Problem 12 - Campbell Biology

Question

You hope to study a gene that codes for a neurotransmitter protein produced in human brain cells. You know the amino acid sequence of the protein. Explain how you might (a) identify what genes are expressed in a specific type of brain cell, (b) identify (and isolate) the neurotransmitter gene, (c) produce multiple copies of the gene for study, and (d) produce large quantities of the neurotransmitter for evaluation as a potential medication.

Step-by-Step Solution

Answer

Sequence RNA from brain cells, use BLAST to find gene, PCR to isolate it, clone in plasmid, express in cells to produce protein.

Step 1: Identify genes expressed in a specific type of brain cell

Use RNA sequencing (RNA-Seq) to determine the RNA present in the brain cells. Brain cells will be lysed to extract the total RNA, then converted into complementary DNA (cDNA) using reverse transcriptase. Sequence the cDNA and compare to a reference genome to identify the active genes.

Step 2: Identify and isolate the neurotransmitter gene

Use the known amino acid sequence of the protein to perform a BLAST search against a human genome database. This will help find the corresponding gene sequence. Extract the gene from the genomic DNA using PCR with specific primers designed for the gene sequence.

Step 3: Produce multiple copies of the gene for study

Insert the isolated gene into a plasmid vector. Use transformation to introduce the plasmid into bacteria (e.g., E. coli) that will then replicate the plasmid, producing many copies of the gene.

Step 4: Produce large quantities of the neurotransmitter

Clone the gene into an expression vector with a strong promoter. Transform this vector into a suitable expression system (e.g., yeast or mammalian cells). Induce the cells to express the neurotransmitter and purify the protein for evaluation as a potential medication.

Key Concepts

RNA sequencing

RNA sequencing (RNA-Seq) is a powerful technique used to study gene expression. To learn what genes are active in specific brain cells, we first isolate all RNA from the cells. This involves lysing the cells to release RNA. Then, we convert the RNA into complementary DNA (cDNA) using an enzyme called reverse transcriptase. The cDNA is then sequenced to figure out the exact order of nucleotides, which can be compared to a reference genome. This comparison allows us to identify which genes are being expressed in the brain cells.

• Extract RNA from brain cells
• Convert RNA to cDNA
• Sequence the cDNA to identify active genes
• Compare sequences to a reference genome

This technique is precise and can give a comprehensive picture of gene expression in a specific cell type.

BLAST search

To identify and isolate the neurotransmitter gene, we can utilize the Basic Local Alignment Search Tool (BLAST). This tool compares a known amino acid sequence to sequences within a database to find similar sequences. For our purposes, we input the amino acid sequence of the neurotransmitter protein into BLAST and search a human genome database. When a matching sequence is found, it indicates the location of the gene that codes for our neurotransmitter protein. Once identified, we can use specific laboratory techniques like Polymerase Chain Reaction (PCR) with specific primers to extract the gene from the human genomic DNA.

• Use a known amino acid sequence
• Perform a BLAST search against a database
• Locate the corresponding gene sequence
• Extract the gene using PCR

BLAST searches are a crucial step in connecting protein activity back to the gene responsible.

gene cloning

Gene cloning is the process of making multiple copies of a gene. Once the neurotransmitter gene has been isolated using techniques like PCR, it can be inserted into a plasmid vector. A plasmid vector is a small, circular piece of DNA that can replicate independently within a bacterial cell. We introduce the plasmid into bacteria, such as E. coli, through a process known as transformation. The bacteria then replicate, making many copies of the plasmid and the gene of interest.

• Insert isolated gene into a plasmid vector
• Transform bacteria with the plasmid
• Bacteria replicate, producing multiple gene copies

Gene cloning is essential for amplifying DNA for detailed study and further use.

expression vectors

To produce large quantities of the neurotransmitter, we use expression vectors. An expression vector is a type of plasmid or viral vector that has additional elements to ensure strong gene expression. This includes a strong promoter to drive gene transcription. The cloned neurotransmitter gene is inserted into the expression vector, which is then introduced into a suitable expression system, like yeast or mammalian cells. These cells are induced to express the gene, producing the neurotransmitter protein. The protein can then be purified and evaluated for its potential as a medication.

• Insert the gene into an expression vector
• Transform into an expression system (yeast/mammalian cells)
• Induce cells to produce the neurotransmitter
• Purify the protein for evaluation

Expression vectors are crucial for producing and studying proteins in large quantities.