DNA Replication & Transcription Lesson PDF

DNA Replication In this lesson, you should be able to: analyze each process involved in the central pinarello dogma of molecular biology.  The Central Dogma The central dogma of molecular biology is an accepted principle stating that the genetic information contained in the DNA is copied into...

DNA Replication In this lesson, you should be able to: analyze each process involved in the central pinarello dogma of molecular biology.  The Central Dogma The central dogma of molecular biology is an accepted principle stating that the genetic information contained in the DNA is copied into daughter cells and transferred to RNA molecules that direct the synthesis of protein molecules. Hence, the flow of genetic messages starts from DNA replication, followed by transcription, and finally, translation. These processes are summarized in the figure below. The Central Dogma Because the DNA contains the genetic information of organisms, it is important that an exact copy can be made and passed on to the next generation of cells. This process, referred to as DNA replication will be discussed deeply in this lesson. Other major processes such as transcription and translation will be discussed in the next chapters. DNA Replication Replication is the process wherein DNA molecules are duplicated during cell division and passed on to each daughter cell. It happens during interphase before mitosis and meiosis take place. Replication is semiconservative, which means that each new copy would contain a strand from the original DNA. The synthesis of a new DNA strand is formed from an old DNA strand as the template and a new complementary strand. DNA Replication DNA Replication To replicate the DNA molecule, the antiparallel strands are separated from one another by disrupting the hydrogen bonds between their nitrogenous bases. Then, each DNA strand is replicated with the help of specialized proteins that utilize free nucleotides present in the surroundings. This process happens very fast and is very accurate! The accuracy and speed of the DNA replication rely on several enzymes and proteins, which will be discussed in the next section. The Molecular Mechanism of DNA Replication Most processes involved in DNA replication are common to most organisms, regardless of whether they are eukaryotes or prokaryotes. The most well-studied molecular mechanism of DNA replication is that of Escherichia coli (E. coli). E. coli is a bacteria, but its replication mechanism is similar to humans. It occurs in three stages - initiation, elongation, and termination. The succeeding discussions will focus on each stage. Stage 1: Initiation How does DNA replication begin? DNA contains not only information that tells your physical and chemical characteristics, but also information on vital processes inside your cells. The information can be deciphered from its sequence. The specific point in the DNA sequence at which replication begins is called the origin of replication. This starting point is represented by sequences consisting of hundreds to thousands of base pairs that are unique to each organism. Stage 2: Elongation DNA polymerase cannot add nucleotides without any existing template. It requires a short nucleotide chain known as primers which are attached to the starting nucleotides in the replication fork by enzymes known as primases. The primase primes the elongation by providing an RNA primer that is usually five to ten nucleotides. The DNA polymerase then extends the primer by adding free nucleotides to the hydroxyl group of the sugar molecule found at the 3’ end. Stage 3: Termination When the two original strands are bound to their own complementary strands, DNA replication stops. The two new identical DNA molecules are complete. The RNA primers can be removed by DNA polymerase I at the end of the process. The nicks are formed after the primers have been removed, and the enzyme DNA ligase seals the fragments synthesized in the lagging strand. After these clean-ups, two DNA molecules are formed. DNA Transcription In this lesson, you should be able to: describe the roles of the three types of RNA; explain the process of splicing mRNA; and differentiate exon from intron. DNA TRANSCRIPTION DNA carries the genetic information of the cell. This information is translated by synthesizing a specific RNA. After synthesizing the said RNA, it is further processed to separate the coding sections from the non-coding sections. The final, mature RNA strand then leaves the nucleus. What are the steps in translating the information from DNA? The Ribonucleic Acid or RNA The RNA is a single-stranded nucleic acid with ribose as the sugar unit. Its bases are cytosine, guanine, adenine, and instead of thymine, it has uracil. In eukaryotes, the RNA is found in the cytoplasm, although RNA synthesis occurs in the nucleus Messenger RNA (mRNA) The messenger RNA (mRNA) carries the genetic sequence information and transfers it from the DNA to the ribosomes. The DNA is confined in the nucleus, whereas proteins are synthesized in the cytoplasm. The mRNA acts as an intermediate messenger that exits the nucleus and transfers the information to the ribosomes to initiate protein synthesis. The mRNA is also called the transcript. It is a strand of RNA produced after transcription. The process of making mRNAs and their structural features will be discussed shortly. Ribosomal RNA (rRNA) The ribosomal RNA (rRNA) is the major component of the ribosomes. It catalyzes the peptide bond formation during protein synthesis. The structure of the ribosome and how it converts the mRNA to a polypeptide is discussed in detail in the next lesson. Ribosomal RNA (rRNA) The ribosomal RNA (rRNA) is the major component of the ribosomes. It catalyzes the peptide bond formation during protein synthesis. The structure of the ribosome and how it converts the mRNA to a polypeptide is discussed in detail in the next lesson. Transfer RNA (tRNA) The transfer RNA (tRNA) serves as the carrier molecules of the amino acids that make up the protein. During translation, it directs a specific amino acid into the ribosome. The ribosome incorporates this amino acid into the polypeptide chain. The amino acid depends on a set of three mRNA bases recognized by the tRNA. The complete process of translation and the structure of tRNA will be discussed in detail in the next lesson. Transcription Transcription is a process of producing RNA as directed by the DNA in the nucleus. The DNA is used as a template to form a single-strand mRNA, which has a base sequence complementary to that of the DNA. A particular segment of the DNA sequence is read by an enzyme called RNA polymerase. This enzyme reads the template strand of the DNA and produces a complementary RNA strand called a primary transcript, or simply a transcript. DNA Translation Objective In this lesson, you should be able to: explain how proteins are synthesized.  Translation and the Codon Table After transcription and post-transcriptional modification, the mature mRNA leaves the nucleus through the nuclear membrane and passes to the ribosomes in the cytoplasm. This mRNA, which is similar to the DNA’s coding strand, is used in translation. Translation is the synthesis of a protein directed by the base sequence of an mRNA. These proteins will carry out the operation encrypted in the DNA sequence and transcribed in the mRNA sequence. To grasp this concept, imagine the analogy shown in the figure below. The DNA serves as the blueprint that contains the information on how life processes should be done. This is similar to a cookbook which contains the necessary information to make a cake. Replication “photocopies” this blueprint, producing an exact copy. This comes in the form of an mRNA. Now, to carry out the operation transcribed in the mRNA sequence, translation takes place and produces proteins that will work out the given task. In the analogy, this process is like adding the ingredients (which represents amino acids) to make a cake (which represents protein). In biology, translation is the process in living cells in which proteins are produced using RNA molecules as templates. The generated protein is a sequence of amino acids. This sequence is determined by the sequence of nucleotides in the RNA. The nucleotides are considered three at a time. Each such triple results in addition of one specific amino acid to the protein being generated. The matching from nucleotide triple to amino acid is called the genetic code. The translation is performed by a large complex of functional RNA and proteins called ribosomes. The entire process is called gene expression. https://www.hgmd.cf.ac.uk/docs/cd_amino.html