Gene Expression & Cell Specialization

By Hopewell Valley Student Publications Network | 06/02/2021

My AP Biology Thoughts

Unit 6 Gene Expression and Regulation

Welcome to My AP Biology Thoughts podcast, my name is Shriya Karthikvatsan and I am your host for episode #110 called Unit 6: Gene Expression and Regulation. Today we will be discussing the mechanisms used by cells to increase or decrease the production of specific gene types, and how this fits into the overarching unit.

Segment 1: Introduction to Gene Expression and Regulation

We will begin by going over a few helpful terms and ideas to provide context for the topic of gene expression and regulation which is a pretty broad topic as a whole
A gene consists of a string of DNA hidden in a cell’s nucleus, and what we will unpack is how it knows when to express itself and cause the production of a string of amino acids called a protein
The overall process is that a string of DNA is expressed to make RNA
Then, something called mRNA is translated from nucleic acid coding to protein coding to form a protein
In terms of regulation, genes can’t control an organism on their own so they must interact with and respond to the organism’s environment
Some genes are always “on” regardless of environmental conditions, and these genes are among the most important elements of the genome because they control the ability of DNA to replicate, express itself, repair itself, and perform protein synthesis
Overall, regulated genes are needed occasionally and get turned “on” or “off”
Regulation differs between prokaryotes and eukaryotes because in prokaryotes, most regulatory proteins are negative and turn genes off
In eukaryotes, cell-cell differences are determined by expression of different sets of genes
This means that an undifferentiated fertilized egg looks and acts quite different from a skin cell, a neuron, or a muscle cell because of differences in the genes each cell expresses
In the next segment we will go into further detail of the specific processes involved in expression and regulation

Segment 2: More About Gene Expression and Regulation

Gene expression begins with transcription which makes mRNA and the overall process is the same in both prokaryotes and eukaryotes
Prokaryotes lack a nuclear envelope, and eukaryotes use an extra step called RNA processing where RNA is edited and introns are edited out and exons are spliced together
It is catalyzed by RNA polymerase which separates DNA strands and links RNA nucleotides at the 3’ end (side notes: prokaryotes have 1 type of polymerase and eukaryotes have 3)
Transcription is initiated when RNA polymerase binds to a promoter and unwinds the DNA strands
Initiation site and a small sequence after are recognized by transcription factors which are proteins that bind to promoter and guide RNA polymerase to bind to TATA box
Then, mRNA carries the genetic code and mRNA itself is a series of codons
In eukaryotes, mRNA processing works by the 5’ end getting a GTP cap and the 3’ end getting a poly-A tail
Also, a splicesome complex of SnRNPs + a protein work together to cut out the introns (intruding codons) and splice the exons (expressed codons) together
Following transcription, translation occurs in the ribosome after mRNA brings the genetic code and it is when tRNA brings the amino acid and the ribosome is able to be completely assembled
Translation is initiated by a small subunit of the ribosome which binds to a recognition site on the mRNA and an anticodon of tRNA initiator binds to a start codon
The next part of translation is elongation in which the anticodon of the next tRNA binds to a codon at the A site and the polypeptide bonds the 2nd amino acid onto the 1st amino acid (this process repeats until a stop codon is reached)
Finally, termination is when the stop codon reaches the A site and a release factor frees the tRNA from the P site and disconnects the polypeptide causing everything to separate
After translation, the protein is modified in ways such as:
The amino acid can be cut off, the protein folds, or sugars and lipids are added
Mutations can also occur which are changes in DNA sequences or the genome
Gene regulation involves:
Promoters are nucleotide sequences where RNA polymerase binds to initiate transcription
Operons are a cluster of related genes with a common metabolic pathway that are controlled together by a “master switch”
An operator is a DNA sequence to which the repressor binds
Regulatory gene—codes for a repressor protein
Repressor—protein that binds to the operator and blocks RNA polymerase
Corepressor—small molecule that binds to the repressor
The two types of operons are:
Tryptophan operon—codes for production of tryptophan (amino acid) and it is repressible meaning the pathway product switches the operon off
Lac operon—codes for proteins needed for take up and metabolism of lactose and it is inducible, meaning the pathway is switched on by the nutrient it uses

Segment 3: Connection to the Course

Overall, gene expression and regulation is so significant because genes encode for proteins and dictate cell function
The thousands of genes expressed in a particular cell determine what that cell can do which allows for life processes to occur and be able to help an organism
It also creates variation which allows for differences in the expression of genes that accounts for phenotypic differences between organisms
The regulation of gene expression conserves energy and space
It would require a significant amount of energy for an organism to express every gene at all times, so it is more energy efficient to turn on the genes only when they are required

Thank you for listening to this episode of My AP Biology Thoughts. For more student-ran podcasts and digital content, make sure that you visit www.hvspn.com.

Music Credits:

“Ice Flow” Kevin MacLeod (incompetech.com)
Licensed under Creative Commons: By Attribution 4.0 License
http://creativecommons.org/licenses/by/4.0/

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Welcome to My AP Biology Thoughts podcast, this is episode #110 called Unit 6: Gene Expression and Regulation.

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