The corn in a tortilla and the corn in an elote have the same DNA in every kernel, but only some cells make the protein that gives pigmented corn its blue color. How does a cell "use" the information stored in its DNA, and why don't all cells make the same proteins? This two-stage process, transcription and translation, is called gene expression.
Transcription: from DNA to mRNA
DNA keeps the instructions inside the nucleus and never leaves it, but the ribosomes that make proteins are in the cytoplasm. How does the information travel without risking the original? By making a working copy.
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Explain in your own words what it means when a cell "expresses a gene", and why the final result of this expression is a protein.
Explain in your own words what it means when a cell "expresses a gene", and why the final result of this expression is a protein.
The template strand of a gene has the sequence 3'-TAC GGA TTT-5'. Write the sequence of the messenger RNA synthesised from it, keeping in mind that in RNA, thymine is…
The template strand of a gene has the sequence 3'-TAC GGA TTT-5'. Write the sequence of the messenger RNA synthesised from it, keeping in mind that in RNA, thymine is replaced by uracil.
Why three bases? With 4 letters, groups of 1 would give only 4 combinations and groups of 2 would give 16: not enough for 20 amino acids. With 3 you get 64, which is enough.
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What is the genetic code? Explain what a codon is and how many bases make it up.
What is the genetic code? Explain what a codon is and how many bases make it up.
Two patients with point mutations in the same codon of their mature mRNA are analyzed; the original codon was GAA (encodes glutamic acid): In patient A, the codon…
Two patients with point mutations in the same codon of their mature mRNA are analyzed; the original codon was GAA (encodes glutamic acid):
In patient A, the codon mutates to GAG (encodes glutamic acid).
In patient B, it mutates to GUA (encodes valine).
a) Based on the properties of the genetic code, justify why patient A shows no alteration while patient B presents a non-functional enzyme.
b) Could a "silent mutation" like the one in patient A exist if the genetic code were not degenerate? Justify your answer by deducing what consequences mutations would have in a hypothetical code with only one codon per amino acid.
The mRNA message uses only 4 letters (A, U, G, C), but proteins are built from 20 amino acids. We already saw the key: it is read three bases at a time. What's left is to see who does the reading.
A summary of the differences between the two stages:
Transcription
Translation
Molecule produced
mRNA
Protein (chain of amino acids)
Location (eukaryotic cell)
Nucleus
Ribosomes in the cytoplasm
Template
DNA template strand
mRNA
Participants
RNA polymerase
Ribosome and tRNA
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Look at the diagram of a eukaryotic cell representing gene expression: labels ?1 and ?2 show where transcription and translation take place, without indicating which is…
Look at the diagram of a eukaryotic cell representing gene expression: labels ?1 and ?2 show where transcription and translation take place, without indicating which is which. State which process occurs at each labelled point and where in the cell it takes place.
A bacterium synthesizes an oligopeptide from the following mature mRNA fragment: 5'-... AUG GCC UUU UAA ...-3' A mutagenic agent causes the deletion (loss) of the first…
A bacterium synthesizes an oligopeptide from the following mature mRNA fragment:
5'-... AUG GCC UUU UAA ...-3'
A mutagenic agent causes the deletion (loss) of the first nucleotide of the codon GCC (the first cytosine), resulting in the sequence: 5'-... AUG GCU UUU AA...-3'.
a) Write the resulting amino acid sequence before and after the mutation.
b) Justify why a reading frameshift mutation (deletion or insertion) usually has much more severe consequences for protein function than a single base substitution mutation.
Why does it matter? If every cell used all its genes at once, there would be no specialized tissues. What sets a cell apart is not the DNA it has, but the mRNAs and proteins it produces.
Back to the beginning: the pigmented corn kernel has the color gene in all of its cells, but it is expressed (transcribed and translated) only in the cells where that protein is needed.
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A student claims: "Since a neuron and a skin cell have completely different functions and shapes, their DNA molecules must be different; for example, the neuron has…
A student claims: "Since a neuron and a skin cell have completely different functions and shapes, their DNA molecules must be different; for example, the neuron has eliminated the genes for skin proteins that it does not need."
a) Refute the student's statement by explaining what we would actually find if we sequenced the genomic DNA of both cell types.
b) If the DNA is identical, predict and justify what differences you would expect to find when analyzing the types of mRNA and proteins present in the cytoplasm of both cells.