Melanie gets poetic in Episode 69. Roses are red, violets are blue. If a promoter is blocked this may be untrue. Make sure to appropriately distinguish between genes and alleles.
Melanie gets poetic in Episode 69. Roses are red, violets are blue. If a promoter is blocked this may be untrue. Make sure to appropriately distinguish between genes and alleles (1:59). Gene expression occurs at many levels, all of which are essential in producing an organism’s phenotype (3:03). The episode concludes with a recap of the Lac Operon - everyone’s favorite bacterial gene expression example (6:17).
The Question of the Day asks (8:31) In the Lac Operon, is the promoter upstream or downstream from the operator?
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap crafting a gene
Let’s Zoom out:
Unit 6 - Gene Expression and Regulation
Topic - 6.4 - 6.6
Big Idea: Information Storage and Transmission
Roses are red, violets are blue. If a promoter is blocked this may be untrue. Unless gene expression is modified….then it could be different...but none of this rhymes. I digress. Today’s episode is all about genes - what they are, how they are tweaked, how they turn on and off, and the effect that has on phenotype.
Let’s Zoom in:
One of the common mistakes that students make when discussing genes and their expression is using the term gene interchangeably with the term allele. They are related, but not synonymous. A gene is a piece of DNA that carries hereditary information. Wait does this mean that part of DNA is not hereditary information? Absolutely - the majority of DNA contains noncoding information. In fact, some estimates claim that as little as 1% of DNA actually codes for functional proteins. However, research through Stanford University and the ENCODE project (Encyclopedia of DNA elements) suggests that as much as 80% of DNA has biological functions. No matter which tomato you tomahto, there are parts of DNA which are non-coding and the cell needs strategies to sort out the junk.
Okay, back to the difference between genes and alleles. Basically an allele is an alternate form of a gene that exists in a population. If I go back to our poetic intro, roses are red, violets are blue and think about Mendel, we can put a few things in context. Genes are located at a specific place on a chromosome known as a locus. And since chromosomes occur in homologous pairs there are two sets of each gene within a cell.Even if a gene has more than two alleles that exist in a population, like with ABO blood typing, only two can be inherited for any one individual. These copies might be the same, homozygous dominant or recessive or they might be different, heterozygous. The expression of these alleles also had great variation according to traits that follow Mendel's rules and others that tend to break them. If these flowers were cultivated by Mendel, the genotype might be heterozygous with a dominant allele capital P and recessive allele little p - phenotype purple.
Gene expression occurs at many levels, all of which are essential in producing an organism’s phenotype. Also, gene regulation is what allows for different cells in multicellular organisms to be, well, different. Eyeballs do not do what elbows do, even though they have the same set of instructions. Eyeballs are reading DNA manual chapter 2 while elbows is a short paragraph in chapter 8. No, the DNA manual isn’t organized into chapters, but rather into chromosomes - but you get the idea.
First, DNA is tightly coiled around histones, forming heterochromatin and euchromatin. Only euchromatin is loose enough to access nucleotides. Second, DNA is double stranded, but only one strand will act as the template for transcription. Which strand will get this honor? Well that depends on which gene is being transcribed at the time. Either way, the enzymatic reading occurs 3’ to 5’. Since enzymes interact and move unidirectionally, we get phrases like “upstream” (before the gene) and “downstream” within or after the gene. So location matters - specifically the location of DNA regulatory sequences that interact with regulatory proteins to control transcription.
A short DNA sequence upstream of the transcription start site is known as a promoter. This is where RNA polymerase and other transcription factors bind to initiate transcription, before moving downstream towards the gene. Other negative regulatory molecules can inhibit gene expression by binding to DNA and blocking transcription. Genes turn on and genes turn off.
Let’s say that transcription factors bind to promotor, RNA polymerase moves downstream and an mRNA transcript is synthesized 5 prime to 3 prime. You still might end up with different protein products. mRNA needs to be processed, with non coding introns spliced out and exons retained. Known as alternative splicing, different sequences can be removed as a result of different cell signalings resulting in unique transcripts from the same gene.
We are going to end today’s episode with a recap of the Lac Operon - everyone’s favorite bacterial gene expression example. First, the name. Lac because these genes involve the products which aid in the digestion of the sugar lactose, and operon, which is a group of genes and regulatory sequences controlled by a single promoter. This is common when genes need to function in unison when multiple proteins are needed by the cell simultaneously. The Lac Operon is an example of an inducible operon, which turns on only in the presence of lactose. This makes sense! Why would a bacteria make enzymes to break down lactose if the sugar wasn’t around? Both inducible and repressible operons respond to environmental changes, which means better efficiency for the cell.
Time for unit connections. Unit 4 - the cell cycle - Don’t forget that
gene expression can be stimulated by signal transduction. For example, cytokines regulate gene expression to allow for cell division and Expression of the SRY gene triggers the male sexual development pathway in animals. Also connection to Unit 5: Heredity with the using probability to analyze single-gene inheritance patterns. Additionally, environmental factors can influence gene expression and phenotypic plasticity. Even unit 7 - since cell specialization is a huge factor in survival. Alright - what about the exam? Make sure you’ve got your vocab ducks in a row before diving into practice problems. Genes vs alleles, transcription vs. translation, DNA vs. RNA, and chromatin vs. chromosome - just to name a few. Science is a language first, and a concept second.
To recap……
Gene regulation results in gene expression and influences cell products and function. Genes are heritable and vary from one cell type to another. Each gene is transcribed and at times modified before translation and phenotypic expression.
Coming up next on the Apsolute RecAP Biology Edition: Helicase unzips
Today’s question of the day is about the Lac Operon
Question of the day: In the Lac Operon, is the promoter upstream or downstream from the operator?