Transcription, Translation & Protein Synthesis, Slides of Molecular biology

Download Transcription, Translation & Protein Synthesis and more Slides Molecular biology in PDF only on Docsity! Transcription, Translation & Protein Synthesis Genes are used by the cell to synthesize proteins. In order to produce a protein, genes must be transcribed and translated by the machinery of the cell. Gene Expression The use of genes to produce proteins is called gene expression. Two main processes are involved in gene expression: transcription and translation. 1. During transcription, DNA in the nucleus of a cell is copied into messenger RNA molecules. 2. The messenger RNA then moves into the cell's cytoplasm and attaches to a ribosome, where it is translated into a protein. Central Dogma of Molecular Biology Once DNA is transcribed into messenger RNA and translated into a protein, the process cannot be reversed. That is, information cannot be transferred from the protein back to the nucleic acid. This is the central dogma of molecular biology. Transcription The sequence of nitrogenous bases in a gene provides the genetic instructions needed to construct a protein. Transcription occurs when a series of chemical signals within the cell causes the gene for a specific protein to "turn on," or become active. During transcription, a segment of DNA is transcribed, or copied, to produce a complementary strand of messenger RNA (mRNA). Transcription occurs in the nucleus of the cell. The three main processes that occur during transcription are described below. 1. Initiation — During initiation, enzymes bind to a DNA sequence and unwind the double helix to expose a strand of nucleotides. 2. Elongation — As the DNA molecule unwinds, an enzyme called RNA polymerase pairs complementary RNA nucleotides with the DNA nucleotides on one of the exposed strands. Adenine (A) on DNA pairs with uracil (U) on RNA, cytosine (C) pairs with guanine (G), and thymine (T) pairs with adenine (A). For example, if the DNA strand reads 'ACG,' the complementary RNA strand would read 'UGC.' 3. Termination — Once the gene is transcribed, the new RNA molecule breaks away and the DNA strands are wound back together. During transcription, a DNA molecule is unwound, and an RNA strand is synthesized using an exposed DNA strand as a template. Now that transcription is completed, the RNA molecule moves to the cytoplasm of the cell, where it will be translated into a protein. Translation Translation occurs in cell organelles called ribosomes. Ribosomes contain proteins and a type of RNA called ribosomal RNA (rRNA). It is a major component of cellular ribosomes and it can act as a catalyst in chemical reactions. During translation, an mRNA strand is used to synthesize a chain of amino acid residues called a polypeptide. When mRNA leaves the nucleus, it travels until it reaches a ribosome. Each three-base segment of the mRNA strand is called a codon. A polypeptide is formed by matching the anticodon of a transfer RNA (tRNA) molecule—each of which carries a specific amino acid—to the corresponding codon on the mRNA strand. Later, the polypeptide will fold into a functional protein. The steps of translation are shown below. 1. A ribosome attaches to the 5' end of the mRNA strand. 2. Transfer RNA (tRNA) molecules carrying amino acids approach the ribosome. 3. The tRNA molecule whose anticodon corresponds to the first codon on the mRNA strand quickly attaches at the ribosome. 4. A new tRNA molecule carrying another amino acid attaches to the next codon on the mRNA strand. 5. As amino acids are added next to each other, peptide bonds form between them. 6. The previous tRNA molecule detaches from the mRNA strand and departs from the ribosome, leaving its amino acid behind. 7. The chain of amino acid residues continues to grow until the ribosome reaches a stop codon at the 3' end of the mRNA strand. This signals that no more amino acids should be added. The result is a polypeptide. Comparing DNA & RNA Both DNA and RNA play a role in storing and transmitting cellular information, but there are key differences in the structures and specific functions of the two types of nucleic acids. DNA Structure vs. RNA Structure The diagram below shows several key structural differences between DNA and RNA. The nitrogenous bases found in each molecule are also shown. DNA and RNA share key similarities, but there are also significant structural differences between them. Molecular Shape  DNA is composed of two nucleotide chains wound together into a double helix.  RNA is composed of one nucleotide chain in a single helix. Nitrogenous Bases  DNA contains the nitrogenous bases cytosine (C), guanine (G), adenine (A), and thymine (T).  RNA contains the nitrogenous bases cytosine (C), guanine (G), adenine (A), and uracil (U). Nucleotide Components  DNA nucleotides each consist of a nitrogenous base, a phosphate group, and a deoxyribose five-carbon sugar.  RNA nucleotides each consist of a nitrogenous base, a phosphate group, and a ribose five-carbon sugar. DNA Function vs. RNA Function Together, DNA and RNA contain all the instructions a cell needs to carry out its life functions. However, they differ in function. Several of these key differences are discussed below.  DNA is the ultimate source of genetic information in the cell. DNA is used as a template to produce RNA during the process of transcription. Before a cell divides, it copies its DNA so its genetic information can be passed on to new cells.  RNA has several unique roles in the cell. There are three main types of RNA:  mRNA, or messenger RNA, is used to produce proteins. It carries information from the nucleus to ribosomes.  tRNA, or transfer RNA, attaches amino acids to the growing polypeptide chain during translation.  rRNA, or ribosomal RNA, is the major structural component of cellular ribosomes. Base Pairing Rules DNA and RNA molecules each contain a unique sequence of nucleotides that ultimately determines their function in the cell. Even though there are only four nitrogenous bases in a strand of DNA or RNA, these bases can be ordered in innumerable ways. This enables the production of the incredible variety of substances, such as proteins, enzymes, and RNA structures, that support an organism's life processes. There are two classes of nitrogenous bases: purines and pyrimidines. Purines Pyrimidines Nitrogenous bases are held together by hydrogen bonds that occur only between complementary bases. Like puzzle pieces, a pyrimidine will pair with only one specific purine, and a purine will pair with only one specific pyrimidine (with the exception of adenine, which pairs with thymine in DNA and uracil in RNA). This is the basis of the genetic code that all living things share. Adenine and thymine bond to one another, and cytosine and guanine bond to one another. The base pairing rules for DNA and RNA are shown below. DNA Base Pairing Rules adenine thymine thymine adenine cytosine guanine guanine cytosine RNA Base Pairing Rules adenine uracil uracil adenine cytosine guanine guanine cytosi Gene Expression Essentials Click on the image below to explore gene expression and the factors that influence protein synthesis. PhET Interactive Simulations University of Colorado https://phet.colorado.edu Transcription & Translation Question 1 . The picture below shows the process of transcription. The process shown above is known as translation and involves the production of proteins from RNA. C. The process shown above is known as cloning and involves the production of RNA from protein molecules. D. The process shown above is known as replication and involves the production of DNA from RNA. Question 6 . The model shows transcription and translation occurring in a eukaryotic cell. What is a limitation of the model in representing the steps of transcription and translation? A. It does not demonstrate that DNA polymerase is involved in transcribing DNA into RNA. B. It does not show that the ribosome stops translating when it encounters a stop codon. C. It does not suggest that transcription and translation are spatially separated in the cell. D. It does not indicate that a particular RNA codon corresponds to a specific amino acid. Question 7 . Proteins play a vital role in all cells. In fact, cells need thousands of proteins in order to function properly. What primarily directs the synthesis of proteins? A. the genetic information stored in DNA B. the folding patterns of strands of RNA C. the energy that is stored in mitochondria D. the cytoplasm found throughout a cell Question 8 . Nadia made a table that describes the function of the three main types of RNA. She labeled the RNA types with 1, 2, and 3. She also labeled the RNA types in an image that shows the structure of each type of RNA and models the process of protein synthesis. She used the labels X, Y, and Z in the image. Which combination correctly identifies a type of RNA from the table and the image? A. X and 1 represent rRNA B. Y and 2 represent mRNA C. Y and 3 represent tRNA D. Z and 2 represent mRNA Question 9 . Lactase is an enzyme produced by cells in the small intestine. Lactase helps break down lactose, a sugar in milk. In humans, lactase is encoded by a single gene. Transcription of this gene produces a primary transcript. The primary transcript is processed and then exported out of the nucleus. In the cytoplasm, the transcript is translated to synthesize lactase. This process is summarized in the diagram below. What are the roles of RNA molecules during the step labeled with an X in the diagram? A. mRNA rRNA tRNA produces amino acids to make lactase makes up the ribosome, which synthesizes lactase carries nucleotides to the mRNA copy of the lactase gene B. mRNA rRNA tRNA forms peptide bonds between amino acids produces amino acids from an RNA template contains instructions to synthesize lactase C. mRNA rRNA tRNA carries instructions to build lactase makes up part of the ribosome, which decodes mRNA carries amino acids to the growing lactase molecule D. mRNA rRNA tRNA contains a copy of the lactase gene folds the amino acid chain to produce the lactase molecule forms peptide bonds between amino acids in lactase Question 10 . The diagram below shows a polypeptide chain folding to become a protein with a compact, 3- dimensional structure.