‘Meiosis and other factors affecting Genetic Variability’ is included in the MCAT syllabus and forms part of the Biological and Biochemical Foundations of Living System section of the test. To prepare for the MCAT test, it’s crucial to familiarize yourself with topics like meiosis and others that are found in the syllabus. Having a good understanding of these topics will help ensure that you achieve a good MCAT score. This overview will explain everything you need to know about meiosis for the MCAT.
Genetic variability refers to the differences that exist within the genomes of individuals, and how these present phenotypically in the organism. It is how individuals have different hair colors, eye colors and skin tones, and currently, research is being conducted into the link between variances in genes and personality types. In essence, it is what makes each person unique. Genetic variability arises from several different causes, including meiosis, environment (both physical, such as a dessert, and emotional, such as life stressors), DNA replication, gene flow, mixing of gametes and more.
Meiosis is the way in which certain cells divide in order to produce sex cells (gametes). There are 4 cells produced from meiosis, and as DNA only replicates once, each cell contains half the amount of genetic information (with n chromosomes) as in a normal body cell. This process is known as reduction division, and the resulting cells are haploid cells. When two haploid cells join through fertilization, they form a diploid cell known as a zygote. Diploid cells are those that contain 46 chromosomes (2n chromosomes). Meiosis contains two phases: meiosis I and meiosis II. These phases each contain the same 4 stages as mitosis, however, there are extra parts that occur throughout, including recombination and recombination, that allows for more genetic variability.
Meiosis and mitosis are both forms of cell replication that results in daughter cells. Both processes contain the stages prophase, metaphase, anaphase and telophase, however, these occur twice in meiosis (first in stage I and again in stage II), whereas in mitosis they only occur once. Mitosis is a highly controlled process that produces 2 genetically identical diploid cells. Meiosis, meanwhile, produces 4 genetically different haploid cells. This is due to extra processes that occur in meiosis alone, including crossing over and recombination, that affect how chromosomes are divided between cells.
Gregor Mendel was a scientist who examined how genes are inherited from the parent to the offspring by using pea plants. He is the biologist associated with the discovery of dominant and recessive inheritance of genes, and he created two laws: his first law of segregation and his second law of independent assortment. Mendel’s first law of segregation states that as each parent has two alleles for the same gene, each allele has an equal chance of being passed to the offspring. Mendel’s second law of independent assortment states that by inheriting one allele, an individual’s chance of inheriting a different allele is not affected. However, this has been proven false.
The theory of independent assortment states that the inheritance of one allele is independent of the inheritance of another allele in meiosis. This theory allows for scientists to create an extension of the Punnett square when looking at inheritance: the dihybrid cross. This is a more advanced Punnett square where multiple characteristics are examined. Although this has proven true in some instances, there have since been genes discovered called linked genes that combat this theory.
Image source: https://commons.wikimedia.org/wiki/File:Dihybrid_cross.svg
Recombination is the process of creating allelic combinations in the offspring that were not present in either parent. This is done through a process called crossing over, during which chromosomal arms will join together (synapsis) and exchange DNA during the end of prophase I.
Image source: https://commons.wikimedia.org/wiki/File:Synapsis_and_Crossing_Over_with_Labels.png
The name ‘tetrad’ refers to when two chromosomes align up next to each other in meiosis I, specifically the prophase stage of meiosis I. It is otherwise known as a divalent and occurs after the chromosomes have condensed. These chromosomes must be homologous with each other, and as the chromosomes each consist of 2 chromatids, the tetrad consists of four chromatids in total. It is required in order for crossing over to occur; therefore, it only occurs in meiosis.
The synaptonemal complex is a structure made up of protein that works to connect two homologous chromosomes, known as a tetrad, in the center of the nucleus during prophase I of meiosis. It allows for chromosomal pairings and prevents double ended DNA breaks from participating in crossing over during meiosis, meaning that crossing over is reciprocal: each chromosome gains the same amount of DNA as it loses. It is what determines the sites for crossing over within the chromosome.
There are two main types of crossovers that occur with recombination: single crossing over and double crossing over. These refer to the number of locations in which synaptonemal complexes are formed between tetrads. A single crossover is when only one complex is formed so genes are crossed over at only one point. It is much more common than double crossing over, however, it also reduces genetic diversity. Double crossing over is much rarer and consists of chromosomes swapping information at two points, therefore two synaptonemal complexes are required. Due to its complexity, its frequency is much lower than that of singular crossing over, however as more information is swapped, the genetic variation is much larger.
Genetic linkage describes the fact that there are genes that are physically closer to each other and are therefore more likely to be inherited together. This is because they are more likely to either be crossed over during prophase or left out of recombination entirely. Due to the theory of independent assortment, the normal frequency of recombination in genes that are unlinked is 50%, but in linked genes this is lower, with the closer the proximity, the lower the chance of recombination.
Sex-linked characteristics refer to those characteristics present on the X chromosome, as the Y chromosome has very few genes on it. Further, females carry two X chromosomes, whereas males have one X and one Y chromosome. Although females carry these two X chromosomes, one of them is made inactive and sent to the edge of the nucleus to become what is known as a Barr body. Different characteristics and mutations can be found on each X chromosome; therefore, this is an instance where females can display a concept called mosaicism. This concept describes how different cells in the body have different chromosomes that are silenced, extra or faulty so the phenotype has two different characteristics. As it is not the same X chromosome in each cell that is silenced, this is true in some X-linked diseases in females.
The Y chromosome in males has very few genes on it, which makes males very susceptible to sex-linked disease, as these affect the X chromosome. In females, if there is a faulty X chromosome and the condition is recessive, they have another normal X chromosome to make up for it. However, this is not the case in males: they are hemizygous. This means that X linked recessive conditions are nearly always found in males. X linked dominant conditions, however, are very rarely found in males and are most prominent in females, with very bad effects. This is thought to be because X linked dominant conditions have severely faulty X chromosomes to the point where males with these conditions will result in miscarriages or stillbirths.
The presence of a Y chromosome is one of the leading factors that determine the sex of an offspring. The Y chromosome contains a gene called the SRY gene that promotes the testes determining factor. This lacks on the X chromosome and triggers the formation of the male Wolffian ducts. The Wolffian ducts will then form an important part of the male testes. This does not occur in females as they lack the SRY gene, and instead the Mullerian ducts will develop, triggering growth of the female reproductive system. The sex of an individual is therefore determined by the Y chromosome, amongst other factors.
Nuclear DNA is inherited equally from the mother and father; however, this is the only part of the cell that is inherited from both parents. In humans, extranuclear DNA and organelles are all inherited from the mother. This includes the cytoplasm, cell membrane and mitochondria amongst others, and is passed down to both male and female offspring. The reason this inheritance is maternal in humans is that male sperm are victim to a concept named cytoplasmic incompatibility, which means that their cytoplasm are not viable for human life and therefore their information cannot be passed down. However, the female ovum can carry and successfully pass on this material. It is worth noting that this inheritance is not maternal in all organisms, however, it is the case in humans.
Mutations are variations in DNA that can have large effects, small effects or no effects. They can occur from several sources, including different environmental changes (for instance: UV light), failure to pick up and replace DNA by the DNA repair mechanisms (for instance: GU not converted back to GT) or from polymerase slippage (for instance: CAG repeat in Huntington’s), amongst others. Mutations are very common, and their effect determines their role in natural selection and evolution.
The most common type of mutation is an SNP (single nucleotide polymorphisms), where the DNA sequences change by one nucleotide. Sometimes, this can cause no change in the amino acid that is translated from this DNA. Other times, it can trigger the DNA translation to be terminated early (known as a nonsense mutation), or areas of mRNA to be spliced prematurely or late. The other option is that it changes the amino acid that results from that DNA translated. This can either be a silent mutation (the new amino acid is chemically identical to the original) or a missense mutation (the new amino acid has a different polarity or charge to the previous one, causing a change in structure and function of the protein). Although these types of mutations can occur through single nucleotide changes, they can also occur through changes in multiple nucleotides.
A further type of mutation is when there are additions or deletions of DNA. Additional mutations occur when either a single nucleotide is added into the DNA sequence or multiple nucleotides are added, and result in what is called a ‘frameshift’ mutation, as the nucleotide triplets are changed by a frame. Deletion mutations are the opposite: certain nucleotides are excised from the DNA sequence and therefore cause a negative frameshift in the way in which the DNA is translated.
Higher than the DNA level changes, chromosomal changes can occur that affect how DNA is read and interpreted. The first is inversions, which are where a section of DNA on the chromosome will be reversed. This subdivided into pericentric inversions (not including centromere) and paracentric inversions (including centromere). The other chromosomal change is a translocation, whereby part of one chromosome will relocate to another chromosome.
Certain mutations can be advantageous to the organism. These can include mutations that result in changes to the organism that suits them better to the environment in which they live, for instance the color of someone’s skin changing the closer to the equator they are. Another possibility is that the mutations will make the individual’s enzymes more efficient so they can break down harmful substrates faster. These mutations are more likely to be preferred through natural selection as they are advantageous to future generations. Deleterious mutations, however, are those that are harmful to the organism. This may include mutations that cause autoimmune diseases or those that prevent the correct formation of enzymes, resulting in their breakdown. These are less likely to be conserved through generations and are therefore more likely to die out over time.
Inborn errors of metabolism refer to those mutations that specifically affect enzymes and the way in which they metabolize different elements within the body. They cover a broad range of disorders, all with varying levels of detriment and treatment options. A lot of these disorders are screened for with newborns, mostly by the heel-prick test, where a baby is pricked on the heel to receive some blood that is then sent off to the lab for testing. Different hospitals cover different testing, although some of the most commonly screened for diseases are phenylketonuria, maple syrup urine disease, MCADD, galactosemia and urea cycle diseases. It is not necessary to know these disorders in depth, only have an awareness that they exist.
Mutations in cell DNA are what cause cancer. This can be point mutations that occur in individuals due to their own individual environment or inherited from family members. Several substances are known as ‘carcinogenic’ due to their role in mutating DNA to a form that causes cancer, for instance, tobacco, some red meats and recently, talcum powder amongst several others. The likelihood of causing cancer depends on the individual substance as well as the organism’s susceptibility. Oftentimes, it is a mixture of several different mutations that causes cancer as opposed to a single mutation. Knudson’s Two Hit Hypothesis states that even if individuals inherit a mutagenic gene, oftentimes they will be heterozygous for this. This means that the other allele can silence or make up for the issues that the cancerous gene can cause, and cancer only occurs when the healthy allele mutates due to environmental pressures. Several mutagens are carcinogens, but not all. However, the majority of carcinogens are mutagenic as the mutations are what are required to form cancers.
Genetic drift is the phenomenon by which different characteristics will become dominant or recessive over time, as well as prominent or rare within a population. If the likelihood of 2 different phenotypes is 50% each but the environment favors phenotype A over phenotype B, phenotype A will become more dominant. This can be eradicated by genetic bottlenecks, during which a large portion of the population is wiped out due to environmental factors, such as an earthquake, that reduces the genetic diversity meaning that the species has to start from scratch. If all but one organism is wiped out, or one organism mutates so far away from its species that it can no longer reproduce, the organism is known as the ‘Founder’ and their offspring and related organisms will have a different genetic structure to the rest of the species.
Crossing over is one of the main ways in which genetic diversity is increased. Portions of DNA consisting of several genes are switched from one chromosome to another, switching the alleles round, affecting the genome that exists within that cell. After crossing over, the chromosomes are known as ‘recombined’. The level of recombination and the genes that are recombined is a reason behind how biological siblings are genetically and phenotypically different (unless they are identical twins), as the egg and sperm that fuse together have unique chromosomes.
Hopefully this has helped you to gain a better understanding of Meiosis for the MCAT exam. For help revising other key areas of the MCAT syllabus, we have blogs covering a wide range of MCAT topics from the excretory system, to the respiratory system, to nerve cells, and many more. To find out more about the exam, including registration, MCAT test dates and fees for 2023, and how to prepare, check out our MCAT Guide or handy MCAT Checklist.