Name____________________________________________ Date:________________ Period:____
DNA Transcription and Translation Standards
1d. Students know the central dogma of molecular biology outlines the flow of information from
transcription of ribonucleic acid (RNA) in the nucleus to translation of proteins on ribosomes in the
cytoplasm.
4a. Students know the general pathway by which ribosomes synthesize proteins, using tRNAs to
translate genetic information in mRNA.
DNA, which is found in the nucleus of eukaryotes, contains the genetic information for encoding proteins. The
DNA sequence specifying a specific protein is copied (transcribed) into messenger RNA (mRNA), which then
carries this message out of the nucleus to the ribosomes located in the cytoplasm. The mRNA message is then
translated, or converted, into the protein originally coded for by the DNA.
CENTRAL DOGMA
Transcription can be explained easily in 4 quick steps.
Step 1: DNA unwinds/"unzips" as the Hydrogen Bonds Break.
Step 2: The free nucleotides of the RNA, pair with complementary DNA bases. As in DNA replication,
DNA is read from 3' → 5' during transcription. The complementary RNA is created from the 5' → 3'
direction. In eukaryotes, RNA polymerase, and therefore the initiation of transcription, requires the
presence of a core promoter sequence in the DNA.
Step 3: RNA sugar-phosphate backbone forms. (Aided by RNA Polymerase.)
Step 4: Hydrogen bonds of the untwisted RNA+DNA "ladder" break, then the RNA leaves the nucleus
through the small nuclear pores. This then goes to the cytoplasm to continue on to protein processing.
DNA does not leave the cell
nucleus, but messenger RNA
(mRNA), complementary to
DNA is transcribed to carry
encoded information from
DNA to the ribosomes (rRNA
and protein) (transcription) in
the cytoplasm. The ribosomes
translate mRNAs to make
protein. Freely floating amino
acids within the cytoplasm are
bonded to specific transfer
RNAs (tRNAs) that then
transport the amino acid to the
mRNA now located on the
ribosome. As a ribosome moves
along the mRNA strand, each
mRNA codon, or sequence of
three nucleotides specifying the
insertion of a particular amino
acid, is paired in sequence with
the anticodon of the tRNA that
recognizes the sequence. Each amino acid is added, in turn, to the growing polypeptide at the specified
position.
Biology Standard 4a-4c
1
Vocabulary
1. mRNA
2. protein [polypeptide chain]
3. amino acid
4. transcription
5. translation
6. Central Dogma of Molecular Biology
7. Ribosomes
8. Codons
9. RNA polymerase
10. Anticodon
11. tRNA
12. start codons
13. stop codons
Explain in your own words how to read the genetic code
Biology Standard 4a-4c
2
4b. Students know how to apply the genetic coding rules to predict the sequence of amino acids from
a sequence of codons in RNA.
The sequence of amino acids in protein is provided by the genetic information found in DNA.
 In prokaryotes, mRNA transcripts of a coding sequence are copied from the DNA as a single
contiguous sequence.
 In eukaryotes, the initial RNA transcript, while in the nucleus, is composed of exons, sequences of
nucleotides that carry useful information for protein synthesis, and introns, sequences that do not.
Before leaving the nucleus, the initial transcript is processed to remove introns and splice exons
together. This splicing activity is also called ligation and uses a ligase enzyme. The processed transcript,
then properly called mRNA and carrying the appropriate codon sequence for a protein, is transported
from the nucleus to the ribosome for translation.
RNA processing is to generate a mature mRNA (for protein genes) or a functional tRNA or rRNA from the
primary transcript.
Processing of pre-mRNA involves the following steps:



Capping - add 7-methylguanylate (m7G) to the 5' end.
Polyadenylation - add a poly-A tail to the 3' end.
Splicing - remove introns and join exons.
In your own words describe three ways that the initial RNA transcript is processed before moving to the
cytoplasm.
Biology Standard 4a-4c
3
Vocabulary
1. Initial mRNA
2. transcript
3. Introns
4. Exons
5. Excised
6. Ligated
7. Ligase
8. Processed transcript
4c. Students know how mutations in the DNA sequence of a gene may or may not affect the
expression of the gene or the sequence of amino acids in the encoded protein.
Mutations are permanent changes in the sequence of nitrogen containing bases in. Mutations occur when base
pairs are incorrectly matched (e.g., A bonded to C rather than A bonded to T) and can, but usually do not,
improve the product coded by the gene. Inserting or deleting base pairs in an existing gene can cause a
mutation by changing the codon reading frame used by a ribosome. Mutations that occur in somatic, or
nongerm, cells are often not detected because they cannot be passed on to offspring. They may, however, give
rise to cancer or other undesirable cellular changes. Mutations in the germline can produce functionally
different proteins that cause such genetic diseases as Tay-Sachs, sickle cell anemia, and Duchenne muscular
dystrophy.
Types of mutation
Point mutaton
Frameshift mutation
Insertion
Deletion
Substitution
Somatic mutation
Germ cell mutation
Biology Standard 4a-4c
4
Tay-Sachs disease is a rare inherited
disorder1 that progressively destroys nerve cells
(neurons) in the brain and spinal cord. The
most common form of Tay-Sachs disease
becomes apparent in infancy. Infants with this
disorder typically appear normal until the age of
3 to 6 months, when their development slows
and muscles used for movement weaken. As
the disease progresses, children with Tay-Sachs
disease experience seizures, vision and hearing
loss, intellectual disability, and paralysis. An eye
abnormality called a cherry-red spot, which can
be identified with an eye examination, is
characteristic of this disorder. Children with this
severe infantile form of Tay-Sachs disease
usually live only into early childhood.
Other forms of Tay-Sachs disease are very rare. Signs and symptoms can appear in childhood, adolescence, or
adulthood and are usually milder than those seen with the infantile form. Characteristic features include muscle
weakness, loss of muscle coordination (ataxia) and other problems with movement, speech problems, and
mental illness. These signs and symptoms vary widely among people with late-onset forms of Tay-Sachs
disease.
Tay-Sachs disease is very rare in the general population. The genetic mutations that cause this disease are more
common in people of Ashkenazi (eastern and central European) Jewish heritage than in those with other
backgrounds. The mutations responsible for this disease are also more common in certain French-Canadian
communities of Quebec, the Old Order Amish community in Pennsylvania, and the Cajun population of
Louisiana.
Mutations in the HEXA gene cause Tay-Sachs disease. The HEXA gene provides instructions for making part
of an enzyme called beta-hexosaminidase A, which plays a critical role in the brain and spinal cord. This
enzyme is located in lysosomes, which are structures in cells that break down toxic substances and act as
recycling centers. Within lysosomes, beta-hexosaminidase A helps break down a fatty substance called GM2
ganglioside. Progressive damage caused by the buildup of GM2 ganglioside leads to the destruction of these
neurons, which causes the signs and symptoms of Tay-Sachs disease.
Because Tay-Sachs disease impairs the function of a lysosomal enzyme and involves the buildup of GM2
ganglioside, this condition is sometimes referred to as a lysosomal storage disorder or a GM2-gangliosidosis.
Sickle cell anemia2 is an inherited blood disorder characterized primarily by chronic anemia and periodic
episodes of pain. The underlying problem involves hemoglobin, a component of red blood cells. Hemoglobin
molecules in each red blood cell carry oxygen from the lungs to body organs and tissues and bring carbon
dioxide back to the lungs. In sickle cell anemia, the hemoglobin is defective. After hemoglobin molecules give
up their oxygen, some may cluster together and form long, rod-like structures. These structures cause red blood
cells to become stiff and assume a sickle shape. Unlike normal red cells, which are usually smooth and donutshaped, sickled red cells cannot squeeze through small blood vessels. Instead, they stack up and cause
blockages that deprive organs and tissues of oxygen-carrying blood. This process produces periodic episodes of
pain and ultimately can damage tissues and vital organs and lead to other serious medical problems. Sometimes
pain lasts only a few hours; sometimes it lasts several weeks, requiring hospitalization. Pain is the principal
1
http://ghr.nlm.nih.gov/condition/tay-sachs-disease
http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/sca.shtml
Biology Standard 4a-4c
5
2
symptom of sickle cell anemia in both children and adults. Normal red blood cells live about 120 days in the
bloodstream, but sickled red cells die after about 10 to 20 days. Because they cannot be replaced fast enough,
the blood is chronically short of red blood cells, a condition called anemia.
Sickle cell anemia Inheritance
Sickle cell anemia is an autosomal recessive genetic disorder caused by a defect in the HBB gene,
which codes for hemoglobin. The presence of two defective genes (SS) is needed for sickle cell
anemia.
Sickle cell anemia Incidence
Sickle cell anemia affects millions throughout the world. It is particularly common among people
whose ancestors come from subSaharan Africa; Spanish-speaking
regions (South America, Cuba,
Central America); Saudi Arabia;
India; and Mediterranean countries
such as Turkey, Greece, and Italy.
In the Unites States, it affects
around 72,000 people, most of whose ancestors
come from Africa. The disease occurs in about 1
in every 500 African-American births and 1 in
every 1000 to 1400 Hispanic-American births.
Duchenne Muscular dystrophy3 is a genetic
conditions characterized by progressive muscle weakness and wasting (atrophy). The Duchenne muscular
dystrophy primarily affect the skeletal muscles, which are used for movement, and the muscles of the heart.
These conditions occur much more frequently in males than in females. Duchenne and Becker muscular
dystrophies together affect 1 in 3,500 to 5,000 newborn males. Between 400 and 600 boys in the United States
are born with these conditions each year. Females are rarely affected by these forms of muscular dystrophy.
Mutations in the DMD gene cause Duchenne and
Becker muscular dystrophy. The DMD gene
provides instructions for making a protein called
dystrophin. This protein helps stabilize and
protect muscle fibers and may play a role in
chemical signaling within cells. Mutations in the
DMD gene alter the structure or function of
dystrophin, or prevent any functional dystrophin
from being produced. Muscle cells without this
protein become damaged as muscles repeatedly
contract and relax with use. The damaged fibers
weaken and die over time, leading to the muscle
weakness and heart problems characteristic of Duchenne and Becker muscular dystrophies.
This condition is inherited in an X-linked recessive pattern. A condition is considered X-linked if the mutated
gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes. In males
(who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the
condition. In females (who have two X chromosomes), a mutation must be present in both copies of the gene
to cause the disorder. Males are affected by X-linked recessive disorders much more frequently than females. A
striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.
3
http://ghr.nlm.nih.gov/condition/duchenne-and-becker-muscular-dystrophy
Biology Standard 4a-4c
6
PRACTICE QUESTIONS
1. A scientist analyzed the base composition present in a certain cell. He
generated the incomplete data shown on the table below: What would
be the expected % Thymine content of the cell?
a. 13%
c. 28%
b. 14%
d. 56%
2. Some events that place during the synthesis of a specific protein are
listed below:
(A) Messenger RNA attaches to a ribosome
(B) DNA serves as a template for RNA production
(C) Transfer RNA bonds to a specific codon
(D) Amino acids are bonded together
(E) RNA moves from the nucleus to the cytoplasm
The correct order of these events is
a. B→E→A→C→D
b. D→A→E→C→B
c. B→C→E→D→A
d. C→B→A→E→D
3. Which of the following relationships is the most similar to the relationship below?
nucleotide: DNA
a. amino acid : protein
b. codons : tRNA
c. mRNA : anticodons
d. carbohydrate : glucose
4. A segment of a DNA strand has the following bases: TAC GAT. What is the complementary strand of
DNA?
a. UAG CAU
c. AUG CUA
b. TAG CAT
d. ATG CTA
5. What is the complementary messenger-RNA sequence for the DNA sequence shown below?
a.
b.
c.
d.
C-A-A-G-G-U
G-T-T-C-C-A
G-U-U-C-C-A
C-A-A-G-G-T
6. Chromosomal mutations occurring in gametes of humans can affect the appearance of offspring because
a. many traits are usually affected
b. only one trait is usually affected
c. these mutations usually speed up embryonic development
d. these mutations usually result in sex-linked Traits
7. One similarity between DNA and messengerRNA molecules is that they both contain
a. the same sugar
b. genetic codes based on sequences of bases
c. a nitrogenous base known as uracil
d. double-stranded polymers
Biology Standard 4a-4c
7
8. Given: A DNA template strand with the nucleotide sequence 3′–ATTGCGTAC–5′. The RNA transcribed
from this template strand will have the nucleotide sequence:
(a) 5′–AUUGCGUAC–3′,
(b) 3′–AUUGCGUAC–5′,
(c) 5′–UAACGCAUG–3′,
(d) 3′–UAACGCAUG–5′,
(e) 5′–TAACGCATG–3′.
9. The replacement of a DNA nucleotide pair with a different nucleotide pair is called a(an):
(a) insertion mutation,
(b) deletion mutation,
(c) thymine dimer mutation,
(d) frameshift mutation,
(e) substitution mutation.
10. The protein subunits of the protein shell that encloses the viral genome are called:
(a) capsomeres,
(b) capsids,
(c) envelopes,
(d) histones,
(e) centromeres.
11. Bacteria do not have:
(a) ribosomes,
(b) a plasma membrane,
(c) a true nucleus,
(d) a genome,
(e) choices “a”, “b”, and “c” are correct, but choice “d” is not correct.
12. A DNA nucleotide is composed of:
(a) a nitrogenous base,
(b) the sugar ribose,
(c) a phosphate,
(d) all of the above choices, “a”, “b”, and “c”, are correct,
(e) choices “a” and “c” are both correct, but choice “b” is not correct.
13. In an experiment we discussed in lecture, bacteria were cultured for several generations in a medium
containing heavy nitrogen (N15). They were then transferred to a medium containing light nitrogen (N14)
and DNA was extracted from some of the cells and centrifuged after one replication cycle and extracted
from other cells and centrifuged after two replication cycles. This experiment proved that:
(a) DNA is the genetic material,
(b) DNA replication is conservative,
(c) DNA replication is semiconservative,
(d) DNA replication is dispersive,
14. In DNA replication, a lagging strand is synthesized:
(a) continuously in the 3′ to 5′ direction as a series of Okazaki fragments,
(b) continuously in the 5′ to 3′ direction as a series of Okazaki fragments,
(c) discontinuously in the 3′ to 5′ direction as a series of Okazaki fragments,
(d) discontinuously in the 5′ to 3′ direction as a series of Okazaki fragments,
(e) choices “a” and “c” are both correct, but choices “b” and “d” are not correct.
Biology Standard 4a-4c
8
15. Molecules of DNA are composed of long chains of
a. amino acids
b. fatty acids
c. monosaccharides
d. nucleotides
16. The primary function of DNA is to
a. make proteins
b. store and transmit genetic information
c. control chemical processes within cells
d. prevent mutations
17. A nucleotide consists of
a. a sugar, a protein, and adenine
b. a sugar, an amino acid, and a starch
c. a sugar, a phosphate group, and a nitrogen-containing base
d. a starch, a phosphate group, and a nitrogen-containing base
18. The part of the molecule for which deoxyribonucleic acid is named is the
a. phosphate group
c. nitrogen base
b. sugar
d. none of the above
19. Purines and pyrimidines are
a. bases found in amino acids
b. able to replace phosphate groups from defective DNA
c. names of specific types of DNA molecules
d. bases found in nucleotides
20. The scientists credited with determining the structure of DNA are
a. Avery and Chargaff
c. Mendel and Griffith
b. Hershey and Chase
d. Watson and Crick
21. The base-pairing rules state that the following are base pairs in DNA:
a. adenine - thymine; guanine – cytosine
b. adenine - thymine; uracil – cytosine
c. adenine - guanine; thymine – cytosine
d. uracil - thymine; guanine – cytosine
22. ATTG : TAAC ::
a. AAAT : TTTG
b. TCGG : AGAT
c. GTCC : CAGG
d. CGAA : TGCG
23. The enzymes responsible for adding nucleotides to the exposed DNA template bases are called
a. Replicases
b. Helicases
c. DNA polymerases
d. Nucleotidases
24. All of the following are TRUE about the structure of DNA except
a. every DNA nucleotide contains a sugar, a phosphate group, and a base
b. short strands of DNA are contained in chromosomes inside the nucleus of a cell
c. DNA consists of two strands of nucleotides joined by hydrogen bonds
d. the long strands of nucleotides are twisted into a double helix
25. The function of rRNA is to
a. synthesize DNA
b. synthesize mRNA
Biology Standard 4a-4c
c. form ribosomes
d. transfer amino acids to ribosomes
9
26. Which of the following types of RNA carries the instructions for making proteins?
a. mRNA
c. tRNA
b. rRNA
d. all of the above
27. RNA differs from DNA in that RNA
a. is sometimes single-stranded
b. contains a different sugar molecule
c. contains the nitrogen base uracil
d. all of the above
28. Which of the following is NOT found in RNA?
a. Adenine
b. Cytosine
c. Thymine
d. guanine
29. In RNA molecules, adenine is complementary to
a. cytosine
b. guanine
c. thymine
d. uracil
30. Given the following strand of mRNA, identify the strand of DNA from which it was made mRNA:
CUCAAGUGCUUC
c. GAGTTCACGAAG
a. CUCAAGUGCUUC
d. AGACCTGTAGGA
b. GAGUUCACGAAG
31. Each nucleotide triplet in mRNA that specifies a particular amino acid is called a(n)
a. Mutagen
c. Anticodon
b. Codon
d. exon
32. codon: nucleotides::
a. ribosomes : binding sites
b. ribosome : DNA molecules
c. RNA : bases
d. DNA : bases
33. During transcription
a. proteins are synthesized
b. DNA is replicated
c. RNA is produced
d. translation occurs
34. Transcription proceeds when RNA polymerase
a. attaches to a ribosome
b. binds to a strand of DNA
c. binds to a strand of RNA
d. attaches to a promoter molecule
35. Given the following sequence of mRNA, what series of amino acids are coded for?
mRNA: CUCAAGUGCUUC
a.
b.
c.
d.
Ser - Tyr - Arg – Gly
Val - Asp - Pro – His
Leu - Lys - Cys – Phe
Pro - Glu - Leu - Val
36. For the same mRNA sequence as in question 35, the anticodons for the codons in the mRNA are
a. GAG - UUC - ACG – AAG
b. GAG - TTC - ACG – AAG
c. CUC - GAA - CGU – CUU
d. CUU - CGU - GAA - CUC
Biology Standard 4a-4c
10
37. Given the following sequence of amino acids, use the genetic code table to determine the DNA
sequence that codes for the amino acids. amino acid sequence: tyrosine - proline - aspartic acid isoleucine - cysteine
a. AUGGGUCUAUAUACG
b. ATGGGTCTATATACG
c. GCAAACTCGCGCGTA
d. ATAGGGCTTTAAACA
38. codons : mRNA ::
a. anticodons : tRNA
b. triplets : DNA
c. rRNA : ribosomes
d. all of the above
39. The codon AUG, which codes for the amino acid methionine, also serves as
a. a lac operon
c. a stop codon
b. a start codon
d. a promoter
40. The codons UAA, UAG, and UGA all code for
a. Arginine
b. Threonine
c. Phenylalanine
d. stop codons
41. With the exception of tryptophan, each amino acid is coded for by more than one codon. This is called
a. Translation
c. Redundancy
b. Reversal
d. sequentialism
42. The fact that the genetic code is almost universal in living organisms is considered to be evidence that
all organisms
a. are evolutionarily related
b. are genetically identical
c. have the same sequence of anticodons
d. none of the above
43. Anticodons are found on
a. rRNA
c. DNA
b. mRNA
d. tRNA
44. The entire genetic code consists of ____ amino acids and ____ codons.
a. 20, 20
c. 30, 60
b. 20, 64
d. 30, 72
45. Given the original DNA strand below what would the final polypeptide chain be?
i. ile-stop
ii. met-ile-gln-val-stop
c. met-pro-trp-gly-arg-leu-stop
d. ile-gln-val-val-stop
46. Which relationship is most similar to the relationship below?
tRNA : ribosome
a. baker : pie
b. delivery truck : factory
Biology Standard 4a-4c
c. key : lock
d. book : publisher
11
The human DNA is up to 80 million base pair long in a chromosome. Thus, the DNA is unwound at multiple places
along its length and DNA replication steps are carried out simultaneously at many places.
Fact File: The human DNA is copied at about 50 base pairs per second. The multiple location of DNA replication
process takes about 1 hour to complete. If this were not the case, then it would take about a month to finish
replicating the entire DNA strand!
The DNA replication process is almost error free with the help of DNA polymerase and other simple DNA replication
enzymes, that proofread the nucleotides being added to the strand. If the nucleotides are not found to be
complementary, then they are removed and a new nucleotide is synthesized. Thus, creating an error free DNA
strand.
Fact File: A billion nucleotides have less than one mistakes. This means that copying 100 dictionaries with 1000
pages word to word, page to page and symbol to symbol, with only one mistake!
In 1950, Erwin Chargaff and colleagues examined the chemical composition of DNA and demonstrated
that the amount of adenine always equals that of thymine, and the amount of guanine always equals that of
cytosine. This observation became known as Chargaff's rule.
a. Based on the structure of DNA, explain the basis of Chargaff's rule.(Why is the amount of adenine and
thymine always equal?)
b. The diagram below represents a single-stranded segment of DNA. Write the complementary DNA strand
that would form from this strand during replication. Use the letters A, C, G, and T to designate the bases: A
= adenine; C = cytosine; G = guanine; T = thymine.
c. Why is Chargaff's rule so important to DNA's ability to replicate itself accurately?
Biology Standard 4a-4c
12