Transcription, Translation and Replication
During this process, the DNA sequence of a gene is copied into RNA. However , there is one important difference: in the newly made RNA, all of the T. Figure 1: Transcription begins when RNA polymerase binds to the DNA Interestingly, this base substitution is not the only difference between DNA and RNA. In the first step, the information in DNA is transferred to a messenger RNA (mRNA ) molecule by way of a process called transcription. During transcription, the.
As a result, each new cell has its own complete genome. This process is known as DNA replication. Replication is controlled by the Watson-Crick pairing of the bases in the template strand with incoming deoxynucleotide triphosphates, and is directed by DNA polymerase enzymes. It is a complex process, particularly in eukaryotes, involving an array of enzymes. A simplified version of bacterial DNA replication is described in Figure 2. This makes it impossible for DNA polymerases to synthesize both strands simultaneously.
A portion of the double helix must first unwind, and this is mediated by helicase enzymes. The leading strand is synthesized continuously but the opposite strand is copied in short bursts of about bases, as the lagging strand template becomes available. The resulting short strands are called Okazaki fragments after their discoverers, Reiji and Tsuneko Okazaki.
Bacteria have at least three distinct DNA polymerases: Pol III can then take over, but it eventually encounters one of the previously synthesized short RNA fragments in its path. The initiation of DNA replication at the leading strand is more complex and is discussed in detail in more specialized texts.
This leads to mismatched base pairs, or mispairs. DNA polymerases have proofreading activity, and a DNA repair enzymes have evolved to correct these mistakes.
Overview of transcription
Occasionally, mispairs survive and are incorporated into the genome in the next round of replication. These mutations may have no consequence, they may result in the death of the organism, they may result in a genetic disease or cancer; or they may give the organism a competitive advantage over its neighbours, which leads to evolution by natural selection.
Transcription Transcription is the process by which DNA is copied transcribed to mRNA, which carries the information needed for protein synthesis.
Transcription takes place in two broad steps. As with DNA replication, partial unwinding of the double helix must occur before transcription can take place, and it is the RNA polymerase enzymes that catalyze this process.
Unlike DNA replication, in which both strands are copied, only one strand is transcribed. The strand that contains the gene is called the sense strand, while the complementary strand is the antisense strand. The mRNA produced in transcription is a copy of the sense strand, but it is the antisense strand that is transcribed.
Transcription ends when the RNA polymerase enzyme reaches a triplet of bases that is read as a "stop" signal. The DNA molecule re-winds to re-form the double helix.
Alternative splicing In alternative splicing, individual exons are either spliced or included, giving rise to several different possible mRNA products. Each mRNA product codes for a different protein isoform; these protein isoforms differ in their peptide sequence and therefore their biological activity.
Several different mechanisms of alternative splicing are known, two of which are illustrated in Figure 6. Splicing is important in genetic regulation alteration of the splicing pattern in response to cellular conditions changes protein expression. Perhaps not surprisingly, abnormal splicing patterns can lead to disease states including cancer.
This process, catalyzed by reverse transcriptase enzymes, allows retroviruses, including the human immunodeficiency virus HIVto use RNA as their genetic material. This is called abortive initiationand is common for both eukaryotes and prokaryotes. Mechanistically, promoter escape occurs through DNA scrunchingproviding the energy needed to break interactions between RNA polymerase holoenzyme and the promoter. Elongation[ edit ] Simple diagram of transcription elongation One strand of the DNA, the template strand or noncoding strandis used as a template for RNA synthesis.
As transcription proceeds, RNA polymerase traverses the template strand and uses base pairing complementarity with the DNA template to create an RNA copy which elongates during the traversal. In eukaryotes, this may correspond with short pauses during transcription that allow appropriate RNA editing factors to bind. These pauses may be intrinsic to the RNA polymerase or due to chromatin structure. Terminator genetics Bacteria use two different strategies for transcription termination — Rho-independent termination and Rho-dependent termination.
This pulls the poly-U transcript out of the active site of the RNA polymerase, terminating transcription.DNA, Hot Pockets, & The Longest Word Ever: Crash Course Biology #11
In the "Rho-dependent" type of termination, a protein factor called " Rho " destabilizes the interaction between the template and the mRNA, thus releasing the newly synthesized mRNA from the elongation complex. An example of such an antibacterial is rifampicinwhich inhibits bacterial transcription of DNA into mRNA by inhibiting DNA-dependent RNA polymerase by binding its beta-subunit, while 8-hydroxyquinoline is an antifungal transcription inhibitor.
DNA replication and RNA transcription and translation (video) | Khan Academy
Regulation of transcription in cancer In vertebrates, the majority of gene promoters contain a CpG island with numerous CpG sites. For example, in colorectal cancers about to genes are transcriptionally inhibited by CpG island methylation see regulation of transcription in cancer. Transcriptional repression in cancer can also occur by other epigenetic mechanisms, such as altered expression of microRNAs. Transcription factories Active transcription units are clustered in the nucleus, in discrete sites called transcription factories or euchromatin.
Such sites can be visualized by allowing engaged polymerases to extend their transcripts in tagged precursors Br-UTP or Br-U and immuno-labeling the tagged nascent RNA.
Transcription factories can also be localized using fluorescence in situ hybridization or marked by antibodies directed against polymerases.