Answer:
A protoplanetary disk is round rotating object which is made of dense gas. This surrounds the newly formed star, Herbig star and T tauri star.
It is also considered as a accreation because the remnants like gases and other materials fall from them on to the star.The extremely illuminating protoplanetary disk are known as the proplyds.
Whereas the nebula can be defined as the densely packed hydrogen and other gases that collapse under the force of gravity. The Nebula starts becoming smaller and smaller because the particles of gases collapse and get closer to each other.
Upon force of gravity their friction and heat increases which slowly collapse the star.
Before DNA can be replicated, the double stranded molecule must be “unzipped” into two single strands. DNA has four bases called adenine (A), thymine (T), cytosine (C) and guanine (G) that form pairs between the two strands. Adenine only pairs with thymine and cytosine only binds with guanine. In order to unwind DNA, these interactions between base pairs must be broken. This is performed by an enzyme known as DNA helicase. DNA helicase disrupts the hydrogen bonding between base pairs to separate the strands into a Y shape known as the replication fork. This area will be the template for replication to begin.
Step 2: Primer Binding
The leading strand is the simplest to replicate. Once the DNA strands have been separated, a short piece of RNA called a primer binds to the 3' end of the strand. The primer always binds as the starting point for replication. Primers are generated by the enzyme DNA primase.
Step 3: Elongation
Enzymes known as DNA polymerases are responsible creating the new strand by a process called elongation. There are five different known types of DNA polymerases in bacteria and human cells. In bacteria such as E. coli, polymerase III is the main replication enzyme, while polymerase I, II, IV and V are responsible for error checking and repair. DNA polymerase III binds to the strand at the site of the primer and begins adding new base pairs complementary to the strand during replication. In eukaryotic cells, polymerases alpha, delta, and epsilon are the primary polymerases involved in DNA replication. Because replication proceeds in the 5' to 3' direction on the leading strand, the newly formed strand is continuous.
Step 4: Termination
Once both the continuous and discontinuous strands are formed, an enzyme called exonuclease removes all RNA primers from the original strands. These primers are then replaced with appropriate bases. Another exonuclease “proofreads” the newly formed DNA to check, remove and replace any errors. Another enzyme called DNA ligase joins Okazaki fragments together forming a single unified strand. The ends of the linear DNA present a problem as DNA polymerase can only add nucleotides in the 5′ to 3′ direction. The ends of the parent strands consist of repeated DNA sequences called telomeres. Telomeres act as protective caps at the end of chromosomes to prevent nearby chromosomes from fusing.
So if that here are the functions of enzymes used:
DNA helicase - unwinds and separates double stranded DNA as it moves along the DNA. It forms the replication fork by breaking hydrogen bonds between nucleotide pairs in DNA.
DNA primase - a type of RNA polymerase that generates RNA primers. Primers are short RNA molecules that act as templates for the starting point of DNA replication.
DNA polymerases - synthesize new DNA molecules by adding nucleotides to leading and lagging DNA strands.
Topoisomerase or DNA Gyrase - unwinds and rewinds DNA strands to prevent the DNA from becoming tangled or supercoiled.
Exonucleases - group of enzymes that remove nucleotide bases from the end of a DNA chain.
DNA ligase - joins DNA fragments together by forming phosphodiester bonds between nucleotides.
Have a nice day
Nancy the neuron has just fired an action potential. Now she cannot fire another action potential until she resets her electrical potential. She is in a refractory period. The refractory period starts after the hyper-polarization of the neuron. In this phase the neurons is inhibited of generating further action potentials, and the recovery to resting potential of the neuron membrane begins with the potassium ions slowly leaking into the cell.
