Central dogma - AP Biology

Abortive initiation is the process by which RNA polymerase starts short cycles of RNA synthesis. During abortive initiation, RNA polymerase releases short RNA strands before the initiation complex leaves the promoter sequence. Abortive initiation is a common process in both eukaryotes and prokaryotes.

An insertion mutation is a mutation due to an additional nucleotide base pair being added to a DNA sequence. In this case, a cytosine nucleotide has been inserted.

A substitution mutation occurs when a base pair is exchanged for a different base pair. In this case, a thymine nucleotide has been substituted for an adenine nucleotide.

An insertion mutation that occurs where an amount of nucleotides that is not a multiple of three is added to a DNA sequence will shift the reading frame.

A deletion mutation occurs when a base pair is removed from the DNA sequence. In this case, a cytosine nucleotide has been deleted.

Okazaki fragments are only produced, and subsequently joined together, in the lagging strand to allow for replication in the opposite direction as replication fork movement. The leading strand, however, allows for continual replication.

All other choices reflect aspects of DNA replication for both the leading and lagging strands.

Helicase, gyrase, and DNA polymerase are all used in the process of DNA replication, which takes place in the nucleus. Helicase is responsible for "unzipping" DNA, separating its two strands and unwinding the double-helix. Gyrase is responsible for relaxing the DNA strands and relieving tensions during unwinding. DNA polymerase synthesizes the the new DNA strands by recruiting nitrogenous bases.

Frameshift mutations involve the insertion or deletion of a nucleotide in a DNA sequence, changing the reading frame of the entire nucleotide sequence after the mutation. As a result, every subsequent codon is also affected, creating a change in the organism's phenotype.

Oftentimes, this results in a premature stop codon, which causes the protein product to be shorter than an unaffected polypeptide.

Point mutations replace a single nucleotide for a different one. This can change a certain codon to code for a different amino acid (missense), the same amino acid (silent), or lead to a stop codon (nonsense). Nonsense mutations are the most severe type of point mutation, as they will cause early termination of the protein.

The polarity of nucleic acids, and therefore DNA molecules, describes the chemical orientation of the macromolecule. Each nucleotide has a end, which refers to the fifth carbon of the deoxyribose sugar ring that has a phosphate group attached to it, and a end, which is the third carbon in the sugar ring that has a hydroxyl group attached to it. DNA polymerase can only add nucleotides to the hydroxyl group of a nucleotide. This results in a new DNA strand elongating in the to direction.

The origin of replication is the sequence of DNA where replication is initiated. The origin of replication often has a high content of adenine and thymine nucleotides because they are only bound together by two hydrogen bonds, making the helix easier to open and unwind. There are multiple origins of replication on each chromosome in eukaryotes, while there is only one origin of replication in prokaryotes. The origin of replication binds to initiator proteins that make up the pre-replication complex, which initiates replication.

In the initiation stage of DNA replication, a number of enzymes are involved. These include the initiator proteins in the pre-replication complex, DNA helicase, single stranded binding proteins, and topoisomerase. Topoisomerase is an enzyme that helps relieve winding and unwinding tension in DNA that arise from the helical structure of the DNA molecule. The DNA ahead of the replication fork often becomes tangled and/or supercoiled. Topoisomerase cuts the DNA to relieve the stress and allow the DNA to relax by unwinding a few times.Later during the replication process, the DNA rewinds and these breaks are resealed.

In the process of DNA replication, DNA polymerase can only add nucleotides to an already existing DNA strand. To overcome this difficulty in synthesizing new DNA strands, the cell utilizes RNA primers. RNA primers are short strands of RNA synthesized by the enzyme primase. Primase is a type of RNA polymerase that does not need a free hydroxyl as a substrate, like DNA polymerase. Thus, it can lay down RNA nucleotides using the DNA parent strands as templates to provide DNA polymerase with the substrate it needs to begin elongation. RNA primers are also used in lagging strand synthesis, since DNA polymerase can only add nucleotides to the end of a nucleotide. Thus all nucleic acid polymerization occurs in the direction. DNA polymerase I removes the RNA primers, replacing them with DNA, then DNA ligase seals the gaps between DNA on the lagging strand.

The lagging strand is the strand of parent DNA that runs in the opposite direction that the replication fork opens. Because DNA polymerase adds nucleotides in direction, RNA primers are added along the length of the newly single parent DNA strand to provide a hydroxyl group onto which DNA polymerase adds nucleotides. DNA polymerase adds nucleotides to the RNA primer until encountering another primer. These segments of newly synthesized DNA are called Okazaki fragments. DNA polymerase I removes RNA primers and replaces them with DNA. Ligase seals the gaps between DNA, forming a continuous strand.

There are three models of DNA replication: the dispersive model, the conservative model, and the semiconservative model. The dispersive model postulates that parental DNA cut into segments, each of which acts as a template for newly synthesized fragments. Together, DNA helices reassemble, containing daughter and parental DNA segments mixed together. The conservative model hypothesizes that the parental DNA double helix acts as a template for the daughter DNA molecule. This model results in one parental double helix and one daughter DNA molecule. The model that describes DNA replication is the semiconservative model. In this model, parental DNA strands separate and act as templates for daughter strands, resulting in two DNA molecules each with one parent and one daughter DNA strand. Thus, each newly synthesized DNA molecule has one parent strand bound to one daughter strand.

The only false statement is that DNA replication of the lagging strand is continuous. The leading strand features continuous replication while the lagging stand is initially made up of discontinuous Okazaki fragments, which are sealed later by ligase.

There are four major types of mutations: deletions, duplications, inversions, and translocations. Deletions occurs when a chromosomal fragment is removed and not replaced. Duplications occur when a chromosomal fragment is aberrantly copied. Inversions occur when a chromosomal fragment reattaches to its original chromosome in reverse orientation. Translocations occur when a chromosomal fragment reattaches to a different part of the same chromosome, or a different chromosome altogether.

Acetylation of histones helps increase genetic expression. Acetylation is the process of adding an acetyl group. Methylation is used by the cell to differentiate between the two DNA strands of a newly synthesized DNA. Essentially, the highly methylated strand is the original or parent strand. This is integral in proofreading the new DNA.

Helicase is the enzyme responsible for breaking the helix and unwinding the DNA into two separate strands. This allows the polymerase enzyme to attach and start adding base pairs for replication. Primase is responsible for setting and synthesizing RNA primers for polymerase attachment.