The Genetic Code

Ch 17 HW

Ch 17 HW

The Genetic Code

DNA is transcribed to messenger RNA (mRNA), and the mRNA is translated to proteins on the ribosomes. A sequence of three nucleotides on an mRNA molecule is called a codon. As you can see in the table, most codons specify a particular amino acid to be added to the growing protein chain. In addition, one codon (shown in blue) codes for the amino acid methionine and functions as a "start" signal. Three codons (shown in red) do not code for amino acids, but instead function as "stop" signals.

U

C

A

G

UUU Phe

UUC U

UUA Leu

UUG

UCU

UAU

UGU

U

Tyr

Cys

UCC

UAC

UGC

C

Ser

UCA

UAA Stop UGA Stop A

UCG

UAG Stop UGG Trp G

CUU

CUC

C

Leu

CUA

CUG

CCU

CAU

CGU

U

His

CCC

CAC

CGC

C

Pro

Arg

CCA

CAA

CGA

A

Gln

CCG

CAG

CGG

G

AUU AUC Ile

ACU ACC

AAU

AGU

U

Asn

Ser

AAC

AGC

C

A AUA

ACA Thr AAA

AGA

A

AUG

Met or start

ACG

Lys

Arg

AAG

AGG

G

GUU

GUC

G

Val

GUA

GUG

GCU

GAU

GGU

U

Asp

GCC

GAC

GGC

C

Ala

Gly

GCA

GAA

GGA

A

Glu

GCG

GAG

GGG

G

Part A Understanding the genetic code

Use the table to sort the following ten codons into one of the three bins, according to whether they code for a start codon, an insequence amino acid, or a stop codon. Drag each item to the appropriate bin.

Hint 1. How to interpret the table of codons

The table of codons shows the start codon in blue and the three stop codons in red. All other codons (shown in black) appear in the middle of the amino acid sequences that make up proteins. To read the table, locate the first letter in the codon on the left side of the table, then locate the second letter along the top, and the third letter down the right side of the table. Follow those letters across and down to identify the amino acid associated with that threeletter codon.

Hint 2. What is the start codon? Identify the start codon. ANSWER:

AUG

Hint 3. What are the stop codons? Enter the three stop codons, separated by commas. ANSWER:

UAG, UGA, UAA

ANSWER:

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Ch 17 HW

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start/methionine

AUG

stop codon

UGA UAG UAA

amino acid

CAC AUC GCA AAA UGC ACU

Correct

Nearly every mRNA gene that codes for a protein begins with the start codon, AUG, and thus begins with a methionine. Nearly every proteincoding sequence ends with one of the three stop codons (UAA, UAG, and UGA), which do not code for amino acids but signal the end of translation.

Part B Translation of mRNA

During translation, nucleotide base triplets (codons) in mRNA are read in sequence in the 5' 3' direction along the mRNA. Amino acids are specified by the

string of codons. What amino acid sequence does the following mRNA nucleotide sequence specify?

5 - AUGGCAAGAAAA - 3

Express the sequence of amino acids using the threeletter abbreviations, separated by hyphens (e.g., MetSerThrLysGly).

Hint 1. How to approach the problem

First, subdivide the sequence into the individual threeletter codons. Then, refer to the table of codons to find the threeletter abbreviation for the amino acid that corresponds to each codon.

Hint 2. An example problem

This chart shows how to decode an example mRNA sequence. Labels indicating directionality (for example, 5' 3') are not needed when writing amino acid sequences.

Example mRNA sequence

5 - AUGUCGACUAAGGGA - 3

Codon sequence

AUG UCG ACU AAG GGA

Amino acid sequence (threeletter abbreviation)

Met

Ser

Thr

Lys

Gly

Hint 3. Can you identify the individual codons in the mRNA sequence?

To identify the amino acids specified by the mRNA sequence, you first need to subdivide the sequence into codons of three nucleotides each. This can be done by placing a space between each codon. Which of the following is the correct division of the codons for the sequence given? Look for the correct placement of spaces.

ANSWER:

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5 - A UGG CAA GAA AA - 3

5 - AU GGC AAG AAA A - 3

5 - AUG GCA AGA AAA - 3

5 - AUGG CAA GAA AA - 3

5 - AU GGCA AGAA AA - 3

Ch 17 HW

Hint 4. Can you decode GCA? Which amino acid does the codon GCA code for? Express your answer using the threeletter abbreviation of the amino acid. ANSWER:

Ala

ANSWER: MetAlaArgLys

Correct

An amino acid sequence is determined by strings of threeletter codons on the mRNA, each of which codes for a specific amino acid or a stop signal. The mRNA is translated in a 5' 3' direction.

Part C The role of DNA in determining amino acid sequences Before a molecule of mRNA can be translated into a protein on the ribosome, the mRNA must first be transcribed from a sequence of DNA.

What amino acid sequence does the following DNA nucleotide sequence specify?

3 - TACAGAACGGTA - 5

Express the sequence of amino acids using the threeletter abbreviations, separated by hyphens (e.g., MetSerHisLysGly).

Hint 1. How to approach the problem

Follow these steps to convert a DNA sequence into an amino acid sequence.

1. First, transcribe the DNA sequence to determine the mRNA sequence. Be sure to remember the following:

The mRNA strand is complementary to the DNA strand. Uracil (U) takes the place of thymine (T) in RNA to pair with A on the DNA. The RNA is assembled in an antiparallel direction to the template strand of DNA. A 3' 5' direction in DNA is transcribed in a 5' 3' direction in RNA. 2. Next, subdivide the mRNA sequence into the individual threeletter codons in the 5' to 3' direction.

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Ch 17 HW 3. Then, refer to the table of codons to identify the threeletter abbreviation for the amino acid that corresponds to each codon.

Hint 2. An example problem

This chart shows how to decode an example DNA sequence. Remember to first determine the mRNA sequence that is complementary to the DNA template strand's sequence. Be sure to write the mRNA sequence in a 5' to 3' direction, and to use U to pair with A.

Example DNA sequence (template strand)

3 - TACGAATCAGCTGTA - 5

Complementary DNA sequence

5 - ATGCTTAGTCGACAT - 3

mRNA sequence

5 - AUGCUUAGUCGACAU - 3

Codon sequence

AUG CUU AGU CGA CAU

Amino acid sequence (threeletter abbreviation) Met Leu Ser Arg His

Hint 3. What mRNA sequence is transcribed from the DNA sequence?

What mRNA nucleotide sequence would be transcribed from the DNA sequence in this problem?

3 - TACAGAACGGTA - 5

ANSWER:

5 - ATGTCTTGCCAT - 3

3 - TACAGAACGGTA - 5

3 - AUGUCUUGCCAU - 5

5 - AUGUCUUGCCAU - 3

ANSWER: MetSerCysHis

Correct

Before mRNA can be translated into an amino acid sequence, the mRNA must first be synthesized from DNA through transcription. Base pairing in mRNA synthesis follows slightly different rules than in DNA synthesis: uracil (U) replaces thymine (T) in pairing with adenine (A). The codons specified by the mRNA are then translated into a string of amino acids.

Chapter 17 PreTest Question 4

Part A What is the function of RNA polymerase?

Hint 1. Compare RNA polymerase to DNA polymerase.

ANSWER:

It adds nucleotides to the 5' end of the growing mRNA molecule. It relies on other enzymes to unwind the double helix. It proceeds slowly along the DNA strand, requiring about a minute to add two nucleotides to the growing mRNA molecule. It unwinds the double helix and adds nucleotides to a growing strand of RNA. All of the above.

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Ch 17 HW

Correct

RNA polymerase has several functions in transcription, including unwinding the DNA double helix and adding RNA nucleotides.

Protein Synthesis (1 of 3): Overview (BioFlix tutorial)

In eukaryotic cells, the nuclear DNA codes for the synthesis of most of the cell's proteins. Each step of protein synthesis occurs in a specific part of the cell. In addition, various forms of RNA play key roles in the processes of protein synthesis. Before beginning this tutorial, watch the Protein Synthesis animation. Pay particular attention to the cellular locations where the various steps of protein synthesis occur in eukaryotic cells, as well as the different types of RNA and how they function. Part A Locations of the processes involved in protein synthesis

In eukaryotic cells, the processes of protein synthesis occur in different cellular locations. Drag the labels to the appropriate targets to identify where in the cell each process associated with protein synthesis takes place.

Hint 1. Some cellular components involved in protein synthesis move through the nuclear pores The nuclear pores are holes in the nuclear envelope that permit many types of large molecules or aggregates of molecules to move between the nucleus and the cytoplasm. Two cellular components that pass through the nuclear pores are ribosomal subunits and fully processed RNA. What can you infer about where these cellular components are made? Hint 2. What occurs during some key processes of protein synthesis? Match these key processes involved in protein synthesis to descriptions of what occurs at each step. ANSWER:

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1. The formation of a strand of RNA that is complementary to one strand of the nuclear DNA is called transcription . 2. The attachment of a free amino acid to a specific RNA molecule is the key step in charging of tRNA . 3. In translation , the nucleotide sequence of a piece of RNA is converted into a sequence of amino acids in a polypeptide. 4. The final RNA template for protein synthesis in eukaryotes is different from the RNA that was produced from the DNA template because of RNA processing .

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Hint 3. Where are cytoplasmic and secreted proteins made?

Ch 17 HW

In a eukaryotic cell, most proteins remain in the cytoplasm after they are synthesized (for example, the enzymes involved in glycolysis). However, some proteins, such as insulin, must be exported (secreted) from the cell to function. Both cytoplasmic and secreted proteins can only be synthesized in the presence of a ribosome. This diagram shows the two kinds of ribosomes:

free ribosomes, which are found in the cytoplasm bound ribosomes, which are found on the membrane of the rough endoplasmic reticulum (ER)

Which statement correctly describes where cytoplasmic and secreted proteins are synthesized? ANSWER:

Cytoplasmic proteins are synthesized on ribosomes bound to the rough ER, whereas secreted proteins are synthesized on free ribosomes. Cytoplasmic proteins are synthesized on free ribosomes, whereas secreted proteins are synthesized on ribosomes bound to the rough ER. Both cytoplasmic and secreted proteins are synthesized on ribosomes bound to the rough ER. Both cytoplasmic and secreted proteins are synthesized on free ribosomes.

ANSWER:

Correct

Part B Roles of RNA in protein synthesis in eukaryotes RNA plays important roles in many cellular processes, particularly those associated with protein synthesis: transcription, RNA processing, and translation. 6/46

Ch 17 HW Drag the labels to the appropriate bins to identify the step in protein synthesis where each type of RNA first plays a role. If an RNA does not play a role in protein synthesis, drag it to the "not used in protein synthesis" bin.

Hint 1. The role of RNA primers DNA synthesis (replication) and RNA synthesis differ in their needs for primer molecules.

In DNA replication, DNA polymerase cannot initiate the formation of a new strand of DNA directly from DNA nucleotides alone. Instead, the process requires an RNA primer to which the nucleotides of the new DNA strand attach. In RNA synthesis, in contrast, RNA polymerase can initiate the formation of a new strand of RNA without any primers. This information should help you sort the "RNA primers" label in this problem.

Hint 2. How do tRNA and rRNA function in protein synthesis? Both tRNA (transfer RNA) and rRNA (ribosomal RNA) play essential roles in protein synthesis. Which two statements correctly describe the roles of tRNA and rRNA in protein synthesis? ANSWER:

rRNA is the major structural component of ribosomes and is involved in binding both mRNA and tRNAs. tRNAs implement the genetic code, translating information from a sequence of nucleotides to the sequence of amino acids that make up a protein. rRNA has many variations, each of which binds a specific amino acid. tRNA transfers a nucleotide sequence from the DNA in the nucleus to the site of protein synthesis in the cytoplasm.

Hint 3. What is the role of mRNA in protein synthesis? mRNA (messenger RNA) plays a key role in protein synthesis as the intermediate between the information encoded by a sequence of bases in DNA (a gene) and the sequence of amino acids that make up the protein product. Which three statements correctly describe the role that mRNA plays in protein synthesis in eukaryotes? ANSWER:

mRNA is the template for protein synthesis in translation. mRNA carries genetic information from the nucleus to the cytoplasm. mRNA links together amino acids, forming a polypeptide chain. mRNA is the immediate product of transcription. mRNA is produced only after the steps of RNA processing.

Hint 4. snRNAs and RNA processing One stage of RNA processing in eukaryotes involves the removal of intronsnoncoding regions interspersed within the coding regions of the pre mRNA. In this RNA splicing process, the machinery that catalyzes the removal of introns (called the spliceosome) is composed of proteins and snRNAs (small nuclear RNAs). The snRNAs (and associated proteins) have two functions in the splicing process:

to bind to specific sequences of RNA that specify the location of the intron in the premRNA, and to catalyze the splicing process itself.

ANSWER:

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Ch 17 HW

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transcription/RNA processing

translation

premRNA snRNA mRNA

tRNA rRNA

not used in protein synthesis

RNA primers

Correct

In eukaryotes, premRNA is produced by the direct transcription of the DNA sequence of a gene into a sequence of RNA nucleotides. Before this RNA transcript can be used as a template for protein synthesis, it is processed by modification of both the 5' and 3' ends. In addition, introns are removed from the premRNA by a splicing process that is catalyzed by snRNAs (small nuclear RNAs) complexed with proteins. The product of RNA processing, mRNA (messenger RNA), exits the nucleus. Outside the nucleus, the mRNA serves as a template for protein synthesis on the ribosomes, which consist of catalytic rRNA (ribosomal RNA) molecules bound to ribosomal proteins. During translation, tRNA (transfer RNA) molecules match a sequence of three nucleotides in the mRNA to a specific amino acid, which is added to the growing polypeptide chain.

RNA primers are not used in protein synthesis. RNA primers are only needed to initiate a new strand of DNA during DNA replication.

Part C Codon size and the genetic code

Life as we know it depends on the genetic code: a set of codons, each made up of three bases in a DNA sequence and corresponding mRNA sequence, that specifies which of the 20 amino acids will be added to the protein during translation. Imagine that a prokaryotelike organism has been discovered in the polar ice on Mars. Interestingly, these Martian organisms use the same DNA RNA protein system as life on Earth, except that

there are only 2 bases (A and T) in the Martian DNA, and there are only 17 amino acids found in Martian proteins.

Based on this information, what is the minimum size of a codon for these hypothetical Martian lifeforms?

Hint 1. What mathematical equation can you use to solve this problem?

A simple mathematical equation can correctly express the maximum number of codons that can be constructed from x different bases, with a codon length of y bases. Recall that for life on Earth,

there are 4 different bases (A, T, G, and C), a codon is 3 bases long, and there are a total of 64 possible codons that specify the 20 different amino acids (some amino acids are specified by more than one amino acid). This chart shows this redundancy in the genetic code for life on Earth.

U

C

A

G

UUU Phe

UUC U

UUA Leu

UUG

UCU

UAU

UGU

U

Tyr

Cys

UCC

UAC

UGC

C

Ser

UCA

UAA Stop UGA Stop A

UCG

UAG Stop UGG Trp G

C CUU Leu CCU Pro CAU

CGU Arg U

His

CUC

CCC

CAC

CGC

C

CUA

CCA

CAA Gln CGA

A

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