Genetics, DNA, and Molecular Biology - GRE

Epigenetics are changes to the genome that result in phenotypic variation that have nothing to do with changes in the actual DNA sequence. All listed answers occur independently of DNA sequence, except for "different exon sequences," which is the actual sequence of an exon. This referces to alternative splicing, an is not related to the modification of DNA or histones.

The correct answer is an increase in gene expression. Histone acetylation removes positive charges on the histones, reducing the affinity of DNA for histones. Remember that DNA is negatively charged due to the phosphate groups on its backbone. DNA and histones are attracted to each other because histones are positively charged due to being rich in basic amino acid residues. Acetylation relaxes the tightly bound DNA allowing transcription factors to bind promoter regions. DNA deacetylation and methylation supress gene transcription by making DNA and histones associate more tightly together, decreasing the ability of transcription factors and/or RNA polymerase to bind the DNA. Histone modifications such as acetylation, deacetylation, and methylation do not directly affect the amount of DNA. If a histone is acetylated on a part of the DNA which codes for the genes for ribosome production, then an increase in ribosomal production and assembly could occur, but genes coding for ribosomes are greatly outnumbered by other genes, and thus, this is not the usual result of acetylating histones.

Methylation and acetylation of histones occurs on lysine residues, thereby decreasing or increasing gene expression, respectively. Methylation increases the affinity for histones and DNA, where acetylation decreases the affinity for histones and DNA. Gene expression is in part controlled by modification of histone proteins, rather non-histone chromosomal proteins.

The promoter is a specific segment of DNA that signals the starting point of transcription. RNA polymerase attaches to the promoter and proceeds to create the mRNA primary transcript.

DNA polymerase binds to the RNA primer to begin DNA replication. Ribosomes bind to the 5' cap on eukaryotic mRNA.

The lac operon is set up in a way so that the lac repressor is able to be transcribed, regardless of glucose and lactose levels. The lac repressor will then attach to the operator, which inhibits transcription. If lactose is present, it will bind to the lac repressor, and make it detach from the operator.

This process allows the operon to be transcribed in the event that glucose is absent. If glucose is absent, but lactose is not present, then the repressor will remain in place and transcription will not take place.

The important thing to remember about the lac operon is that it is transcribed when glucose is absent from the cell, but lactose is present and can be utilized. As a result, the operon's transcription would be high if there are both high levels of lactose available, and very little amounts of glucose.

DNA is stored in loosely wound euchromatin before mitosis. During mitosis, the DNA condenses into chromosomes, which are made of heterochromatin. It becomes more dense during prophase, and stays that way until the end of mitosis. Euchromatin is more lightly packed than heterochromatin.

Mitosis follows the following sequence: prophase, metaphase, anaphase, telophase, cytokinesis. Interphase refers to the time period between mitotic divisions. During interphase, most DNA is euchromatin, but some regions remain as heterochromatin to prevent unwanted transcription; thus DNA exists as both types of chromatin during interphase, but only as heterochromatin during mitosis. Matching euchromatin to telophase is the answer, as this is a false statement.

Nucleosomes are the basic, repeating units of eukaryotic chromatin. They consist of chromosomal DNA wrapped around special DNA-binding proteins called histones. There are many examples of non-chromosomal DNA, such as plasmids, but they do not contain nucleosomes. Nuclear import is controlled by importin proteins.

Histones are proteins that bind and package DNA. The strand of DNA is wound around histone proteins, condensing it to fit in the nucleus and acting to moderate gene expression. Chromatin is the term given to the complex of DNA associated with histones. A nucleosome is the smallest repeating unit of chromatin, formed from eight histone proteins and two loops of coiled DNA. Telomerase is an enzyme responsible for maintaining the integrity of the telomeres.

Euchromatin is the name given to chromatin that appears lighter when viewed under a microscope. It is actually relatively decondensed chromatin that is available for active transcription. Because it is decondensed it is more accessible to RNA polymerase and, therefore, easier to transcribe. In contrast, heterochromatin is tightly wound, dense DNA that is inaccessible by RNA polymerase and is considered inactive.

Translation is the process of synthesizing proteins from mRNA transcripts and does not directly involve DNA or chromatin.

Chromatin is not present in all eukaryotic and prokaryotic DNA; most prokaryotic DNA is circular and does not require the complex folding of eukaryotic chromatin. Chromatin exists in more compacted states than 10nm. In particular, the 30nm version is commonly recognized as heterochromatin (DNA that is not being actively transcribed). Packaging can also be more condensed during certain stages of mitosis. Nucleosomes are the smallest units of chromatin and are strands of DNA wrapped in proteins known as histones.

Patterns of methylation and acetylation of these histones have been shown to repress and activate gene expression, respectively, and are important factors in regulating gene expression and epigenetics.

Human beings have somatic (body) cells that are diploid. This means that each cell has two copies of each of the 23 chromosomes: one from the father and one from the mother. As a result, the karyotype of a human being would show 23 pairs of chromosomes, for a total of 46. Diploid cells contain two non-identical copies of the same genes. All diploid cells will contain two separate alleles for each gene in the genome, represented by the two homologous chromosomes.

An important note to make is that human germ (sex) cells are haploid, meaning that the chromosomes are not paired in sperm cells and egg cells.

A nucleosome is defined as a core region of histones plus one stretch of linker DNA. This gives a "beads on a string" shape, which can be further packaged into chromatin. These nucleosomes contain a DNA wrapped histone octamer in the core region, and a linker histone in the linker DNA region. The histone octamer has 2 each of H2A, H2B, H3, and H4 histones. The linker DNA has an H1 histone.

Karyotypes describe whole chromosome structure, including the number and size of chromosomes, position of centromeres, distribution of heterochromatin versus euchromatin, and the presence of satellite chromosomes that are found near the centromeres. However, a karyotype is unable to label specific gene sequences and determine their chromosomal locations. Most karyotypes depict chromosomes of a cell in metaphase.

Since colorblindness is a recessive disease, all copies of the X-chromosome must have the diseased allele in order for the person to be colorblind. Daughters have two copies of the X-chromosome: one from the mother and the other from the father. Males only have one copy of the X-chromosome (from the mother) and a Y-chromosome from the father.

Since we know that the father has normal vision, he does NOT carry the colorblind allele. Since the daughters for this couple can only potentially receive one colorblind allele (from the mother), all of their daughters will have normal vision. This means that there is a zero percent chance for colorblindness in their daughters.

The cross would look like this, taking Xb as the colorblind allele:

Parents: XXb x XY

Offspring: XX or XXb (normal daughters), XY (normal son), YXb (colorblind son)

The chance of a colorblind daughter will be zero, but the chance of a colorblind son will be 50%.

X-linked recessive inheritance dictates that expression of themutant phenotype will only occur if the individual is homozygous for the mutation on the X-chromosomes. Therefore, a female must have inherited two mutant X-chromosomes to have hemophilia A, while a male only requires one mutant X-chromosome to have the disorder. By virtue of the father not having hemophilia A, we know the daughter is inheriting at least one wild-type X-chromosome, and therefore there is zero chance she will be homozygous and have hemophilia A.

Incomplete dominance indicates that there is no dominant allele. In these cases, the phenotype associated with inheriting one copy of each allele (the heterozygotes, Bb) is often a blending of the phenotypes associated with homozygosity of each allele. As such, a genotype of BB will result in black hair, bb will produce white hair, and Bb will result in grey hair.

The incorrect answers are too limited in scope to be cases of incomplete dominance. The correct answer identifies that there will be three unique phenotypes.

Because the disorder is autosomal dominant, the statement "If an individual does not have the disorder, they can still pass on the mutant gene if one of their parents has the disorder" must be false.

If the indivdual in question does not have the disorder, that means they did not inherit ANY copies of the mutant gene, and therefore cannot pass it on.

For autosomal dominant disorders, the individual only needs to inherent a single copy of a mutated allele to then show symptoms of that disorder. If it were recessive, both alleles would have to be mutant. X-linked recessive is incompletely correct for males since they only have one X-chromosome, and incorrect for females since 2 copies of the X-chromosome are needed, and thus 2 copies of the allele. Complex inheritance describes situations beyond a single gene, and Mendelian inheritance is not a specific method of inheritance. Note that Y-linked disorders are passed from father to son, and since males only have one copy of the Y-chromosome, if there is a genetic mutation on the Y-chromosome, the individual will be affected.

Epigenetics are changes to the genome that result in phenotypic variation that have nothing to do with changes in the actual DNA sequence. All listed answers occur independently of DNA sequence, except for "different exon sequences," which is the actual sequence of an exon. This referces to alternative splicing, an is not related to the modification of DNA or histones.