The topics covered in each section contain important terms and concepts. To prepare for the MCAT, it is advantageous to know what each term is and understand how it relates to other terms. For example, DNA and RNA are important biological molecules; DNA stores the genetic information and is a template for RNA.
Biology is the study of living organisms. This field of study can be subdivided into molecular biology, cellular biology, organismal biology, and anatomy and physiology. Biochemistry is the study of chemical reactions in living organisms. You will have 95 minutes in which to complete 59 questions on this part of the MCAT.
While simply knowing definitions of terms will not guarantee a passing score on the MCAT, it certainly is a starting point. Read all you can about the following things that will be included on the exam and also make sure you understand how these things interact during the various life processes.
A molecule is a group of two or more atoms covalently bonded together. Molecules can comprise one type of atom, like hydrogen gas (H2), or more complex assemblies, like glucose (C6H12O6). Examples of important biological molecules are deoxyribonucleic acid (DNA), ribonucleic acid (RNA), adenosine triphosphate (ATP), proteins, and carbohydrates.
Biomolecules are molecules that are involved in biological processes and are required for life. Biomolecules can include macromolecules such as nucleic acids, proteins, carbohydrates, and lipids. Smaller biomolecules include metabolites, signaling molecules, and vitamins. Biomolecules interact to perform all the biological processes required for living, including generating ATP to fuel cellular functions.
Macromolecules are large molecules that are generated by combining smaller monomeric subunits. In biology, the four major classes of macromolecules are nucleic acids, proteins, carbohydrates, and lipids. Macromolecules can be linear (nucleic acids, carbohydrates, and proteins) or branched (carbohydrates).
Proteins are macromolecules made of amino acids. Humans use 20 amino acids to make all proteins. Peptide bonds link amino acids into a linear chain, which can then fold into a 3D structure. The 3D structure is important for the functionality of proteins; denatured, or unfolded proteins, are often not functional. Proteins can act as scaffolds to organize cellular compartments. Receptors and enzymes are two important classes of proteins that are involved in signaling and catalyzing biological reactions, respectively.
DNA stores the genetic information of an organism and RNA is made using DNA as a template (transcription). RNA is then translated into proteins. DNA and RNA are nucleic acids that consist of linear strands of nucleotides. Each nucleotide is made of a covalently bonded deoxyribose (DNA) or ribose (RNA) sugar, a phosphate group, and a nitrogen-containing nucleobase. DNA has the nucleobases cytosine (C), guanine (G), adenine (A), or thymine (T). For RNA, the nucleobases are C, G, A, and uracil (U). The double helix of DNA is formed from two antiparallel strands of polynucleotides that associate through H bonds of the nucleobases. A bonds with T and G bonds with C. RNA can be linear or form complex structures that form when A forms hydrogen bonds with U or G forms H bonds with C.
Lipids include fatty acids, sterols, phospholipids, and waxes. Phospholipids are an important component of cell membranes and sterols are the building blocks of several hormones.
Carbohydrates exists as monosaccharides, disaccharides, or polysaccharides. Monomeric monosaccharides can polymerize via glycosidic bonds’ Carbohydrates are cyclic, linear, or branched. Carbohydrates, like glucose, are important for glycolysis, which provides energy to cells.
Biotechnology applies our knowledge of biology for practical purposes. For example, we can now amplify genes using polymerase chain reactions (PCR) and determine the gene sequence. Thus, we can identify mutations that cause disease. Further, restriction enzymes cut certain DNA sequences and we can use restriction enzymes to clone genes into plasmids and use transformed bacteria to make therapeutic molecules such as insulin.
In cells, DNA is organized to prevent tangling. DNA is wrapped around proteins called histones and this structure is called chromatin. Humans have 23 pairs of chromosomes; 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes. Females have 2 X chromosomes and males have an X and a Y chromosome.
Adenosine triphosphate (ATP) is made of covalently bonded adenine ring, ribose, and three phosphate groups. ATP is typically generated during glycolysis and fermentation. It is an important cofactor for many enzymatic reactions and provides energy to fuel these reactions. Enzymes hydrolyze ATP, which results in ADP and removal of the terminal phosphate.
The plasma membrane separates the cellular components from the environment and the cytoskeleton provides structure for the cell. The cytoplasm contains important organelles that include mitochondria (energy production), lysosomes (contains degradative enzymes), the Golgi apparatus (organization of molecules to be packaged and secreted), the endoplasmic reticulum (protein production), and peroxisomes (breakdown of fatty acids). The nucleus contains the chromosomes.
In humans, cells are organized into tissues; tissues are assembled into organs, and organs function in organ systems. Different cells often perform specialized functions in tissues. For example, goblet cells in the colon secrete mucus and epithelial cell do not. Bacteria can also form multicellular communities called biofilms.
Prokaryotes are single-cell organisms that lack membrane-bound organelles. The two domains of prokaryotes are Archaea and Bacteria. Theses organisms have a variety of shapes, which include rods (bacilli), spheres (cocci), and spirals. Some of these organisms move using flagella. Bacterial chromosomes are circular and some bacteria have extra DNA encoded on plasmids.
Eukaryotes are the third domain of life. All eukaryotic organisms have membrane-bound organelles, including a nucleus; thus, eukaryotic cells are larger than prokaryotic cells. Eukaryotes can reproduce asexually or sexually.
Viruses must replicate inside other organisms. Viruses attach to a host cell and can insert their genome directly into the host cell (bacteriophage). Alternatively, the entire virus is taken into the cell and uncoats to release its genome from the capsid. Viruses hijacks the host cellular machinery to replicate its genome, which consists of either RNA or DNA, and to assembly new viral particles. The viral genome is packaged into a protein capsid. Enveloped viruses coat this protein capsid with a layer of the host cell membrane. The assemble virus is then released from the cell to infect new hosts.
Meiosis is a type of cell division and is important for sexual reproduction. During meiosis, chromosomes are replicated, so that there are identical sister chromatids. The mother cell (four copies of each chromosome) then divides and each daughter cell (two copies of each chromosome) undergoes another division. Thus, the four new cells (haploid) have one copy of each chromosome and half the original DNA of the original cell (diploid). This is how gametes such as ova and sperm are produced. When gametes fuse, they form a zygote.
Homeostasis is the process by which living organisms strive to maintain a steady state/ equilibrium. Organisms need to sense their environment and respond appropriately. On the cellular level, this includes maintaining electrical and chemical gradients across cell membranes. Homeostasis is also important on a larger scale. For example, in response to high extracellular glucose levels that occur after consuming food, the human body produces insulin, which signals for cells to increase their glucose uptake. Thus, glucose levels are restored to normal.
Nerve cells, or neurons, have a cell body, an axon, and several dendrites. The cell body is where the nucleus and organelles are located. Axons are long protrusions from the cell body and can be myelinated. Electrical and chemical signals traverse the axon to the synapse where these signals are relayed to other cells. Dendrites are small protrusions that receive cell signals.