Understand steps of replication - AP Biology

During DNA replication, helicase is responsible for unwinding the DNA helix and topoisomerase cleaves portions of the sugar-phosphate backbone to release tension in the strands. DNA polymerase then enters the replication bubble created by helicase. The bubble has two sides, each with a leading strand and a lagging strand. The leading strand at one side of the bubble is the lagging strand at the other, since DNA is anti-parallel. DNA polymerase can only synthesize in the 5'-to-3' direction; the strand oriented in the 3'-to-5' direction at the replication fork is known as the lagging strand since it must be replicated in pieces in the reverse direction. These pieces are known as Okazaki fragments.

DNA ligase is the protein responsible for fusing breaks in the sugar-phosphate backbone. It repairs the bonds broken by topoisomerase and creates phosphodiester bonds between Okazaki fragments.

An adenine-thymine sequence would be more likely to be found near the replication origin. Adenine and thymine pair with two hydrogen bonds, while cytosine and guanine pair with three hydrogen bonds. This makes adenine and thymine regions easier to break apart. Since helicase must break the hydrogen bonds in order to create the replication fork at the replication origin, it makes sense that this event would occur in a region where there were weaker forces between the two DNA strands.

Remember, guanine always pairs with cytosine and adenine always pairs with thymine.

The role of the topoisomerase enzyme is to unwind the DNA, allowing enzymes such as DNA helicase access to the nucleotide sequence. DNA helicase is responsible for breaking the hydrogen bonds responsible for holding double-stranded DNA together. This creates the replication fork and allows DNA polymerase access to the nitrogenous base sequence. The role of DNA polymerase is to place nucleotides once the DNA has been unwound, synthesizing the daughter DNA strands. DNA ligase binds nucleotide fragments together during synthesis. Telomerase is responsible for lengthening the telomeres at the ends of chromosomes.

DNA polymerase III is the enzyme that attaches to the RNA primer and adds DNA nucleotides complementary to the template strand in order to create the new, growing strand.

Primase is responsible for synthesizing the RNA primer. Helicase unwinds the DNA and creates the replication fork for other enzymes to bind. Ligase repairs breaks in the sugar-phosphate backbone and binds Okazaki fragments together. DNA polymerase I has a number of functions, including replacing the RNA primer with DNA nucleotides.

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.

Telomerase catalyzes the lengthening of chromosomes. Without telomerase, chromosomes would shorten with each round of replication, until the chromosome shortens, cutting into an important gene. At this time, the cell would not be able to carry out replication and/or make a gene product essential to its survival. If telomerase is overactive, cells' chromosomes would not naturally shorten over time, and they may continue to lengthen and divide uncontrollably (cancer).

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.

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.

This is really just a math equation. We need to double the amount of DNA each time it goes through a replication cycle.

Begin: 4

Cycle 1: 8

Cycle 2: 16

Cycle 3: 32

Cycle 4: 64

Cycle 5: 128

Cycle 6: 256

Cycle 7: 512

Cycle 8: 1024

Cycle 9: 2048

Cycle 10: 4096

After ten cycles, we would have 4096 copies from our original 4.

A shortcut calculation would be .

This is why PCR amplification is so effective.

The complementary strand will be going in the opposite direction (3'-5'). As a result, you will need to flip the direction in order for it to be complementary to the original strand. When pairing bases, remember that guanine (G) and cytosine (C) are paired with one another, and adenine (A) and thymine (T) are paired.

5'-ACTTGACT-3' Switch the direction.

3'-TCAGTTCA-5' Find the complement pairs.

5'-AGTCAAGT-3'

DNA polymerase always reads DNA strands in the 3'-to-5' direction, creating a complimentary 5'-to-3' strand. As a result, the parent strand oriented in the 3'-to-5' can be replicated seamlessly, but the strand that is unraveled in the 5'-to-3' direction will require multiple attachment points for DNA polymerase so that the whole strand can be replicated in the reverse direction.

These multiple segments of replication are called Okazaki fragments, and can only be found on the lagging strand, which is replicated more slowly.

Okazaki fragments are found on the lagging strand during replication. Because these fragments will not be attached together following strand synthesis, a protein is required to combine the fragments. DNA ligase will follow DNA polymerase on the lagging strand, and combine the fragments in order to create a complete strand.

DNA polymerase is responsible for recruiting and joining nucleotides in the 3'-to-5' direction, but cannot fuse Okazaki fragments on the lagging strand. Primase lays down an RNA primer to recruit DNA polymerase prior to replication. Helicase unwinds the DNA helix in order to expose the template strands. RNA polymerase is involved in transcription, and plays no active role in DNA replication.

Single-strand binding protein (SSB) binds the newly separated DNA strands to ensure that it does not reanneal during replication. This keeps the strands separate so that replication can occur.

All of the other answers describe the functions of other proteins. Primase synthesizes the RNA primers, which helps to recruit DNA polymerase. The structural basis for the replication of the leading and lagging strands ensures that replication follows the same rate on both strands.

DNA topoisomerases are the cell's solution to the "winding" problem. The double helical nature of DNA results in tension during the replication process that would interfere with the process. DNA topoisomerases cut the phosphate backbone to relieve this tension, and allow DNA to replicate properly.

The primary function of the ribosomes bound to the rough endoplasmic reticulum is to synthesize proteins for transport to the cell exterior or extracellular matrix. These ribosomes produce polypeptides that are packaged into vesicles by the Golgi apparatus and transported to the membrane. The vesicle then fuses with the membrane, either releasing proteins out of the cell or incorporating them into the cell membrane.

Nuclear ribosomes synthesize replication and transcription proteins into the nucleus, while cytoplasmic ribosomes produce cytoplasmic proteins.