AP Biology : Understand steps of replication

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #71 : Central Dogma

In DNA replication, which enzyme is responsible for separating the two strands of DNA apart?

Possible Answers:

Single-stranded binding proteins

Polymerase III

Helicase

Primase

Polymerase I

Correct answer:

Helicase

Explanation:

The correct answer is Helicase because it is involved with the separation of the two strands of DNA by breaking the hydrogen bonds between them. Primase lays down the RNA primer so that the polymerase enzymes can attach and start adding complementary base pairs. SSBs are single stranded binding proteins which anchor the separated strands and keep them from annealing to each other.

Example Question #942 : Ap Biology

A protein that ultimately functions in the plasma membrane of a cell is most likely to have been synthesized __________.

Possible Answers:

in the plasma membrane

by free cytoplasmic ribosomes

in the rough endoplasmic reticulum

by ribosomes on the nuclear envelope

in the mitochondria

Correct answer:

in the rough endoplasmic reticulum

Explanation:

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.

Example Question #9 : Transcription

Which of the following steps of DNA replication is inaccurate?

Possible Answers:

A replication bubble with two replication forks is formed

The DNA strands are separated by the enzyme helicase

The primase enzyme lays down RNA primers

Okazaki fragments are joined by DNA polymerase

The enzyme topoisomerase creates breaks in the DNA backbone

Correct answer:

Okazaki fragments are joined by DNA polymerase

Explanation:

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.

Example Question #72 : Central Dogma

What is heterochromatin?

Possible Answers:

DNA that can be transcribed

DNA that cannot be transcribed

DNA that is being translated

DNA that is not being translated

Correct answer:

DNA that cannot be transcribed

Explanation:

Heterochromatin is “dark” chromatin that represents DNA that is not active in transcription. The fact that it is “dark” implies that it is condensed and inaccessible by polymerases. Heterochromatin is created when DNA is tightly wound around histones. This tight winding prevents transcription proteins from interacting with the DNA. Heterochromatin is most common in the nucleus during mitosis, when no transcription is taking place. In contrast, euchromatin is capable of being transcribed and is most common during interphase, when most cellular growth and production occurs.

Translation occurs outside of the nucleus and uses mRNA as a template, not DNA.

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