From Gene to Polypeptide
-- The Roles of the Base-Pairing Rules and the Genetic Code
Dr. Ingrid Waldron, Department of Biology, University of Pennsylvania, 2011
1. Complete the following flowchart to describe how a gene influences a person's characteristics. Use the terms: DNA, protein, RNA.
nucleotide sequence in the ______of a gene
à nucleotide sequence in messenger ______
transcription
à amino acid sequence in a polypeptide which folds into a ______
translation
à structure and function of the protein
(e.g. normal hemoglobin vs. sickle cell hemoglobin)
à person's characteristics or traits
(e.g. normal health vs. sickle cell anemia)
2. The double helix structure of DNA, transcription and translation all depend on base-pairing rules that match each type of nucleotide in DNA or RNA with another nucleotide which has a complementary base. The base-pairing rules are slightly different in each case because the base T in DNA is replaced by U in RNA. Complete the following tables to describe these base-pairing rules.
Base-pairing Rules in DNA Double Helix
Base in one strand of DNA / C / G / A / TComplementary base in the other strand of DNA
Base-pairing Rules in Transcription
Base in DNA of a gene / C / G / A / TComplementary base in mRNA
Base-pairing Rules in Translation
Base in mRNA codon / C / G / A / UComplementary base in tRNA anti-codon
3. The first row of the table gives the nucleotide sequence in the coding strand of a hypothetical mini-gene. Use the base-pairing rules to complete the table.
Coding or Sense strand of DNA/ GAGCAT
Template or Anti-Sense strand of DNA
Transcription of template strand of DNA
produces mRNA
Rewrite the mRNA molecule as a pair of
triplet codons. (Each codon codes for
one amino acid in the polypeptide.) / codon 1: / codon 2:
Anti-codons in the tRNA molecules which will
bring the correct amino acids for translation
4. Which strand of DNA has the same nucleotide sequence as mRNA (taking into account that T in DNA is the equivalent of U in mRNA)?
5. Complete the following sentences to describe how the base-pairing rules contribute to transcription and translation.
During transcription, each nucleotide in the ______strand of DNA is matched by
(coding/template)
the base-pairing rules to a nucleotide in the mRNA molecule.
During translation, each ______in the mRNA molecule is matched by the
(anti-codon/codon)
base-pairing rules to the ______in a tRNA molecule which brings the correct
(anti-codon/codon)
amino acid for that position in the growing polypeptide.
6. Use the genetic code chart to complete the following sentence.
Translation of the mini-gene shown in question 3 will produce a polypeptide consisting of the amino acids ______and ______, linked together by a covalent bond.
(Notice that the genetic code chart shows the amino acid for each codon in mRNA,
not the anti-codon of the tRNA molecules that carry the amino acids.)
7. Suppose that a mutation occurred in the gene shown in question 3, and transcription and translation produced this polypeptide: glutamic acid linked to glutamine. What change or changes in the mRNA would result in the production of this polypeptide?
What change or changes in the template strand of the DNA would result in the production of this mRNA?
Teachers are encouraged to copy this student handout for classroom use. A Word file (which can be used to prepare a modified version if desired), teacher notes, comments, and links to additional activities are available at http://serendip.brynmawr.edu/exchange/bioactivities.