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    Gene Expression

    In multicellular organisms, nearly all cells have the same DNA, but different cell types express distinct proteins. Learn how cells adjust these proteins to produce their unique identities.

    Gene Expression

    Genes encode proteins and proteins dictate cell function. Therefore, the thousands of genes expressed in a particular cell determine what that cell can do. Moreover, each step in the flow of information from DNA to RNA to protein provides the cell with a potential control point for self-regulating its functions by adjusting the amount and type of proteins it manufactures.

    At any given time, the amount of a particular protein in a cell reflects the balance between that protein's synthetic and degradative biochemical pathways. On the synthetic side of this balance, recall that protein production starts at transcription (DNA to RNA) and continues with translation (RNA to protein). Thus, control of these processes plays a critical role in determining what proteins are present in a cell and in what amounts. In addition, the way in which a cell processes its RNA transcripts and newly made proteins also greatly influences protein levels.

    How Is Gene Expression Regulated?

    The amounts and types of mRNA molecules in a cell reflect the function of that cell. In fact, thousands of transcripts are produced every second in every cell. Given this statistic, it is not surprising that the primary control point for gene expression is usually at the very beginning of the protein production process — the initiation of transcription. RNA transcription makes an efficient control point because many proteins can be made from a single mRNA molecule.

    Transcript processing provides an additional level of regulation for eukaryotes, and the presence of a nucleus makes this possible. In prokaryotes, translation of a transcript begins before the transcript is complete, due to the proximity of ribosomes to the new mRNA molecules. In eukaryotes, however, transcripts are modified in the nucleus before they are exported to the cytoplasm for translation.

    Eukaryotic transcripts are also more complex than prokaryotic transcripts. For instance, the primary transcripts synthesized by RNA polymerase contain sequences that will not be part of the mature RNA. These intervening sequences are called introns, and they are removed before the mature mRNA leaves the nucleus. The remaining regions of the transcript, which include the protein-coding regions, are called exons, and they are spliced together to produce the mature mRNA. Eukaryotic transcripts are also modified at their ends, which affects their stability and translation.

    Of course, there are many cases in which cells must respond quickly to changing environmental conditions. In these situations, the regulatory control point may come well after transcription. For example, early development in most animals relies on translational control because very little transcription occurs during the first few cell divisions after fertilization. Eggs therefore contain many maternally originated mRNA transcripts as a ready reserve for translation after fertilization (Figure 1).

    On the degradative side of the balance, cells can rapidly adjust their protein levels through the enzymatic breakdown of RNA transcripts and existing protein molecules. Both of these actions result in decreased amounts of certain proteins. Often, this breakdown is linked to specific events in the cell. The eukaryotic cell cycle provides a good example of how protein breakdown is linked to cellular events. This cycle is divided into several phases, each of which is characterized by distinct cyclin proteins that act as key regulators for that phase. Before a cell can progress from one phase of the cell cycle to the next, it must degrade the cyclin that characterizes that particular phase of the cycle. Failure to degrade a cyclin stops the cycle from continuing.

    Figure 1: An overview of the flow of information from DNA to protein in a eukaryote

    First, both coding and noncoding regions of DNA are transcribed into mRNA. Some regions are removed (introns) during initial mRNA processing. The remaining exons are then spliced together, and the spliced mRNA molecule (red) is prepared for export out of the nucleus through addition of an endcap (sphere) and a polyA tail. Once in the cytoplasm, the mRNA can be used to construct a protein.

    © 2010 Nature Education All rights reserved.

    Figure Detail

    How Do Different Cells Express the Genes They Need?

    Only a fraction of the genes in a cell are expressed at any one time. The variety of gene expression profiles characteristic of different cell types arise because these cells have distinct sets of transcription regulators. Some of these regulators work to increase transcription, whereas others prevent or suppress it.

    Normally, transcription begins when an RNA polymerase binds to a so-called promoter sequence on the DNA molecule. This sequence is almost always located just upstream from the starting point for transcription (the 5' end of the DNA), though it can be located downstream of the mRNA (3' end). In recent years, researchers have discovered that other DNA sequences, known as enhancer sequences, also play an important part in transcription by providing binding sites for regulatory proteins that affect RNA polymerase activity. Binding of regulatory proteins to an enhancer sequence causes a shift in chromatin structure that either promotes or inhibits RNA polymerase and transcription factor binding. A more open chromatin structure is associated with active gene transcription. In contrast, a more compact chromatin structure is associated with transcriptional activity (Figure 2).

    Source : www.nature.com

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    What are the three steps in gene expression that ultimately affect protein levels in a eukaryotic cell?

    2021-04-02 Michael Wilson

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    What are the three steps in gene expression that ultimately affect protein levels in a eukaryotic cell?

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    Eukaryotic gene expression is regulated at the levels of transcription, RNA processing, translation, and post-translation. Proteins called transcription factors bind to DNA and control transcription. Different types of transcription factors can increase or decrease transcription.

    What are two ways repressors can work?

    What are two ways in which repressors can interfere with transcription? Some can bind to the binding side of activators, thus preventing them from binding to DNA and so transcription cannot be activated. Some can order the chromatin structure to coil up tightly and that makes them unavailable for transcription.

    How is the lactase gene regulated?

    It is accepted that lactase gene expression is primarily regulated at the transcriptional level33,34,35, and in non-human mammals Cdx2, Gata4/6 and Hnf1α TFs collectively activate this gene.

    How is the LCT gene turned off?

    These studies imply that after early childhood, the lactase gene is usually shut off by DNA methylation. The SNPs that alter the DNA sequence in the control region, however, prevent this methylation from happening. This, in turn, results in the production of lactase because the gene is kept on.

    Why is the LCT gene important?

    The LCT gene provides instructions for making an enzyme called lactase. This enzyme helps to digest lactose, a sugar found in milk and other dairy products.

    When did we become lactose tolerant?

    The pattern was the same for all mammals: At the end of infancy, we became lactose-intolerant for life. Two hundred thousand years later, around 10,000 B.C., this began to change. A genetic mutation appeared, somewhere near modern-day Turkey, that jammed the lactase-production gene permanently in the “on” position.

    How did we become lactose tolerant?

    An Evolutionary Whodunit: How Did Humans Develop Lactose Tolerance? : The Salt Thousands of years ago, ancient farmers gained the ability to consume milk as adults without getting an upset stomach. A remarkable mutation let some of them digest lactose sugar.

    What two sugars are lactose broken into digested?

    Lactose consists of two sugars: glucose and galactose. An enzyme in our small intestine called lactase quickly breaks down the lactose into its two parts. Only after the two sugars have been separated can they be absorbed by our bowel.

    Can you become lactose intolerant all of a sudden?

    Lactose intolerance can start suddenly, even if you’ve never had trouble with dairy products before. Symptoms usually start a half-hour to two hours after eating or drinking something with lactose.

    What is the best lactose free milk?

    The major lactose-free milk options

    Soy milk. Let’s start with the most common substitute.

    Rice milk. Rice milk tends to be sweeter than other lactose-free milks, with a thin and watery consistency.

    Almond milk. Coconut-based milk. Cashew milk. Hazelnut milk. Hemp milk. Oat milk. Facebook Twitter LinkedIn Reddit WhatsApp Telegram Messenger Share

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    Source : www.restaurantnorman.com

    Regulation after transcription (article)

    Alternative splicing, miRNAs and siRNAs, translation initiation factors, & protein modifications.

    Gene regulation in eukaryotes

    Regulation after transcription

    Alternative splicing, miRNAs and siRNAs, translation initiation factors, & protein modifications.

    Google ClassroomFacebookTwitter


    Key points:

    Even after a gene has been transcribed, gene expression can still be regulated at various stages.

    Some transcripts can undergo alternative splicing, making different mRNAs and proteins from the same RNA transcript.

    Some mRNAs are targeted by microRNAs, small regulator RNAs that can cause an mRNA to be chopped up or block translation.

    A protein's activity may be regulated after translation, for example, through removal of amino acids or addition of chemical groups.


    The genes that a eukaryotic cell turns "on" largely determine its identity and properties. For instance, a photoreceptor cell in your eye can detect light because it expresses genes for light-sensitive proteins, as well as as genes for neurotransmitters that allow signals to be relayed to the brain.

    In eukaryotic cells like photoreceptors, gene expression is often controlled primarily at the level of transcription. However, that doesn't mean transcription is the last chance for regulation. Later stages of gene expression can also be regulated, including:

    RNA processing, such as splicing, capping, and poly-A tail addition

    Messenger RNA (mRNA) translation and lifetime in the cytosol

    Protein modifications, such as addition of chemical groups

    In the sections below, we’ll discuss some common types of gene regulation that occur after an RNA transcript has been made.

    Regulation of RNA processing

    When a eukaryotic gene is transcribed in the nucleus, the primary transcript (freshly made RNA molecule) isn't yet considered a messenger RNA. Instead, it's an "immature" molecule called a pre-mRNA.

    The pre-mRNA has to go through some modifications to become a mature mRNA molecule that can leave the nucleus and be translated. These include splicing, capping, and addition of a poly-A tail, all of which can potentially be regulated – sped up, slowed down, or altered to result in a different product.

    Alternative splicing

    Most pre-mRNA molecules have sections that are removed from the molecule, called introns, and sections that are linked or together to make the final mRNA, called exons. This process is called splicing.

    In the process of alternative splicing, different portions of an mRNA can be selected for use as exons. This allows either of two (or more) mRNA molecules to be made from one pre-mRNA.

    Diagram of a pre-mRNA being spliced into two different variants. There are four possible exons in the pre-mRNA: 1, 2, 3, and 4

    Variant 1 contains exons 1, 2, and 4, but not exon 3.

    Variant 2 contains exons 1, 3, and 4, but not exon 2.

    Image modified from "Eukaryotic Post-transcriptional Gene Regulation: Figure 1," by OpenStax College, Biology (CC BY 3.0).

    Alternative splicing is not a random process. Instead, it's typically controlled by regulatory proteins. The proteins bind to specific sites on the pre-mRNA and "tell" the splicing factors which exons should be used. Different cell types may express different regulatory proteins, so different exon combinations can be used in each cell type, leading to the production of different proteins.

    Small regulatory RNAs

    Once an mRNA has left the nucleus, it may or may not be translated many times to make proteins. Two key determinants of how much protein is made from an mRNA are its "lifespan" (how long it floats around in the cytosol) and how readily the translation machinery, such as the ribosome, can attach to it.

    A recently discovered class of regulators, called small regulatory RNAs, can control mRNA lifespan and translation. Let's see how this works.


    microRNAs (miRNAs) were among the first small regulatory RNAs to be discovered. A miRNA is first transcribed as a long RNA molecule, which forms base pairs with itself and folds over to make a hairpin.

    Next, the hairpin is chopped up by enzymes, releasing a small double-stranded fragment of about

    22 22 22 nucleotides ^1 1

    start superscript, 1, end superscript

    . One of the strands in this fragment is the mature miRNA, which binds to a specific protein to make an RNA-protein complex.

    Diagram of where miRNAs come from and how they regulate targets.

    First, a microRNA precursor is transcribed from a microRNA gene. The precursor folds into a hairpin, which is then processed by enzymes so it is as short duplex (double-stranded) RNA that's imperfectly complementary. One strand of this duplex is the miRNA, which associates with a protein to form an miRNA-protein complex.

    The miRNA directs the protein complex to mRNAs that are partially or fully complementary to the miRNA. When the miRNA is perfectly complementary to the mRNA, the mRNA is often cut in two by an enzyme in the protein complex. When the miRNA is not perfectly complementary to the mRNA, the miRNA-protein complex may remain bound to the mRNA and block translation.

    Source : www.khanacademy.org

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