DNA REPLICATION:
Before the lagging-strand DNA exits the replication factory, its RNA primers must be removed and the Okazaki fragments must be joined together to create a continuous DNA strand. The first step is the removal of the RNA primer. RNAse H, which recognizes RNA-DNA hybrid helices, degrades the RNA by hydrolyzing its phosphodiester bonds. Next, the sequence gap created by RNAse H is then filled in by DNA polymerase which extends the 3' end of the neighboring Okazaki fragment. Finally, the Okazaki fragments are joined together by DNA ligase that hooks together the 3' end of one fragment to the 5' phosphate group of the neighboring fragment in an ATP- or NAD+-dependent reaction.
REPLICATION IN ACTION
We are now ready to review the steps of DNA replication.
The process begins when the helicase enzyme unwinds the double helix to expose two single DNA strands and create two replication forks. DNA replication takes place simultaneously at each fork. The mechanism of replication is identical at each fork. Remember that the proteins involved in replication are clustered together and anchored in the cell. Thus, the replication proteins do not travel down the length of the DNA. Instead, the DNA helix is fed through a stationary replication factory like film is fed through a projector.
Single-strand binding proteins, or SSBs, coat the single DNA strands to prevent them from snapping back together. SSBs are easily displaced by DNA polymerase.
The primase enzyme uses the original DNA sequence as a template to synthesize a short RNA primer. Primers are necessary because DNA polymerase can only extend a nucleotide chain, not start one.
DNA polymerase begins to synthesize a new DNA strand by extending an RNA primer in the 5' to 3' direction. Each parental DNA strand is copied by one DNA polymerase. Remember, both template strands move through the replication factory in the same direction, and DNA polymerase can only synthesize DNA from the 5' end to the 3' end. Due to these two factors, one of the DNA strands must be made discontinuously in short pieces which are later joined together.
As replication proceeds, RNAse H recognizes RNA primers bound to the DNA template and removes the primers by hydrolyzing the RNA.
DNA polymerase can then fill in the gap left by RNase H.
The DNA replication process is completed when the ligase enzyme joins the short DNA pieces together into one continuous strand.
| 5593514980 | DNA Replication | the process of making identical copies of DNA before cell division | 0 | |
| 5593514981 | Semi Conservative | Each half of an original DNA molecule serves as a templete for a new strand, and the two new DNA molecules each have one old and one new strand. | 1 | |
| 5593514982 | helicase | An enzyme that unwinds the double helix of DNA and separates the DNA strands in preparation for DNA replication. | 2 | |
| 5593514983 | DNA Polymerase | Enzyme involved in DNA replication that joins individual nucleotides to produce a DNA molecule | 3 | |
| 5593514984 | DNA ligase | enzyme which connects the individual okazaki fragments on the lagging strand by forming covalent bonds | 4 | |
| 5593514985 | replication fork | The Y-shaped region that results when the two strands separate | 5 | |
| 5593514986 | antiparallel | Having two strands that run parallel to each other, but one is upside down. | 6 | |
| 5593514987 | pairs with adenine | thymine | 7 | |
| 5593514988 | pairs with cytosine | guanine | 8 | |
| 5593514989 | pairs with thymine | adenine | 9 | |
| 5593514990 | pairs with guanine | cytosine | 10 | |
| 5593514991 | double helix | Shape of DNA | 11 | |
| 5593514992 | Watson and Crick | Figured out structure of DNA was a double helix | 12 | |
| 5593514993 | Avery | Proposed that DNA could transmit disease, not protein & that hereditary material was probably made of DNA, not protein | 13 | |
| 5593514994 | Chargaff | Determined that the amounts of A in a DNA molecule are equal to the amounts of T and the amount of G is equal to the amount of C | 14 | |
| 5593514995 | Rosalind Franklin | Woman who generated x-ray images of DNA, she povided Watson and Crick with key data about DNA | 15 | |
| 5593514996 | Hershey and Chase | Used radioactive material to label DNA and protein; infected bacteria passed on DNA; helped prove that DNA is genetic material not proteins | 16 | |
| 5593514997 | type of bond between bases | hydrogen | 17 | |
| 5593514998 | nucleotide | A building block of DNA, consisting of a five-carbon sugar covalently bonded to a nitrogenous base and a phosphate group. | 18 | |
| 5593514999 | nucleotide | nucleotide |  | 19 |
| 5593515000 | double helix | DNA's structure is a double helix |  | 20 |
| 5593515001 | helicase | helicase unzips or unwinds the DNA strands |  | 21 |
| 5593515002 | antiparallel | one strand runs 5' to 3' while the other is upside down |  | 22 |
| 5593515003 | DNA polymerase | structure D is polymerase; it is adding nucleotides to the new DNA strand |  | 23 |
| 5593515004 | semiconservative | the new DNA strand contain one strand that is old and one that is new |  | 24 |
| 5593515005 | DNA in nucleus | DNA is located in the nucleus (of eukaryotic cells like plants and animals) |  | 25 |
