Three of the 64 codons terminate protein synthesis and release the polypeptide from the
translation machinery. These triplets are called stop codons. Another codon, AUG, also has a
special function. In addition to specifying the amino acid methionine, it also serves as the start
codon to initiate translation. The reading frame for translation is set by the AUG start codon
near the 5' end of the mRNA. The genetic code is universal, meaning that virtually all species
use the same genetic code for protein synthesis, which is powerful evidence that all life on
Earth shares a common origin.
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SUMMARY
Figure. The making of a protein following the instructions encoded in DNA, via the processes
of Transcription and Translation. DNA holds all of the genetic information necessary to build a
cell’s proteins. The nucleotide sequence of a gene is ultimately translated into an amino acid
sequence of the gene’s corresponding protein.
CC licensed content
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Chapter 3. Authored by: OpenStax College. Provided by: Rice University. Located at:
http://cnx.org/contents/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@7.1@7.1.. Project:
Anatomy & Physiology. License: CC BY: Attribution. License Terms: Download for free at
http://cnx.org/content/col11496/latest/.
Chapter 2. Authored by: OpenStax College. Provided by: Rice University. Located at:
http://cnx.org/contents/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@7.1@7.1.. Project:
Anatomy & Physiology. License: CC BY: Attribution. License Terms: Download for free at
http://cnx.org/content/col11496/latest/.
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Name _____________________________
WORKSHEET: Replication – Transcription – Translation
Fill in the blanks – either the DNA sequence, the mRNA sequence, or the amino acid sequence.
1) DNA
2) DNA
3’- G _ T G A _ T _ G A C _ A _ T -5’
Coding
... _ G A _ _ G _ A _ _ G T _ C _ ...
Template
3’- G C A A T G G G T A C A C A A T G A C G -5’
Coding
5’- C G T T A C C C A T G T G T T A C T G C -3’
Template
mRNA ... _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ -3’
3) DNA
5’- G C A A T G G G T A C A C A A T G A C G -3’
Coding
3’- C G T T A C C C A T G T G T T A C T G C -5’
Template
mRNA ... _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ -3’
Protein
4) DNA
Met
___
___
___
5’- G G A _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ -3’
Coding
3’- _ _ T _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ -5’
Template
mRNA 5’- G G A _ _ C G G G U G C A U U A A C C G ...
Protein
Met
___
___
___
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