The leading strand only needs this RNA primer once at the very beginning. So maybe the real scumbag here is the DNA polymerase. Since the lagging strand runs in the opposite direction, it has to be copied as a series of segments. Here that awesome little enzyme RNA primase does its thing again, laying down an occasional short little RNA primer that gives the DNA polymerase a starting point to then work backwards along the strand. This is done in a ton of individual segments, each 1, to 2, base pairs long and each starting with an RNA primer, called Okazaki fragments after the couple of married scientists who discovered these step of the process in the s.
These allow the strands to be synthesised in short bursts. And that is why I say the lagging strand is such a scumbag! DNA replication gets it wrong about one in every 10 billion nucleotides. It turns out that DNA polymerase can also proofread, in a sense, removing nucleotides from the end of a strand when they discover a mismatched base because the last thing we want is an A when it would have been a G!
And this, my friends, is why scientists are not exaggerating when they call DNA the most celebrated molecule of all time. So you might as well look this episode over a couple of times and appreciate it for yourself.
Thank you to all the people here at Crash Course who helped make this episode awesome. You can click on any of these things to go back to that section of the video. If you have any questions, please, of course, ask them in the comments or on Facebook or Twitter. Home browse videos DNA structure and replication. DNA structure and replication Hank from Crash Course introduces that wondrous molecule deoxyribonucleic acid—also known as DNA—and explains how it replicates itself in our cells.
Mind blown yet? Hey, you wanna make one? These chromosomes are packed together tightly with proteins in the nucleus of the cells. DNA gets the first part of its name from our first ingredient, the sugar molecule, which is called deoxyribose. But all the really significant stuff, the genetic coding that makes you YOU, is found among the four nitrogenous bases: adenine A , thymine T , cytosine C and guanine G.
Now, it is time to make ourselves one of these famous double helices. Okay, here goes. The terminal DNA segment cannot be replicated. The DNA strand gets shorter and shorter as the number of cell division rounds increases. To protect themselves against the rapid shortening of the DNA, eukaryotic chromosomes possess sequence repeats telomeres at their extremities, which do not code for proteins. Since the telomeres do not contain any important information, the key parts of the DNA are protected.
The telomeres get shorter each time a cell divides. The length of these so-called telomere caps defines the number of possible divisions and hence the lifespan of a cell. Some cell types for example maturing sex cells or certain tumour cells contain the enzyme telomerase, which prevents telomere shortening and thus protects the cell from cell ageing and programmed cell death. A cell must at all costs prevent errors from occurring during the replication of DNA. The strict order of the copying process is therefore essential.
In eukaryotic cells, the DNA is kept as clearly arranged as possible: genome regions that are not undergoing replication are densely packed in chromosomes. Scientists also assume that the chromosomes in eukaryotic cells are spanned over a cytoskeleton consisting of protein tubes and wires.
The replication enzymes are bound to this so-called nuclear matrix and motor proteins pull the genome past them. It appears that bacterial cells have similar mechanisms. Many bacteria divide once every thirty minutes, others replicate even faster. Eukaryotic cells only replicate their genome when new cells have to be created. This happens as a result of external signals, for example tissue loss or inflammation.
The life cycle of eukaryotic cells underlies an accurately defined sequence of activities that can be divided into different phases: 1 st resting gap phase G1 , DNA synthesis phase S , 2 nd gap phase G2 and mitosis M. The cells duplicate their genome in the S phase. During the gap between DNA synthesis S and mitosis M , the cells check whether the DNA has been replicated and errors have been repaired in order to determine whether they are ready to proceed to mitosis and divide. This sequence of activities is controlled by enzymes of the group of cyclin-dependent kinases CDKs.
If errors occur during the control of the cell cycle, the cells might divide more quickly and more frequently as is the case with many cancer cells. Viruses also reproduce, but they cannot do so on their own. They use the replication apparatus of the host cells, and have additionally developed a number of special characteristics. Scientists differentiate viruses according to the genome type — there are DNA and RNA viruses: viruses may have single-stranded or double-stranded linear RNA, single-stranded or double-stranded linear DNA, single-stranded or double-stranded circular DNA and other variations.
Some viruses contain some of the enzymes required for their replication, for example the influenza virus, whose envelope not only contains an RNA genome but also an RNA polymerase. How is DNA replicated? What triggers replication? Figure 1: Helicase yellow unwinds the double helix. The initiation of DNA replication occurs in two steps. First, a so-called initiator protein unwinds a short stretch of the DNA double helix. Then, a protein known as helicase attaches to and breaks apart the hydrogen bonds between the bases on the DNA strands, thereby pulling apart the two strands.
As the helicase moves along the DNA molecule, it continues breaking these hydrogen bonds and separating the two polynucleotide chains Figure 1. How are DNA strands replicated? Figure 3: Beginning at the primer sequence, DNA polymerase shown in blue attaches to the original DNA strand and begins assembling a new, complementary strand.
Figure 4: Each nucleotide has an affinity for its partner. A pairs with T, and C pairs with G. The color of the rectangle represents the chemical identity of the nitrogenous base. A grey horizontal cylinder is attached to one end of the rectangle in each nucleotide and represents a sugar molecule. The nucleotides are arranged in two rows and the nitrogenous bases point toward each other.
A set of four nucleotides are in both the upper and lower rows. From left to right, the nucleotides in the top row are adenine green , cytosine orange , thymine red , and guanine blue. From left to right, the complementary nucleotides in the bottom row are: thymine red , guanine blue , adenine green , and cytosine orange.
Figure 5: A new DNA strand is synthesized. This strand contains nucleotides that are complementary to those in the template sequence. How long does replication take? More on replication. How does DNA polymerase work?
DNA Replication. Lawrence C. Brody, Ph.
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