AP Biology : Understanding Transcription Processes

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Transcription

If a DNA template (the strand actively being transcribed) begins with AAGGCTCGGAA, what will the nascent RNA template begin with?

Possible Answers:

AUGUGUTTCUG

CCTTAGATTCC

UUCCGAGCCUU

AAGGCTCGGAA

TTCCGAGCCTT

Correct answer:

UUCCGAGCCUU

Explanation:

Simply create a complementary strand of RNA with matching base pairs, but substitute uracil (U) for thymine (T), since RNA does not use thymine base pairs.

The answer choice starting with AUG might seem appropriate since it is the start codon, however, realize that the rest of the base pairs do not match up to the DNA template. Further, RNA templates do not begin right at the start codon; there are promotoer and enhancer regions of DNA that are transcribed well before the first exon is transcribed with its AUG start codon.

Example Question #943 : Ap Biology

Eukaryotic DNA is packed in a chromatin structure, making it hard for DNA to be transcribed. What proteins are associated with the packing of DNA? 

Possible Answers:

Actin

Myosin

Histones

Collagen

Polymerases

Correct answer:

Histones

Explanation:

Histones are associated with DNA packed in chromatin. Acetylation of these histones allows for DNA transcription. Other proteins mentioned play no part in DNA chromatin structure.

Polymerases are active during DNA repair and transcription. Collagen is a fibrous protein associated with the extracellular matrix. Actin and myosin are myofilaments active in muscle contraction.

Example Question #1 : Understanding Transcription Processes

In transcription, the promoter is __________.

Possible Answers:

an RNA sequence that signals the start of translation

a protein that increases the transcription levels for a given gene

a protein that guides RNA polymerase to the starting point

a sequence of DNA that designates the starting point for RNA polymerase

Correct answer:

a sequence of DNA that designates the starting point for RNA polymerase

Explanation:

Transcription requires a DNA sequence that signals the RNA polymerase where to begin transcribing a given gene. The promoter is the DNA sequence that allows the RNA polymerase to bind to the right spot on the DNA and begin transcription. This prevents transcription of partial proteins or protein fragments that would be non-functional and possibly harmful to the cell.

Example Question #946 : Ap Biology

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

Possible Answers:

by ribosomes on the nuclear envelope

in the rough endoplasmic reticulum

by free cytoplasmic ribosomes

in the plasma membrane

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 #1 : Transcription

Where does transcription take place in eukaryotic cells?

Possible Answers:

The rough endoplasmic reticulum

The nucleus

The cytoplasm

Ribosomes

Correct answer:

The nucleus

Explanation:

Transcription is the process of synthesizing RNA from a DNA template. In eukaryotic cells, chromosomal DNA is contained within the nucleus. Transcription requires access to this DNA, and therefore must occur in the nucleus. Resulting RNA molecules are then shuttled out of the nucleus to be used in other processes.

Example Question #2 : Understanding Transcription Processes

Where does transcription occur in a cell?

Possible Answers:

Nucleus

Nucleolus

Rough endoplasmic reticulum

Mitochondria

Lysosome

Correct answer:

Nucleus

Explanation:

The DNA gets transcribed into RNA inside the nucleus. This is where DNA is housed; DNA never leaves the nucleus (except during mitosis, during which the nuclear envelope will briefly disappear so that two cells can be formed). After DNA gets transcribed into RNA, the RNA is modified and eventually transported out of the nucleus as mRNA, which is now ready for translation.

Translation occurs on ribosomes, which can either be bound to the rough endoplasmic reticulum or free-floating in the cytoplasm. The nucleolus is a structure within the nucleus where ribosomal RNA (rRNA) is produced and ribosomal subunits are assembled. Mitochondria are essential for cellular respiration and ATP synthesis. Lysosomes are responsible for digesting wastes and defective proteins.

Example Question #4 : Understanding Transcription Processes

What is the result of a silent mutation?

Possible Answers:

There is no change to the peptide product

Inclusion of a different amino acid

A shift in the reading frame that results in a nearly completely different protein

Early termination of translation

Correct answer:

There is no change to the peptide product

Explanation:

As the name suggests, silent mutations are point mutations that actually have no visible effect on the protein. This is due to the degeneracy of the genetic code. Several codons actually insert the same amino acid. It is possible to mutate a codon so that it actually inserts the same amino acid. For example, if the codon UCU were mutated to UCG, it will still recruit the amino acid serine.

The other answers describe other types of mutations. Missense mutations are point mutations that result in the swapping of one amino acid for another. Nonsense mutations cause early termination. Frameshift mutations shift the reading frame of the codon sequence, severely altering the protein composition.

Example Question #4 : Transcription

Which of the following is directly produced during gene transcription by RNA polymerase?

Possible Answers:

mRNA

htRNA

miRNA

rRNA

Correct answer:

htRNA

Explanation:

When RNA polymerase binds to a template strand of DNA, it recruits complementary ribonucleotides to form a strand of RNA. This strand of RNA, however, is incomplete and must undergo post-transcriptional modification to become a mature mRNA product. The initial RNA transcript is known as heteronuclear RNA, or htRNA.

Introns are removed for the htRNA and a 5'cap and poly-A tail are added to convert it to mRNA.

Example Question #6 : Understanding Transcription Processes

Which of the following steps of DNA replication is inaccurate?

Possible Answers:

A replication bubble with two replication forks is formed

The primase enzyme lays down RNA primers

Okazaki fragments are joined by DNA polymerase

The enzyme topoisomerase creates breaks in the DNA backbone

The DNA strands are separated by the enzyme helicase

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 #3 : Understanding Transcription Processes

What is heterochromatin?

Possible Answers:

DNA that can be transcribed

DNA that is being translated

DNA that is not being translated

DNA that cannot be transcribed

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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