For anyone else who wants a slightly better transcript :) DNA replication begins at a single defined DNA sequence of 245 base pairs called oriC. A protein called DnaA increases in concentration as a cell grows and gets very cell division. This protein, as a complex with ATP, controls the onset of initiation by binding to specific nine base pairs repeat at the oriC. The binding distorts the DNA leading to the opening of the adjacent 13 base pairs repeats in the DNA. The opening of the DNA allows protein complexes to enter the replication bubble and bind to the single-stranded DNA. Each complex consists of the DNA helicase (DnaB) and a DNA helicase loader (DnaC). The DNA helicase loaders open the DNA helicase protein rings and place the rings around the single-stranded DNA. The loaders are then released. The helicase uses energy from ATP hydrolysis to unwind the DNA helix at each of the 2 replication forks. Each DNA helicase recruits an enzyme called DNA primase, which synthesizes an RNA primer on the DNA template. An RNA primer has on its end a 3’ hydroxyl group, which is required as a starting point for DNA polymerase to add DNA nucleotides. The main replication polymerase in E. coli is called DNA polymerase III. DNA polymerase III complexes are ferried to the replication forks by protein complexes called clamp loaders. Clamp loaders also carry other protein complexes called sliding clamps. The clamp loaders place the sliding clamp onto the DNA. It then places an attached DNA polymerase III complex next to the sliding clamp. The sliding clamp holds the DNA polymerase in position on the 3’ prime end of the growing strand as the polymerase synthesizes new DNA. Nucleotides with complementary bases to the template strands are added one by one in the 5’ to 3’ direction. The synthesis of DNA in the direction of the fork occurs continuously to the end of the template. This new strand is called the leading strand. In contrast, the other strand, called the lagging strand, is built in fragments called Okazaki fragments. Note that the template strands are antiparallel, with their 3’ and 5’ ends oriented in opposite directions. Because DNA polymerase can add nucleotides only in the 5’ to 3’ direction, the leading strand grows continuously in the direction of the replication fork, but the lagging strand can grow only in short segments as the parent DNA molecule unzips. Many proteins participate in DNA replication, including those called single-strand DNA binding (SSB) proteins, which quickly coat exposed single-stranded regions of DNA and protect the single-stranded DNA from attack by nucleases. DNA replication continues as the DNA pol on the lagging strand meets the 5’ end of the next premier, causing the polymerase and the sliding clamp to disengage. After the DNA helicase has moved ~1,000bp, a second RNA primer is synthesized at the fork. The sliding clamp loader adds a new sliding clamp to the primer, and then adds the DNA pol to begin synthesis on a new Okazaki fragment. The cycle continues for the length of the template strand Note that the lagging strand now consists of Okazaki fragments with a segment of RNA at one end. The RNA is cleaved by an enzyme called RNase H. Another enzyme called DNA pol I uses the 3’ OH group of the adjacent Okazaki fragment to fill in the large gap with nucleotides. Finally, DNA ligase closes the remaining nicks, leaving a continuous DNA molecule. In this way, an E. coli chromosome is replicated at 2 replication forks all the way around the circular molecule.
I just grasped the distinction between paracentric and pericentric inversions in under 30 seconds. It's crazy to consider that my professor spent an entire hour explaining this, and I still couldn't grasp the concept.
Thanks for creating this video. It helped me picture how this part of embryo developpment works, for an unknown reason I always have problems with the formation of the archenteron.
Here's what he said (I hope I did no mistakes): By the early 1960s scientists understood that DNA directs the synthesis of RNA In turn experiments implicated RNA as the template in protein synthesis. In 1961 Marshall Nirenberg and Heinrich Matthaei published experiments that aimed proving the RNA connection. The experiments also have serendipitous effect of revealing clues about the genetic code. The scientists began their experiments by preparing extracts from E.coli cells. The extracts were prepared so they contained ribosomes, amino acids and other components required for protein synthesis. However, the extracts lacked mRNA Without RNA no protein synthesis occurred. When the investigators added RNA to the tube, the extract produced protein. Nirenberg and Matthaei experimented with a number of different sources of RNA and even used a synthetic RNA that contained nucleotides with only the base uridine called poly-U they found that the extract produced protein and the protein consisted entirely of phenyalanine amino acids In later experiments Nirenberg and his colleagues found that a template RNA consisting of only adenine resulted in a protein made only of lysisne amino acids a poly c RNA resuilted in a protein of prolene amino acids from these experiments it was clear that RNA was required for protein synthesis and that the sequence of the RNA distated the amino acids in the rpotein from their experiments the scientists knwe that poly-U RNA directed the incoporation of phenylalanine into a protein howerver they didnts know how many uracil containing nucleotides were required to create a code word now known as a codon was one nucleotide enough or were two three, or four required? proteins contain 20 different amino acids while mRNA contains only four different nucleotides if an mRNA codon is one two, three or four nucleotides long, how many different codons can be formed from different arrangements of 4 nucleotides? would this be enough to code for 20 different amino acids? to determine a codon's length Nirenberg and colleague Phillip later devised a clever and quick assay the assay involved ribosomes and tRNAs charged with radioactive phenylalanine they knew that amino acyl tRNA molecules participated in protein synthesis and under certain conditions could be fount attached to ribosomes the assay also included poly-U RNA in addition to the reaction mixture the assay relied on the function of a membrane filter made of chemical cellulose nitrate this filter has special properties: if ribosomes are poured onto the filter, the ribosomes stick while the fluid goes through the filter a washing step does not dislodge the ribosomes although the ribosomes stick to the filter, neither the tRNA molecules nor the poly-U RNA molecule stick each type of molecule can flow through the porous filter during the washing step The ability of the ribosomes but not the other molecules to bind to the filter was the key to the assay For their experiments the investigators created mixtures that included ribosomes and tRNA molecules charge with radioactive phenylalanine One of the mixtures also received poly-U RNA After giving these mixtures time to incubate, the mixtures were poured on the filters and the washed After the washing step, they found that only one of the filters was radioactive Based on your knowledge of the sticking properties of the ribosomes poly-U RNA and phenylalanine tRNA to the filter One combination of molecules has adhered to the radioactive filter. Nirenberg then later reported their result in graph form. At a variety of temperatures Poly-U produced a positive reaction In the absence of poly-U there was virtually no reaction regardless of the temperature tested They used they assay to determine the number of nucleotides in the Poly-U required to bring together ribosomes and tRNA charged with phenylalanine They used RNA molecules with 12 uracil containing nucleotides and found reactivity They also found reactivity with six, five, four and three nucleotides However tow nucleotides produced almost no reactivity Assume that the minimal number of nucleotides that produce reactivity is also the number of nucleotides in a codon Nirenberg and colleagues wnt on to characterize a number of trinucleotides Only a subset of which are shown here They found that threonine charged tRNA bind to a Cu Alanine charged tRNA bind to GCU Prolene charged tRNAs bind to -CCA And serine charged tRNAs bind to UCG From such experiments we know the genetic code, which consists of 64 codons Each codon specifies an amino acid with the exception of three stop codons When a ribosome encounters a stop codon, no tRNA will bind to the codon and protein synthesis terminates Another codon called a atart codon is unique. Because it is always found at the beginning of a protein coding sequence in mRNA Using the genetic code table construct an RNA molecule thatwould code for this sries of amino acids in this protein drag blablabla Notice the more than 1 codon will work for most of these amino acids The genetic code is there for redundant. However the code is not ambiguous For example CCU specifies prolene but it does not specify other amino acid such as leucine or alanine.
Thank goodness!! The dull reading material in most texts, which is quite frankly, poorly developed, renders the learning process null and void! This video conveyed in almost 3 minutes what one hour of reading did not! Thank you so very, very much!
