Imprinted Gene Mcat

• Nondisjunction occurs when chromosomes fail to segregate during meiosis.

Imprinted Gene Mcat

Alleles segregate randomly in gametes introducing genetic variation in a variety of ways. This contributes to variation in populations and offspring. This can be done by an independent assortment of chromosomes in meiosis or by the crossing over of chromosomes.

Independent assortment generates genetic variation. For instance, a cell with two copies (one from mom, one from dad) of each chromosome, undergoes independent assortment that shuffles these chromosomes, and then only one copy of each into the gamete. This way, the gamete may have chromosome one from mom, chromosome 2 from dad, chromosome 3 from dad, and so on. The mechanism of independent assortment during meiosis is the following:

1. During metaphase I, homologous chromosome pair up along the metaphase line in random orientation – sometimes the mom’s chromosome is on the left, sometimes it’s on the right.
2. During anaphase I, the homologous chromosomes are pulled apart. Those on the left will be put into one daughter cell; those on the right will be put into another.

Because of independent assortment, genes on different chromosomes are randomized. However, genes on the same chromosome can not be randomized by this mechanism. The segregation of alleles into gametes can be influenced by linkage. The physically closer the genes are on the chromosome, the more linked they are. However, because of the process of recombination , or “crossover,” it is possible for two genes on the same chromosome to behave independently, or as if they are not linked. Although crossing over is a mechanism that reduces linkage, crossing over is only efficient when the genes are physically apart from each other on the chromosome. When the genes are further apart on the chromosome, crossing over makes them less linked.

Nondisjunction occurs when chromosomes fail to segregate during meiosis; when this happens, gametes with an abnormal number of chromosomes are produced. This can occur in Anaphase 1 and Anaphase 2 and can lead to gametes with too many or too few chromosomes.

Genetic recombination is the process that introduces genetic diversity into the gametes during meiosis. The two processes that makeup recombination are independent assortment and crossing over .

The crossover events are the first source of genetic variation in the nuclei produced by meiosis, occur during prophase I , and may consist of a single, double, or multiple crossovers.

• A single crossover is when homologous chromosomes are aligned, and chromatids from two different chromosomes can exchange segments resulting in genetic recombination.
• In double crossovers , chromatids from two homologous chromosomes come in contact at two points.

Crossover. Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.

The synaptonemal complex , the protein complex that glues the tetrad together, first forms at specific locations and then spreads to cover the entire length of the chromosomes. It supports the crossing over. At the end of prophase I, the pairs are held together only at the chiasmata and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible. Tetrads form for crossing over to occur. It breaks apart when the homologous chromosomes separate in meiosis I.

Genes that are located on the sex chromosomes are called sex-linked genes. Sex-linked genes generally refer to genes on the X chromosome in humans, because the Y chromosome has very few genes. Because males only have on X chromosome, they only inherit one allele of sex-linked genes, making them more susceptible to diseases caused by sex-linked genetic mutations. This is different from imprinted genes, in which genes are expressed in a parent-specific manner based on inherited epigenetic modifications.

Practice Questions

Khan Academy

MCAT Official Prep (AAMC)

Biology Question Pack, Vol. 1 Passage 12 Question 76

Biology Question Pack, Vol. 1 Passage 14 Question 91

Biology Question Pack, Vol. 1 Passage 14 Question 92

Biology Question Pack, Vol. 1 Passage 14 Question 93

Practice Exam 3 B/B Section Question 11

Practice Exam 3 B/B Section Passage 7 Question 40

Sample Test B/B Section Question 18

Key Points

• Independent assortment generates genetic variation.

• Nondisjunction occurs when chromosomes fail to segregate during meiosis.

• Crossing over is a mechanism that reduces linkage. However, crossing over is only efficient when the genes are physically apart from each other on the chromosome.

• Genetic recombination is the process that introduces genetic diversity into the gametes during meiosis.

• The crossover events occur during prophase I and may consist of a single, double, or multiple crossovers.

• The synaptonemal complex is the protein complex that glues the tetrad together; tetrads form for crossing over to occur. It breaks apart when the homologous chromosomes separate in meiosis I.

• Sex-linked genes are genes located on sex chromosomes, this mainly refers to genes located on the X chromosome

Key Terms

Gene : a unit of heredity; the functional groups of chromosomes that determine specific characteristics by coding for particular proteins

Chromosome : a structure in the cell nucleus that contains DNA, histone protein, and other structural proteins

Allele : one of several alternative forms of the same gene occupying a given position on a chromosome

Linkage : the property of genes of being inherited together

Recombination : the formation of genetic combinations in offspring that are not present in the parents

Crossing over : the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes

Tetrad : two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process

Chromatid : either of the two strands of a chromosome that separate during meiosis

Independent assortment: when alleles of two (or more) different genes get sorted into gametes independently of one another

Meiosis: cell division in sexually-reproducing organisms used to produce haploid gametes

Metaphase: the second stage of cell division, between prophase and anaphase, during which the chromosomes become attached to the spindle fibers

Anaphase: the stage of meiotic or mitotic cell division in which the chromosomes move away from one another to opposite poles of the spindle

Prophase: the first stage of cell division, before metaphase, during which the chromosomes become visible as paired chromatids and the nuclear envelope disappears

Chiasmata: a point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands.

Synaptonemal complex: is a protein structure that forms between homologous chromosomes to mediate synapsis and recombination during meiosis

Sex-linked genes: genes located on sex chromosomes, mainly refers to genes located on the X chromosome in humans

Imprinted genes: genes that are expressed in a parent-specific manner, based on epigenetic mechanisms

DNA for the MCAT: Everything You Need to Know

Learn key MCAT concepts about DNA, plus practice questions and answers

(Note: This guide is part of our MCAT Biochemistry series.)

Part 1: Introduction

Part 2: Structure of DNA

a) Nitrogenous bases

b) Phosphate backbone

c) G-C content

d) DNA synthesis

Part 3: The Central Dogma

a) Transcription

b) Translation

c) Exceptions to the central dogma

Part 4: Epigenetics and imprinting

a) Histone structure and function

b) X inactivation

Part 5: Disorders and cancers

a) Oncogenes and tumor suppressor genes

b) Chromosomal defects

Part 6: High-yield terms

Part 7: Passage-based questions and answers

Part 8: Standalone questions and answers

Part 1: Introduction

Understanding DNA and its function is crucial on the MCAT, especially as DNA is the biological basis of life. DNA, or deoxyribonucleic acid, is one of the two main molecules of nucleic acids (the other being RNA, or ribonucleic acid). DNA stores and selectively expresses the genetic information our cells need to function. DNA is carefully replicated and inherited from one generation of cells to the next to avoid any errors. Errors in DNA replication can vary significantly, from unnoticeable mutations to life-threatening illnesses, such as cancer. As a result, proper DNA regulation is vital to cell survival.

This guide will take a tour through the structure and function of DNA and what can go wrong. When studying DNA, it’s easy to confuse the names of the bases and building blocks of DNA as well as the different modifications DNA undergoes in the cell. It’s helpful to create a list outlining why each component or modification of DNA is necessary for its function. Throughout this guide, you will also see bolded terms that will be important to recall on the exam.

Part 2: Structure of DNA

Let’s take a close look at the structure of DNA. DNA exists as a double helix—an identifying structure that is unique to this biomolecule. In a double helix, two strands of DNA wrap around each other and form a twisted ladder pattern. There are two important parts of a DNA molecule: the nucleotides and the sugar phosphate backbone. Four unique nucleotides that bond with each other to create the rungs of the ladder: adenosine monophosphate (adenine), guanosine monophosphate (guanine), cytidine monophosphate (cytosine), and deoxythymidine monophosphate (thymine). Each nucleotide has a unique structure shown below.

What do each of these four nucleotides have in common? All of the bases have a very similar structure. Each nucleotide consists of a phosphate group (PO4-), a nitrogenous base, and a pentose sugar that links the other two groups together. In contrast, nucleosides are molecules that only consist of a nitrogenous base and a pentose sugar.

a) Nitrogenous bases

The nucleotides have different nitrogenous bases. There are two classes of nitrogenous bases: purines and pyrimidines. While both purines and pyrimidines are aromatic nitrogenous bases, they differ in size and ring structure. Purines are made of two rings and are larger than pyrimidines, which contain only one ring.

Take a close look at the structure of nitrogenous bases of the nucleotides. Adenine and guanine have purine bases, while cytosine and thymine have pyrimidine bases. Adenine and guanine are often broadly classified as purine nucleotides, while cytosine and thymine are classified as pyrimidine nucleotides. It’s important to remember adenine and guanine still have differences in the organic structure: for instance, in the number of attached carbonyl (C=O) bonds and where they are located. The same goes for cytosine and thymine. Memorizing each nucleotide’s structure will be crucial for test day.

Each nucleotide follows strict rules for bonding with other nucleotides. In DNA, purine bases bond with pyrimidine bases instead of other purines, and vice versa. In particular, guanine and cytosine almost exclusively bond together while adenine and thymine bond with each other. This bonding is known as Watson-Crick base pairing. Scientists have discovered exceptions to these rules, but you won’t be tested on them.

When guanine and cytosine bond, three hydrogen bonds form. When adenine and thymine bond, two hydrogen bonds form. We’ve illustrated these bonds below.

These hydrogen bonds provide stability to the DNA, making the guanine and cytosine pair and adenine and thymine pair favorable. An easy way to remember which nucleotides bond together is by memorizing the letters AT and GC together. This will remind you that adenine (A) bonds with thymine (T), while guanine (G) bonds with cytosine (C).

b) Phosphate backbone

Imagine having a ladder with only rungs. It wouldn’t function as a ladder at all! A ladder needs its side rails to work. Similarly, DNA needs a backbone to hold the double helix structure together.

The DNA backbone is known as a sugar phosphate backbone. The sugar phosphate backbone consists of a repeating pattern of sugar and phosphate groups bonded together.

Recall the structure of a nucleotide and that each nucleotide base contains a phosphate group and pentose sugar. The phosphate group is attached to the 5’ carbon of the pentose sugar. In the sugar phosphate backbone, the phosphate group of one nucleotide creates a phosphodiester bond with the 3’ carbon of the pentose sugar on the adjacent nucleotide. Try to trace the locations of the phosphodiester bond on the diagram below.

Phosphodiester bonds exist between adjacent nucleotides to create the backbone of DNA. An important fact to know about DNA is that its backbone is negatively charged. Can you see why? The backbone is negatively charged because the phosphate groups carry charged oxygen atoms. This negatively charged backbone creates an attractive force between the aqueous, polar environment and the DNA molecule.

Consider what might happen if the negatively charged backbone were on the interior of the molecule and the aromatic bases were on the exterior of the molecule. This would lead to a very unfavorable energy conformation—the two negative charges on either side of the backbone would repel each other, and the aromatic bases would not be soluble in water at all!

c) G-C content

One of the most important ways to analyze the nucleotide composition of DNA is to calculate its G-C content. G-C content is a measure of the percentage of nucleotide bases that contain guanine or cytosine in a fragment of DNA. Why might this percentage be important?

Recall from our discussion of guanine and cytosine bonds that G-C bonds are more stable than A-T bonds. When guanine and cytosine bond, three strong hydrogen bonds are created. Breaking a single hydrogen bond takes a significant amount of energy, let alone three.

G-C content is important because it determines the melting point of DNA, as well as its accessibility by polymerases. A higher G-C content means that there are a greater number of guanine-cytosine base pairs holding the two DNA strands together. This means there are also more hydrogen bonds. A greater amount of energy is needed to dissociate the two strands, leading to a higher melting point. A lower G-C content means the opposite. Less energy is needed to dissociate the DNA strands, lowering the melting point and making the DNA more accessible to polymerases.

The MCAT may ask you to calculate nucleotide composition using Chargaff’s rules. Chargaff’s rules state that the ratio of purine nucleotides to pyrimidine nucleotides in DNA is 1-to-1. In fact, the ratio of guanine nucleotides to cytosine nucleotides and the ratio of adenine nucleotides to thymine nucleotides are also each 1-to-1.

Chargaff’s rules hold true for any piece of DNA we come across. Why? Recall that adenine and thymine bond together while guanine and cytosine bond together. Since these bases always bond in this pattern—and DNA consists of only bonded nucleotides—there must be a 1-to-1 ratio of adenine to thymine and guanine to cytosine. To understand the first part of Chargaff’s rules, let’s look at an example.

Consider the DNA strand pictured below. If we count the number of purine and pyrimidine nucleotides, we get a 1 to 1 ratio. If we count all of the nucleotides, we get a 1-to-1 ratio of guanine to cytosine and a 1-to-1 ratio of adenine to thymine.