MCAT Biology : Biology
Study concepts, example questions & explanations for MCAT Biology
All MCAT Biology Resources
Example Questions
Example Question #1783 : Mcat Biological Sciences
Which of the following will be the complimentary pairing of this coded gene during transcription?
3’ GCTAGC 5’
5’ CGATCG 3’
5’ CGAUCG 3’
3’ CGATCG 5”
3’ CGAUCG 5’
5’ CGAUCG 3’
During transcription, a complimentary RNA strand is constructed from a DNA template. It is important to realize that in RNA, the nucleotide base uracil is used instead of thymine as a complementary pair with adenine. The complementary strand is constructed 5’ to 3’ , but is lined up anti-parallel to the original strand.
Example Question #12 : Dna And Rna Sequencing
Choose the transcript created if RNA polymerase transcribes the following template strand.
5'-ACTTGCAGGCC-3'
5'-GGCCTGCAAGT-3'
5'-UGAACGUCCGG-3'
5'-TGAACGTCCGG-3'
5'-GGCCUGCAAGU-3'
5'-GGCCUGCAAGU-3'
When transcribing from a template strand, here are a few things to remember:
1. RNA polymerase reads the strand in the 3' to 5' direction.
2. In the RNA transcript, thymine is replaced with uracil.
In order to double check, make sure that the two strands are complementary when antiparallel to one another.
Template: 5'-ACTTGCAGGCC-3'
Transcript: 3'-UGAACGUCCGG-5'
Example Question #1401 : Biology
What is the mRNA transcript of the given DNA sequence?
3' ACCTGTTAC 5'
3' TGGACAATG 5'
5' UGGACAAUG 3'
5' TGGACAATG 3'
3' UGGACAAUG 5'
5' UGGACAAUG 3'
mRNA is transcribed antiparallel to the DNA template, meaning that the 5' end of the mRNA sequence should align with the 3' end of the DNA sequence. Pairing rules dictate that cytosine will pair with guanine and adenine with uracil, rather than thymine. Using these conventions, we can decipher the mRNA sequence that correlates to the given DNA template.
DNA: 3' ACCTGTTAC 5'
RNA: 5' UGGACAAUG 3'
Example Question #13 : Dna And Rna Sequencing
If a gene has a sequence of 5'-AGCTGCCTT-3', what would be the complementary mRNA sequence that leaves the nucleus to be translated?
3'-UCGACGGAA-5'
5'-UCGACGGAA-3'
5'-UCGUCGGAA-3'
5'-AGCTGCCTT-3'
3'-TCGACGGAA-5'
3'-UCGACGGAA-5'
The correct answer is 3'-UCGACGGAA-5'.
In order to arrive at this answer, it is important to note that we are starting with DNA and finding the complementary mRNA. We must remember that there is no thymine in RNA; instead of thymine, RNA has uracil. The last thing to remember is that the mRNA strand will be anti-parallel, meaning that the 5' end of the DNA sequence must match up with the 3' end of the RNA sequence. Cytosine and guanine will form pairs. Adenine bases in DNA will pair to uracil bases in RNA, and thymine bases in DNA will bind to adenine in RNA.
DNA: 5'-AGCTGCCTT-3'
RNA: 3'-UCGACGGAA-5'
Example Question #12 : Dna And Rna Sequencing
The base sequence of one strand of tRNA is v. What is the corresponding sequence of DNA?
5'-GATCGATCGATA-3'
5'-AGTCGATCTAGC-3'
5'-TCAGCTAGATCG-3'
5'-AGUCGAUCUAGC-3'
3'-AGUCGAUCUAGC-5'
5'-AGTCGATCTAGC-3'
Both tRNA and DNA are complementary to mRNA, meaning that they will have the exact same sequence in the exact same direction with only one distinction: tRNA will use uracil where DNA uses thymine.
The given tRNA strand is 5'-AGUCGAUCUAGC-3'.
The corresponding DNA strand will be 5'-AGTCGATCTAGC-3'.
Example Question #11 : Dna And Rna Sequencing
You are a student researcher cloning a gene that is around 1500 bases long into a vector for recombinant expression. Starting with cDNA, you succesfully clone and transfect bacterial cells to propogate the plasmid. You sequence the plasmid to check and make sure that the target gene has been succesfully incoorporated into the vector. After checking the sequence, you notice that there is a single nucleotide that has been switched from an A to a G several hundred basepairs after the start codon. You express the protein anyway. After subjecting the protein to SDS-page, you learn that it exists at its expected length, but in a functional asssay, the protein seems to have lost its function. What type of mutation is this called?
Frameshift mutation
Nonsense mutation
Missense mutation
Silent mutation
Missense mutation
Nonsense mutations result in a premature stop codon, which would have resulted in a shorter peptide. A silent mutation means that there is a change in the nucleotide sequence, but not in the amino acid sequence. Thus, this is not likely the case seeing as there is a loss of function in the cloned and recombinantly expressed protein. We have no reason to believe that a frameshift mutation occurred. Rather, what the student observed was a point mutation that resulted in a missense reading of the gene.
Example Question #1401 : Biology
Human chromosomes are divided into two arms, a long q arm and a short p arm. A karyotype is the organization of a human cell’s total genetic complement. A typical karyotype is generated by ordering chromosome 1 to chromosome 23 in order of decreasing size.
When viewing a karyotype, it can often become apparent that changes in chromosome number, arrangement, or structure are present. Among the most common genetic changes are Robertsonian translocations, involving transposition of chromosomal material between long arms of certain chromosomes to form one derivative chromosome. Chromosomes 14 and 21, for example, often undergo a Robertsonian translocation, as below.
A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:
Though an individual with aberrations such as a Robertsonian translocation may be phenotypically normal, they can generate gametes through meiosis that have atypical organizations of chromosomes, resulting in recurrent fetal abnormalities or miscarriages.
In the DNA component of a chromosome, changes such as methylation result in alterations in how DNA is processed without changing its sequence. This is known as __________.
transcription
epigenetic modification
alternative splicing
translation
RNA interference
epigenetic modification
Epigenetic modification is defined as the modifications that lead to alternative gene expression without changes in the sequence of the nucleic acid itself. The other choices are incorrect, though "alternative splicing" is a tempting option. This also leads to changes in gene production, but derives from splicing out of different regions of the DNA sequence itself and rejoining it to other sections. Epigenetics does not rely on any sequence changes at all.
Example Question #361 : Cell Biology, Molecular Biology, And Genetics
The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.
The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.
Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.
The passage indicates that genomic imprinting can be the result of silencing genes by adding methyl groups to DNA sequences. Which of the following is true of methyl groups?
They directly inhibit DNA transcription
They directly inhibit ribosome assembly
They directly inhibit protein translation
They directly inhibit nuclear pore transport
They reduce protein stability
They directly inhibit DNA transcription
The best answer here is the inhibition of transcription. The question indicates that methyl groups are added directly to the DNA structure. The only step to get from DNA to protein that makes direct use of the DNA structure is transcription.
If methyl groups were added to mRNA, then inhibition of protein translation would be a better answer.
Example Question #3 : Regulation Mechanisms And Epigenetics
Human chromosomes are divided into two arms, a long q arm and a short p arm. A karyotype is the organization of a human cell’s total genetic complement. A typical karyotype is generated by ordering chromosome 1 to chromosome 23 in order of decreasing size.
When viewing a karyotype, it can often become apparent that changes in chromosome number, arrangement, or structure are present. Among the most common genetic changes are Robertsonian translocations, involving transposition of chromosomal material between long arms of certain chromosomes to form one derivative chromosome. Chromosomes 14 and 21, for example, often undergo a Robertsonian translocation, as below.
A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:
Though an individual with aberrations such as a Robertsonian translocation may be phenotypically normal, they can generate gametes through meiosis that have atypical organizations of chromosomes, resulting in recurrent fetal abnormalities or miscarriages.
Histones are important components of chromosomes that help to form the scaffolding around which DNA wraps while organizing. Considering the structure of DNA, what is most likely true of histones?
Histones are fixed structures, and associated with DNA in every phase of the cell cycle
They are basic proteins and undergo acetylation to vary DNA binding
They are neutral proteins and interact with DNA via van der Walls forces
They are acidic proteins because DNA is generally negatively charged
Histones interact with DNA mainly through covalent interactions
They are basic proteins and undergo acetylation to vary DNA binding
Histones are basic proteins; DNA is acidic, allowing them to interact via dipole intermolecular bonds. One must understand that the transition from heterochromatin to euchromatin involves the tightening of DNA into chromosomes, a process driven in part by modification of the histones via acetylation. While you can answer this question by just deducing the choice from the acidic nature of DNA, it would not make sense for histones to be inert, as DNA morphology must change so dramatically through the cell cycle. Additionally, we would not expect covalent interactions due to this need to vary structure. Covalent bonds are more permanent.
Example Question #364 : Cell Biology, Molecular Biology, And Genetics
The cell is the most basic functional unit of life. Everything that we consider to be living is made up of cells, and while there are different kinds of cells, they all have some essential features that link them all together under the category of "life." One of the most important parts of a cell is the membrane that surrounds it, seperating it from the rest of the environment.
While organisms from the three main domains live in incredibly different environments, they all possess similar cell membranes. This phospholipid bilayer protects the cell, giving it a way to allow certain things in while keeping other things out. Though organisms from different domains have different kinds of fatty linkages in their membranes, they all serve this essential purpose.
Membranes contain all kinds of essential proteins and signal molecules that allow the inside of the cell to respond to the outside of the cell. In a multicellular eukaryote, this ability can be used to allow cells to communicate. In a bacterial colony, an extracellular signal could be used to signal other bacteria. Signals cascade through a series of molecular pathways that go from the outside of the cell all the way to the nucleus and back out again, giving the cell control on a genetic level. This allows cellular responses to be quick and effective, and it also allows the cell to control how long it stays in that state.
What is an important aspect of the control a cell has over its molecular responses?
DNA produced from RNA degrades quickly.
RNA produced from DNA degrades quickly.
DNA produced from RNA is very stable and lasts a long time.
Both RNA and DNA can be created or degraded at will.
RNA produced from DNA is very stable and lasts a long time.
RNA produced from DNA degrades quickly.
Whenever a cell responds to a signal, it transcribes DNA into RNA which can then be translated into protein. That protein is the molecule that will affect the cell somehow and cause it to react. RNA is very unstable on its own, and will only last a short amount of time in the cell. Therefore, when a cell wants to stop having that particular reaction, it can simply stop making RNA, and the signal will be stopped quickly.
Certified Tutor
All MCAT Biology Resources
