Central DogmaGenetics lec 2EAzad Mohammad Mehdi ,
DNA replication DNA replication. The double helix is unwoundand each strand acts as a template for the next strand. Bases are matched to synthesize the new partner strands. DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. It has been suggested that Replication fork be merged into this article or section. (Discuss) Proposed since May 2009.
• In a cell, DNA replication begins at specific locations in the genome, called "origins Unwinding of DNA at the origin, and synthesis of new strands, forms a replication fork. In addition to DNA polymerase, the enzyme that synthesizes the new DNA by adding nucleotides matched to the template strand, a number of other proteins are associated with the fork and assist in the initiation and continuation of DNA synthesis.• . The polymerase chain reaction (PCR), a common laboratory technique, employs such artificial synthesis in a cyclic manner to amplify a specific target DNA fragment from a pool of DNA.
• DNA structure• DNA usually exists as a double-stranded structure, with both strands coiled together to form the characteristic double-helix. Each single strand of DNA is a chain of four types of nucleotides having the bases: adenine, cytosine, guanine, and thymine. A nucleotide is a mono-, di-, or triphosphate deoxyribonucleoside; that is, a deoxyribose sugar is attached to one, two, or three phosphates. Chemical interaction of these nucleotides forms phosphodiester linkages, creating the phosphate- deoxyribose backbone of the DNA double helix with the bases pointing inward. Nucleotides (bases) are matched between strands through hydrogen bonds to form base pairs. Adenine pairs with thymine, and cytosine pairs with guanine.DNA structure
DNA polymeraseDNA polymerases are a family ofenzymes that carry out all forms of DNAreplication However, a DNA polymerasecan only extend an existing DNA strandpaired with a template strand; it cannotbegin the synthesis of a new strand. Tobegin synthesis, a short fragment ofDNA or RNA, called a primer, must becreated and paired with the templateDNA strand.DNA polymerase then synthesizes anew strand of DNA by extending the 3end of an existing nucleotide chain,adding new nucleotides matched to thetemplate strand one at a time via thecreation of phosphodiester bonds.
Replication process The replication fork is a structure that forms within thenucleus during DNA replication. It is created by helicases, which break the hydrogen bonds holding the two DNA strands together. The resulting structure has two branching "prongs", each one made up of a single strand of DNA. These two strands serve as the template for theleading and lagging strands, which will be created as DNA polymerase matches complementary nucleotides to the templates; The templates may be properly referred to as the leading strand template and the lagging strand template.
Transcription (genetics)• Transcription is the process of creating a complementary RNA copy of a sequence of DNA.  Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes. During transcription, a DNA sequence is read by an RNA polymerase, which produces a complementary, antiparallel RNA strand. As opposed to DNA replication, transcription results in an RNA complement that includes uracil (U) in all instances where thymine (T) would have occurred in a DNA complement.
Transcription is explained easily in4 or 5 steps, each moving like awave along the DNA.• RNA Polymerase moves the transcription bubble, a stretch of unpaired nucleotides, by breaking the hydrogen bonds between complementary nucleotides.• RNA Polymerase adds matching RNA nucleotides that are paired with complementary DNA bases.• RNA sugar-phosphate backbone forms with assistance from RNA polymerase.• Hydrogen bonds of the untwisted RNA+DNA helix break, freeing the newly synthesized RNA strand.• If the cell has a nucleus, the RNA is further processed (addition of a 3 poly-A tail and a 5 cap) and exits through to the cytoplasm through the nuclear pore complex.• Transcription is the first step leading to gene expression. The stretch of DNA transcribed into an RNA molecule is called a transcription unit and encodes at least one gene. If the gene transcribed encodes a protein, the result of transcription is messenger RNA (mRNA), which will then be used to create that protein via the process of translation. Alternatively, the transcribed gene may encode for either ribosomal RNA (rRNA) or transfer RNA (tRNA), other components of the protein-assembly process, or other ribozymes]
As in DNA replication, DNA is read from 3 → 5 during transcription. Meanwhile, the complementary RNA is created from the 5 → 3 direction. This means its 5 end is created first in base pairing. Although DNA is arranged as two antiparallel strands in a double helix, only one of the two DNA strands, called the template strand, is used for transcription. This is because RNA is only single-stranded,as opposed to double-stranded DNA. The other DNA strandis called the coding (lagging) strand, because its sequence is the same as the newly created RNA transcript (except for the substitution of uracil for thymine). The use of only the 3 → 5 strand eliminates the need for the Okazaki fragments seen in DNA replication] Transcription is divided into 5 stages: pre-initiation, initiation, promoter clearance, elongation
Translation (biology) In molecular biology and genetics, translation is the third stage of protein biosynthesis (part of the overall process of gene expression). Intranslation, messenger RNA (mRNA) produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein. In Bacteria, translation occurs in the cells cytoplasm, where the large and small subunits of the ribosome are located, and bind to the mRNA. In Eukaryotes, translation occurs across the membrane of the endoplasmic reticulum in a process called vectorial synthesis. The ribosome facilitates decoding by inducing the binding of tRNAs with complementary anticodon sequences to that of the mRNA. The tRNAs carry specific amino acids that are chained together into a polypeptide as the mRNA passes through and is "read" by the ribosome in a fashion reminiscent to that of a stock ticker and ticker tape.
• Translation proceeds in four phases: activation, initiation, elongation and termination (all describing the growth of the amino acid chain, or polypeptide that is the product of translation). Amino acids are brought to ribosomes and assembled into proteins.• In activation, the correct amino acid is covalently bonded to the correct transfer RNA (tRNA). The amino acid is joined by its carboxyl group to the 3 OH of the tRNA by an ester bond. When the tRNA has an amino acid linked to it, it is termed "charged". Initiation involves the small subunit of the ribosome binding to the 5 end of mRNA with the help of initiation factors (IF). Termination of the polypeptide happens when the A site of the ribosome faces a stop codon (UAA, UAG, or UGA). No tRNA can recognize or bind to this codon. Instead, the stop codon induces the binding of a release factor protein that prompts the disassembly of the entire ribosome/mRNA complex.