Muscle Bio Week 8.docx

***Week 8, Module 8: Basics of Gene Transcription and Protein Synthesis***   - Info needed to synthesise proteins is coded in the sequence of bases (or nucleotides) that make up the DNA code within a cell\'s nucleus. - DNA of the nucleus controls every aspect of the cell\'s structure a...

***Week 8, Module 8: Basics of Gene Transcription and Protein Synthesis***   - Info needed to synthesise proteins is coded in the sequence of bases (or nucleotides) that make up the DNA code within a cell\'s nucleus. - DNA of the nucleus controls every aspect of the cell\'s structure and function, eg: the nucleus can accelerate the rate of glycolysis in the cell by increasing the synthesis of glycolytic enzymes within the cytoplasm. - Skeletal muscle cells are special when compared to many other cells within the body -- one example being they are multi-nucleated cells. When we consider this means that muscle cells have more of the 'blueprints' required to make proteins (encoded in the nuclei), this helps to explain the adaptability (plasticity) and regenerating qualities of skeletal muscle. - New proteins are made by activating genes encoded in the DNA that resides in the nucleus of the cell. - when exercise is repeated over time, repeated 'bursts' of gene activation (expression), coupled with the translation of gene transcripts (messenger RNA or mRNA) into specific proteins, culminate in the adaptations associated with either endurance or resistance training. These internal, molecular level changes is what underpins the external adaptations that we see.   *DNA, RNA and Protein* [DNA] - Deoxyribonucleic acid - Double helix structure. Comprised of the nucleotide bases: Adenine, Cytosine, Thymine and Guanine. - Located in the nucleus of each cell. Mitochondria also contain DNA that encodes 13 mitochondrial specific proteins. - The genetic code is called a '***triplet code***' because a sequence of three nucleotide bases (ACG, or AAA, or TAG) specifies the identity of a single amino acid. - The information within this code contains the blueprint for \~30,000 genes, and allows the synthesis of somewhere from 100,000 to 200,000 different proteins. On average, each gene consists of 3000 bases, but this varies greatly and some genes contain as many as 2.4 million bases [RNA] - Ribonucleic acid - Molecules essential for the regulation and expression of genes - Single stranded. - Used to copy genetic info from DNA 3 types of RNA 1. Messenger RNA (mRNA): copies genetic info from DNA, this process is called transcription. The copied DNA is then given to the cytosol in the cell, to the ribosomes, where they specify which amino acids need to join together to make new proteins. 2. Transfer RNA (tRNA): links the mRNA and amino acid sequence by carrying amino acids to the cellular 'factory' where new proteins are made (ribosomes). The tRNA does this by containing the amino acid linked to a three-nucleotide sequence (codon), which is complementary and specific to the codon on the mRNA. 3. Ribosomal RNA (rRNA):  combine to form two subunits of a ribosome, known as the large and small subunits. During translation, an mRNA strand is 'sandwiched' between the small and large subunits, and the ribosome catalyses the formation of a peptide bond between the two amino acids that are contained in the rRNA.   [Transcription from DNA to mRNA] - Transcription happens in the nucleus. It\'s the process of copying part of DNA to make an RNA molecule. - Transcription means to copy or rewrite. - The forming of mRNA in this process is catalysed by the enzyme RNA polymerase. This enzyme reads the DNA and transfers the genetic code of target genes to an mRNA molecule using ribonucleotides to the mRNA. - Only one strand of DNA is copied and RNA formed is referred to as the primary gene transcription. - RNA is made up of guanine, cytosine, adenine and uracil. - Guanine will bind to Cytosine, Cytosine will bind to Guanine, Adenine will bind to Uracil, and Uracil bind to Adenine   [Translation from mRNA to Protein ] - This occurs on the ribosomes residing within the cytosol and is the site of protein synthesis. - The mRNA is converted into a sequence of amino acids to form a polypeptide chain. - The meaning of 'translate' is to present the same information in a different language; in this case, a message written in the language of nucleic acids (the mRNA) is translated by ribosomes and tRNA into the language of proteins (sequences of amino acids) Translation has 3 phases: 1. Initiation: the translation of an mRNA into a peptide chain is \'initiated\' by the formation of a complex containing a tRNA carrying the Methionine amino acid (which is the first amino acid in all proteins), a protein known as an initiation factor, and another type of high-energy phosphate molecule known as GTP (guanine triphosphate).Once formed, this complex then binds with other initiation factors, and attaches to the ribosome so that translation can begin. 2. Elongation: the process whereby the peptide chain is \'elongated\' via the addition of more amino acids to the growing peptide chain. The first phase of the process involves the tRNAs bringing each amino acid to the ribosome, and the second involves the movement of the ribosome along the mRNA strand, so that the mRNA code can continue to be read and the peptide chain can grow. 3. Termination: The translation process is terminated when a \'stop\' codon (UAA, UAG, or UGA) on the mRNA strand is encountered within the ribosome. This process culminates with the release of a newly-formed polypeptide (i.e., protein) chain from the ribosome.   *Control of Gene Expression and Protein Synthesis* - As a physiological stress, exercise can affect the degree of expression of genes (and therefore levels of specific proteins) within skeletal muscle. A gene can be induced (its product increases) or it can be repressed (its product decreases). The steps below correspond to the numbered picture on the right: 1. Transcription of DNA to RNA - Most important control point of gene expression. - Exercise enhances transcription of genes encoding structural proteins, transport proteins and enzymes. 2. Conversion of primary RNA to mRNA (RNA processing) - Once RNA is copied to a new strand from DNA, it can undergo alteration. This leads to a new stand of mRNA and the synthesis of a different protein. 3. mRNA transport from the nucleus into the cytosol 4. mRNA stability (delay of degradation by ribonucleases) - Lifespan of mRNA varies between minutes to hours. This lifespan can be viewed as another level of control and how long it survives if dictated by the degradative enzymes called ribonucleases. - How long the molecules last can be regulated by proteins that stabilise mRNA, whilst others destabilise it. - The long the mRNA is in the cell, the more proteins it can make. 5. Translation of mRNA to protein - Levels of a certain protein within the cell are clearly dependent on whether or not the mRNA encoding that protein is translated at the ribosome.  - It is important to consider, however, that abundance of a given protein can occur without any apparent increase in its corresponding mRNA.  - This suggests that either the rate at which existing mRNA are translated is increased, or that there is a decrease in the rate at which the given protein is degraded within the cell. 6. Protein stability - Proteins are more stabile than mRNA, but can also be degraded. This refers to the ratio of protein synthesis to protein breakdown. - Protein degradation is carried out by 3 major enzymes known as proteases: calpain (calcium activated neutral protease) system, autophagy-lysosomal system and ubiquitin proteasome pathway. The activation of these enzymes increases in catabolic conditions, including diseased states such as cancer or AIDS. 7. Post translational modification of proteins - Similar to how RNA can be altered following transcription to produce alternate mRNA 'codes', proteins can also undergo alterations after being translated from mRNA. This can change the resultant protein structure or function.