Science Study Guide for the TEAS

Biology: Cells and Genetics

Biology is the study of life. It is studied at several levels, from the organic molecules that make up cells, to the interaction of organisms, to the complex cycles of matter that move in and out of living things and through the global system.

Cells

Cells are the components that make up all living organisms. All animal and plant cells contain a nucleus, cytoplasm, a cell membrane, mitochondria, and ribosomes. However, plant cells contain three extra components: chloroplasts, a cell wall, and a permanent vacuole. Different cells are designed to perform different functions; for example, red blood cells are designed to carry oxygen to other cells in animals, while a plant’s leaf cells are designed to absorb light.

Organizational Hierarchy of the Body

The body is organized into levels. The most basic level is the cell. Cells make up tissues, tissues make up organs, organs make up organ systems, and organ systems make up the organism. Some organisms are unicellular and only exist at the cellular level, but for multicellular organisms, this hierarchy appears.

Cells

Cells are the basic unit of structure and function of all organisms. Cells may be differentiated to perform specific functions for a multicellular organism, but they have the same basic components.

Tissues

Tissues are groups of cells working together to perform a specific function. There are four main types of tissues in the body: epithelial, connective, muscular, and nervous tissues. Epithelial tissue exists in sheets and does not have a dedicated blood supply; instead, it relies on diffusion from nearby capillaries. It serves two purposes: either as a covering, like skin, or to produce secretions, like glands. Connective tissue connects different structures within the body and usually has its own blood supply. The three types of muscle tissue (skeletal, cardiac, and smooth) are all involved in allowing movement. Finally, nervous tissue makes up the brain and nervous system.

Organ Systems

Just as groups of cells working together make tissues, groups of tissues working together make organs. A group of organs working together then makes an organ system. For example, the nervous system is made up of the brain, the spinal cord, and the nerves. There are 11 major organ systems in the human body: respiratory, circulatory or cardiovascular, digestive or gastrointestinal, muscular, nervous, integumentary, endocrine, urinary, reproductive, immune, and skeletal. It is important to understand the general anatomy and physiology of all these systems.

Parts of a Cell

A eukaryotic cell, found in plants, animals, fungi, and protists, has several organelles that help the cell perform its functions.

Retrieved from: https://www.ncbi.nlm.nih.gov/books/NBK65951/figure/CDR0000044060__3/

Cell Membrane

The cell membrane surrounds and defines the cell. It helps maintain homeostasis by regulating what substances enter and leave the cell. In other words, it is selectively permeable. It is made up of a double layer of phospholipids, various proteins, cholesterol, and carbohydrate chains.

Cytoplasm

The cytoplasm is the fluid inside the cell in which the organelles are suspended. It is mostly water, with other dissolved ions and particles.

Golgi Apparatus

The Golgi apparatus modifies and packages the proteins that are created by the ribosomes into sacs called vesicles.

Lysosome

Lysosomes contain enzymes that break down cell parts that are no longer needed. It is also responsible for apoptosis, or programmed cell death.

Mitochondrion

Mitochondria are small, bean-shaped structures with a double membrane and their own DNA. The theory of endosymbiosis states that they were once independent unicellular organisms that were engulfed by a larger cell and kept for their energy-producing properties. They break down food for energy in a process called cellular respiration.

Nucleus

The nucleus (plural: nuclei) holds the DNA and the nucleolus, the place where the cell’s ribosomes are made. It has a nuclear membrane with nuclear pores that allow the passage of materials in and out.

Ribosome

Cells contain thousands, or even millions, of ribosomes. They are responsible for synthesizing proteins using coded instructions from the DNA in the nucleus.

Rough Endoplasmic Reticulum

The endoplasmic reticulum (ER) transports proteins through the cell. The rough ER has ribosomes attached to its surface. The ER extends from just outside the nucleus all the way to the cell membrane in a series of membranes and tubes.

Smooth Endoplasmic Reticulum

The smooth ER does not have ribosomes attached to its surface. It is important in synthesizing lipids and detoxifying various substances.

Vacuole

The vacuole is a storage compartment for food, water, waste, or other materials. Plants have a large central vacuole that stores water. This serves as a water source for the cell and also provides the plant cell with support.

Mitosis and Meiosis

Cells can only come from pre-existing cells, and they duplicate in a process called mitosis. Before mitosis, the cell’s DNA is replicated to ensure each new cell has a complete copy of the DNA. Meiosis is the process that creates gametes, or sex cells, for organisms that reproduce sexually.

The Phases of Mitosis

During prophase, the nuclear membrane dissolves and spindle fibers, which help organize the chromosomes, begin to appear from the centrioles. The chromosomes line up in the middle of the cell in metaphase, where the spindle fibers attach to the center of each replicated chromosome. The spindle fibers shorten during anaphase, pulling the replicated chromosomes apart. During telophase, the chromosomes are at opposite ends of the cell and a new nuclear membrane begins to form around each set.

The Phases of Meiosis

One notable difference between mitosis and meiosis is that meiosis occurs twice. The first division includes prophase I–telophase I, and involves the separation of the homologous chromosomes (a set of chromosomes with similar genetic information, one inherited from each parent). The second division includes prophase II–telophase II, and involves the separation of the replicated chromosomes. The result is four genetically different gametes.

Genetics and Proteins

Heredity is the mechanism by which genetic information is passed from parent to child. This genetic information can code for certain traits, such as eye color. The genetic information that is passed on is held in the DNA, which is made of nucleic acids. Over many generations, the passing on of some traits and the omission of others can lead to the evolution of a species.

Chromosomes

DNA strands in cells are very long and thin and are curled into structures called chromosomes. The DNA in chromosomes carries the genetic code in a series of nucleotide bases. A gene is a section of DNA that codes for a specific protein. Humans have 23 pairs of chromosomes (46 total chromosomes) in each cell, except for sperm and ova, which have only one set of each chromosome (23 in total). Normally, two strands of DNA are complementary and form a double helix, but when cell division takes place, the two strands of the helix separate and are copied, eventually resulting in two sets of double helixes and two sets of chromosomes—one for each of the new cells.

Genes

Genes are made up of DNA, and each chromosome may contain many genes. Genes contain the genetic instructions with which to make proteins. Every person has two copies or alleles of every gene, one inherited from each of their parents. Most genes are the same in every person, but some, such as the genes that determine eye color, are different. Genes can either be dominant or recessive, and if an organism has one of each, the dominant gene is the one that will be expressed. For example, a person might have a dominant gene for green eyes and a recessive gene for gray eyes. As a result, they will have green eyes. Incomplete dominance and codominant genes also exist.

DNA

DNA stands for deoxyribonucleic acid and is the fundamental genetic building block in humans and all other life. Most of the DNA is held in the nucleus of the cells. It is made up of four molecules that match up in pairs: adenine goes with thymine and guanine goes with cytosine. From just these four chemicals, every gene, chromosome, and protein can be made. It is important to know that DNA can replicate itself, which allows for the creation of new cells.

RNA

RNA, or ribonucleic acid, is single-stranded and shorter than DNA. There are three types of RNA. Messenger RNA (mRNA) serves as a messenger between DNA and the ribosomes. Transfer RNA (tRNA) reads the code in mRNA and transfers the appropriate amino acids. Ribosomal RNA (rRNA) is in the ribosome and provides a binding site for translation.

Transcription and Translation

Transcription occurs when a strand of mRNA is created using DNA as a template. This strand then travels out of the nucleus to a ribosome. There, the mRNA nucleotides are read three bases at a time by the tRNA in a process called translation to create a chain of amino acids that becomes a protein.

Relationship between Chromosomes, Genes, RNA, and DNA

Chromosomes are bundles of DNA. The genes on the chromosomes are segments of DNA that code for proteins, and proteins determine the traits of the organism. The genetic code is copied by mRNA, then translated into an amino acid (protein) sequence.

Genetic Mutations

When there is a mistake in the order of nucleotides of DNA, this is called a mutation. Mutations occur randomly, but there are also environmental factors that cause mutations to occur, such as exposure to radiation or chemicals. If the mutation occurs in a gene sequence, this can result in a protein with the incorrect amino acid, which leads to loss of function. Cancer is caused by specific genetic mutations, as are all genetic disorders.

Mendel’s Laws of Inheritance

Johann Gregor Mendel discovered the laws of heredity when studying pea plants. They are as follows:

• The law of segregation: Each inherited trait comes from a pair of genes, one from each parent.

• The law of independent assortment: Genes on different chromosomes are kept separate from each other so that the inheritance of one trait does not depend on the inheritance of another.

• The law of dominance: If a pair of genes is made up of one dominant and one recessive gene, the dominant gene will be expressed.

Genotype and Phenotype

The phenotype of an organism is the physical trait it expresses. Mendel’s pea plants were either tall or short. The genotype is the set of alleles the organism carries. Each organism has two alleles for a trait, and the combination of the two determines the phenotype.

Dominant and Recessive Traits

When one allele for a trait overpowers the expression of another allele, we say that allele is dominant. In Mendel’s pea plants, the tall allele (T) overpowered the short allele (t), so plants that had one of each (Tt) were tall. The trait that gets covered up is recessive.

Alleles

An allele is one version of a trait. For example, Mendel’s pea plants could have the tall allele for height or the short allele. The different alleles carried by individuals in a population account for the variations that exist among them. If an individual carries two of the same alleles (TT or tt), they are homozygous, and if they carry two different ones (Tt), they are heterozygous.

Meiosis and Genetic Variation

The process of meiosis increases genetic variation. The law of independent assortment states that the alleles an individual carries separate during meiosis, and each gamete will have a different mix of the parent’s chromosomes. A process called crossing over, in which homologous chromosomes exchange segments of information during prophase I, increases the variation even more.

Using a Punnett Square

A Punnett square can help predict the probable outcome of the offspring of two parents regarding a certain trait. Below is a Punnett square showing the outcome of two pea plants who are heterozygous for the tall trait. The outcome is a 75% chance of tall offspring and a 25% chance of short offspring.