Students who are well-versed in this content can move through this chapter quickly. For students who need a deeper review, additional learning resources will be recommended in the chapter summary. It is recommended to start with the final summary and the review questions at the end of each chapter to identify learning needs.
Key terminology
*This unit will summarize additional key terminology at the end of each chapter.
Adenine
Adenine (A) is one of the four nucleotide bases in DNA, with the other three being cytosine (C), guanine (G) and thymine (T). Within a double-stranded DNA molecule, adenine bases on one strand pair with thymine bases on the opposite strand. The sequence of the four nucleotide bases encodes DNA’s information.
Allele
An allele is one of two or more versions of DNA sequence (a single base or a segment of bases) at a given genomic location. An individual inherits two alleles, one from each parent, for any given genomic location where such variation exists. If the two alleles are the same, the individual is homozygous for that allele. If the alleles are different, the individual is heterozygous.
Amino Acid
An amino acid is the fundamental molecule that is the building block for proteins. There are 20 different amino acids. A protein consists of one or more chains of amino acids (called polypeptides) whose sequence is encoded in a gene. Some amino acids can be synthesized in the body, but others (essential amino acids) cannot and must be obtained from a person’s diet.
Aneuploidy
Aneuploidy is an abnormality in the number of chromosomes in a cell due to loss or duplication. Aneuploidy would be any number of chromosomes other than the usual 46 in humans.
Autosomal Dominant Disorder
Autosomal dominant is a pattern of inheritance characteristic of some genetic disorders. “Autosomal” means that the gene in question is located on one of the numbered, or non-sex, chromosomes. “Dominant” means that a single copy of the gene variant (from one parent) is enough to cause the disorder. A child of a person affected by an autosomal dominant condition has a 50% chance of being affected by that condition via inheritance of a dominant allele. By contrast, an autosomal recessive disorder requires two copies of the gene variant (one from each parent) to cause the disorder. Huntington’s disease is an example of an autosomal dominant genetic disorder.
Autosomal Recessive Disorder
Autosomal recessive is a pattern of inheritance characteristic of some genetic disorders. “Autosomal” means that the gene in question is located on one of the numbered, or non-sex, chromosomes. “Recessive” means that two copies of the gene variant (one from each parent) are required to cause the disorder. In a family where both parents are carriers and do not have the disease, roughly a quarter of their children will inherit two disease-causing alleles and have the disease. By contrast, an autosomal dominant disorder requires only a single copy of the gene variant from one parent to cause the disorder. Sickle cell anemia is an example of an autosomal recessive genetic disorder.
Autosome
An autosome is one of the numbered chromosomes, as opposed to the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Autosomes are numbered roughly in relation to their sizes. The largest autosome — chromosome 1 — has approximately 2,800 genes; the smallest autosome — chromosome 22 — has approximately 750 genes.
Base Pair
A base pair consists of two complementary DNA nucleotide bases that pair together to form a “rung of the DNA ladder.” DNA is made of two linked strands that wind around each other to resemble a twisted ladder — a shape known as a double helix. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G) or thymine (T). The two strands are held together by hydrogen bonds between pairs of bases: adenine pairs with thymine, and cytosine pairs with guanine.
Cancer
Cancer is a disease in which some of the body’s cells grow uncontrollably. There are many different types of cancer, and each begins when a single cell acquires a genomic change (or mutation) that allows the cell to divide and multiply unchecked. Additional variants can cause the cancer to spread to other sites. Such variants can be caused by errors during DNA replication or result from DNA damage due to environmental exposures (such as tobacco smoke or the sun’s ultraviolet rays). In certain cases, variants in cancer genes are inherited, which increases a person’s risk of developing cancer.
Carrier
A carrier, as related to genetics, is an individual who “carries” and can pass on to its offspring a genomic variant (allele) associated with a disease (or trait) that is inherited in an autosomal recessive or sex-linked manner, and who does not show symptoms of that disease (or features of that trait). The carrier has inherited the variant allele from one parent and a normal allele from the other parent. Any offspring of carriers is at risk of inheriting a variant allele from their parents, which would result in that child having the disease (or trait).
Central Dogma
The central dogma of molecular biology is a theory first proposed by Francis Crick in 1958. It states that genetic information flows only in one direction, from DNA to RNA to protein. Scientists have since discovered several exceptions to the theory.
Chromosome
Chromosomes are threadlike structures made of protein and a single molecule of DNA that serve to carry the genomic information from cell to cell. In plants and animals (including humans), chromosomes reside in the nucleus of cells. Humans have 22 pairs of numbered chromosomes (autosomes) and one pair of sex chromosomes (XX or XY), for a total of 46. Each pair contains two chromosomes, one coming from each parent, which means that children inherit half of their chromosomes from their mother and half from their father. Chromosomes can be seen through a microscope when the nucleus dissolves during cell division.
Codon
A codon is a DNA or RNA sequence of three nucleotides (a trinucleotide) that forms a unit of genomic information encoding a particular amino acid or signaling the termination of protein synthesis (stop signals). There are 64 different codons: 61 specify amino acids and 3 are used as stop signals.
Codominance
Codominance, as it relates to genetics, refers to a type of inheritance in which two versions (alleles) of the same gene are expressed separately to yield different traits in an individual. That is, instead of one trait being dominant over the other, both traits appear, such as in a plant or animal that has more than one pigment color.
Cytosine
Cytosine (C) is one of the four nucleotide bases in DNA, with the other three being adenine (A), guanine (G) and thymine (T). Within a double-stranded DNA molecule, cytosine bases on one strand pair with guanine bases on the opposite strand. The sequence of the four nucleotide bases encodes DNA’s information.
Deoxyribonucleic Acid (DNA)
Deoxyribonucleic acid (abbreviated DNA) is the molecule that carries genetic information for the development and functioning of an organism. DNA is made of two linked strands that wind around each other to resemble a twisted ladder — a shape known as a double helix. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G) or thymine (T). The two strands are connected by chemical bonds between the bases: adenine bonds with thymine, and cytosine bonds with guanine. The sequence of the bases along DNA’s backbone encodes biological information, such as the instructions for making a protein or RNA molecule.
Diploid
Diploid is a term that refers to the presence of two complete sets of chromosomes in an organism’s cells, with each parent contributing a chromosome to each pair. Humans are diploid, and most of the body’s cells contain 23 chromosomes pairs. Human gametes (egg and sperm cells), however, contain a single set of chromosomes and are said to be haploid.
DNA Replication
DNA replication is the process by which the genome’s DNA is copied in cells. Before a cell divides, it must first copy (or replicate) its entire genome so that each resulting daughter cell ends up with its own complete genome.
Dominant Traits and Alleles
Dominant, as related to genetics, refers to the relationship between an observed trait and the two inherited versions of a gene related to that trait. Individuals inherit two versions of each gene, known as alleles, from each parent. In the case of a dominant trait, only one copy of the dominant allele is required to express the trait. The effect of the other allele (the recessive allele) is masked by the dominant allele. Typically, an individual who carries two copies of a dominant allele exhibits the same trait as those who carry only one copy. This contrasts to a recessive trait, which requires that both alleles be present to express the trait.
Epistasis
Epistasis is a circumstance where the expression of one gene is modified (e.g., masked, inhibited or suppressed) by the expression of one or more other genes.
Exome
An exome is the sequence of all the exons in a genome, reflecting the protein-coding portion of a genome. In humans, the exome is about 1.5% of the genome.
Exon
An exon is a region of the genome that ends up within an mRNA molecule. Some exons are coding, in that they contain information for making a protein, whereas others are non-coding. Genes in the genome consist of exons and introns.
Gamete
A gamete is a reproductive cell of an animal or plant. In animals, female gametes are called ova or egg cells, and male gametes are called sperm. Ova and sperm are haploid cells, with each cell carrying only one copy of each chromosome. During fertilization, a sperm and ovum unite to form a new diploid organism.
Genetic code
Genetic code refers to the instructions contained in a gene that tell a cell how to make a specific protein. Each gene’s code uses the four nucleotide bases of DNA: adenine (A), cytosine (C), guanine (G) and thymine (T) — in various ways to spell out three-letter “codons” that specify which amino acid is needed at each position within a protein.
Gene Regulation
Gene regulation is the process used to control the timing, location and amount in which genes are expressed. The process can be complicated and is carried out by a variety of mechanisms, including through regulatory proteins and chemical modification of DNA. Gene regulation is key to the ability of an organism to respond to environmental changes.
Genomic Variation
Genomic variation refers to DNA sequence differences among individuals or populations. A variant is a change in the DNA sequence of an organism. Some variants influence biological function (such as a mutation that causes a genetic disease), while others have no biological effects. Variants can result from errors in DNA replication during cell division, exposure to mutagens or a viral infection. Germline variants (that occur in eggs and sperm) can be passed on to offspring, while somatic variants (that occur in body cells) are not passed on.
Genotype
A genotype is a scoring of the type of variant present at a given location (i.e., a locus) in the genome. It can be represented by symbols. For example, BB, Bb, bb could be used to represent a given variant in a gene. Genotypes can also be represented by the actual DNA sequence at a specific location, such as CC, CT, TT. DNA sequencing and other methods can be used to determine the genotypes at millions of locations in a genome in a single experiment. Some genotypes contribute to an individual’s observable traits, called the phenotype.
Germ Line
Germ line refers to the sex cells (eggs and sperm) that sexually reproducing organisms use to pass on their genomes from one generation to the next (parents to offspring). Egg and sperm cells are called germ cells, in contrast to the other cells of the body, which are called somatic cells.
Guanine
Guanine (G) is one of the four nucleotide bases in DNA, with the other three being adenine (A), cytosine (C) and thymine (T). Within a double-stranded DNA molecule, guanine bases on one strand pair with cytosine bases on the opposite strand. The sequence of the four nucleotide bases encodes DNA’s information.
Haploid
Haploid refers to the presence of a single set of chromosomes in an organism’s cells. Sexually reproducing organisms are diploid (having two sets of chromosomes, one from each parent). In humans, only the egg and sperm cells are haploid.
Heterozygous
Heterozygous, as related to genetics, refers to having inherited different versions (alleles) of a genomic marker from each biological parent. Thus, an individual who is heterozygous for a genomic marker has two different versions of that marker. By contrast, an individual who is homozygous for a marker has identical versions of that marker.
Histone
A histone is a protein that provides structural support for a chromosome. Each chromosome contains a long molecule of DNA, which must fit into the cell nucleus. To do that, the DNA wraps around complexes of histone proteins, giving the chromosome a more compact shape. Histones also play a role in the regulation of gene expression.
Homologous Recombination
Homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of DNA. During the formation of egg and sperm cells (meiosis), paired chromosomes from the male and female parents align so that similar DNA sequences can cross over, or be exchanged, from one chromosome to the other. This exchanging of DNA is an important source of the genomic variation seen among offspring.
Homozygous
Homozygous, as related to genetics, refers to having inherited the same versions (alleles) of a genomic marker from each biological parent. Thus, an individual who is homozygous for a genomic marker has two identical versions of that marker. By contrast, an individual who is heterozygous for a marker has two different versions of that marker.
Inherited
Inherited, as related to genetics, refers to a trait or variants encoded in DNA and passed from parent to offspring during reproduction. Inheritance is determined by the rules of Mendelian genetics.
Intron
An intron is a region that resides within a gene but does not remain in the final mature mRNA molecule following transcription of that gene and does not code for amino acids that make up the protein encoded by that gene. Most protein-coding genes in the human genome consist of exons and introns.
Linkage
Linkage, as related to genetics and genomics, refers to the closeness of genes or other DNA sequences to one another on the same chromosome. The closer two genes or sequences are to each other on a chromosome, the greater the probability that they will be inherited together.
Locus
A locus, as related to genomics, is a physical site or location within a genome (such as a gene or another DNA segment of interest), somewhat like a street address. The plural of locus is loci.
Lyonization
Lyonization (also called X-inactivation) refers to the normal phenomenon in which one of the two X chromosomes in every cell of a female individual is inactivated during embryonic development. This inactivation prevents females from having twice as many X chromosome gene products as males, who possess only a single copy of the X chromosome. Lyonization is named after Mary F. Lyon, the British geneticist who discovered the phenomenon.
Meiosis
Meiosis is a type of cell division in sexually reproducing organisms that reduces the number of chromosomes in gametes (the sex cells, or egg and sperm). In humans, body (or somatic) cells are diploid, containing two sets of chromosomes (one from each parent). To maintain this state, the egg and sperm that unite during fertilization must be haploid, with a single set of chromosomes. During meiosis, each diploid cell undergoes two rounds of division to yield four haploid daughter cells — the gametes.””Mendel, Johann (Gregor)””Gregor Mendel was an Austrian monk in the 19th century who worked out the basic laws of inheritance through experiments with pea plants. In his monastery garden, Mendel performed thousands of crosses with pea plants, discovering how characteristics are passed down from one generation to the next — namely, dominant and recessive traits. Mendel’s early experiments provided the basis of modern genetics.
Mendelian Inheritance
Mendelian inheritance refers to certain patterns of how traits are passed from parents to offspring. These general patterns were established by the Austrian monk Gregor Mendel, who performed thousands of experiments with pea plants in the 19th century. Mendel’s discoveries of how traits (such as color and shape) are passed down from one generation to the next introduced the concept of dominant and recessive modes of inheritance.
Messenger RNA (mRNA)
Messenger RNA (abbreviated mRNA) is a type of single-stranded RNA involved in protein synthesis. mRNA is made from a DNA template during the process of transcription. The role of mRNA is to carry protein information from the DNA in a cell’s nucleus to the cell’s cytoplasm (watery interior), where the protein-making machinery reads the mRNA sequence and translates each three-base codon into its corresponding amino acid in a growing protein chain.
Mitosis
Mitosis is the process by which a cell replicates its chromosomes and then segregates them, producing two identical nuclei in preparation for cell division. Mitosis is generally followed by equal division of the cell’s content into two daughter cells that have identical genomes.
Mutagen
A mutagen is a chemical or physical agent capable of inducing changes in DNA, formerly called mutations, now termed variants. Examples of mutagens include tobacco products, radioactive substances, x-rays, ultraviolet radiation and a wide variety of chemicals. Exposure to a mutagen can produce DNA changes that cause or contribute to certain diseases.
Non-Coding DNA
Non-coding DNA corresponds to the portions of an organism’s genome that do not code for amino acids, the building blocks of proteins. Some non-coding DNA sequences are known to serve functional roles, such as in the regulation of gene expression, while other areas of non-coding DNA have no known function.
Nucleic Acids
Nucleic acids are large biomolecules that play essential roles in all cells and viruses. A major function of nucleic acids involves the storage and expression of genomic information. Deoxyribonucleic acid, or DNA, encodes the information cells need to make proteins. A related type of nucleic acid, called ribonucleic acid (RNA), comes in different molecular forms that play multiple cellular roles, including protein synthesis.
Nucleosome
A nucleosome is the basic repeating subunit of chromatin packaged inside the cell’s nucleus. In humans, about six feet of DNA must be packaged into a nucleus with a diameter less than a human hair, and nucleosomes play a key role in that process. A single nucleosome consists of about 150 base pairs of DNA sequence wrapped around a core of histone proteins. In forming a chromosome, the nucleosomes repeatedly fold in on themselves to tighten and condense the packaged DNA.
Oncogene
An oncogene is a gene variant that has the potential to cause cancer. Before an oncogene becomes altered, it is called a proto-oncogene, and it plays a role in regulating normal cell division. Cancer can arise when a proto-oncogene is altered, changing it into an oncogene and causing the cell to divide and multiply uncontrollably. Some oncogenes work like an accelerator pedal in a car, pushing a cell to divide again and again. Others work like a faulty brake in a car parked on a hill, also causing the cell to divide unchecked.
Phenotype
Phenotype refers to an individual’s observable traits, such as height, eye color and blood type. A person’s phenotype is determined by both their genomic makeup (genotype) and environmental factors.
Proto-oncogene:
A normal gene that controls cell division by regulating the cell cycle that becomes an oncogene if it is altered.
Ribonucleic Acid (RNA)
Ribonucleic acid (abbreviated RNA) is a nucleic acid present in all living cells that has structural similarities to DNA. Unlike DNA, however, RNA is most often single-stranded. An RNA molecule has a backbone made of alternating phosphate groups and the sugar ribose, rather than the deoxyribose found in DNA. Attached to each sugar is one of four bases: adenine (A), uracil (U), cytosine (C) or guanine (G). Different types of RNA exist in cells: messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA). In addition, some RNAs are involved in regulating gene expression. Certain viruses use RNA as their genomic material.
Sex Chromosome
A sex chromosome is a type of chromosome involved in sex determination. Humans and most other mammals have two sex chromosomes, X and Y, that in combination determine the sex of an individual. Females have two X chromosomes in their cells, while males have one X and one Y.
Somatic Cells
Somatic cells are the cells in the body other than sperm and egg cells (which are called germ cells). In humans, somatic cells are diploid, meaning they contain two sets of chromosomes, one inherited from each parent. DNA mutations in somatic cells can affect an individual, but they cannot be passed on to their offspring.
Stop Codon
A stop codon is a sequence of three nucleotides (a trinucleotide) in DNA or messenger RNA (mRNA) that signals a halt to protein synthesis in the cell. There are 64 different trinucleotide codons: 61 specify amino acids and 3 are stop codons (i.e., UAA, UAG and UGA).
Trait
A trait, as related to genetics, is a specific characteristic of an individual. Traits can be determined by genes, environmental factors or by a combination of both. Traits can be qualitative (such as eye color) or quantitative (such as height or blood pressure). A given trait is part of an individual’s overall phenotype.
Telomere
A telomere is a region of repetitive DNA sequences at the end of a chromosome. Telomeres protect the ends of chromosomes from becoming frayed or tangled. Each time a cell divides, the telomeres become slightly shorter. Eventually, they become so short that the cell can no longer divide successfully, and the cell dies.
Thymine
Thymine (T) is one of the four nucleotide bases in DNA, with the other three being adenine (A), cytosine (C) and guanine (G). Within a double-stranded DNA molecule, thymine bases on one strand pair with adenine bases on the opposite strand. The sequence of the four nucleotide bases encodes DNA’s information.
Transcription
Transcription is the process of making an RNA copy of a gene sequence. This copy, called a messenger RNA (mRNA) molecule, leaves the cell nucleus and enters the cytoplasm, where it directs the synthesis of the protein, which it encodes.
Translation
Translation is the process of translating the sequence of a messenger RNA (mRNA) molecule to a sequence of amino acids during protein synthesis. The genetic code describes the relationship between the sequence of base pairs in a gene and the corresponding amino acid sequence that it encodes. In the cell cytoplasm, the ribosome reads the sequence of the mRNA in groups of three bases to assemble the protein
Tumor Suppressor Gene
A tumor suppressor gene encodes a protein that acts to regulate cell division, keeping it in check. When a tumor suppressor gene is inactivated by a variant, the protein it encodes is not produced or does not function properly, and as a result, uncontrolled cell division may occur. Such variants may contribute to the development of a cancer.
Uracil
Uracil (U) is one of the four nucleotide bases in RNA, with the other three being adenine (A), cytosine (C) and guanine (G). In RNA, uracil pairs with adenine. In a DNA molecule, the nucleotide thymine (T) is used in place of uracil.
X Chromosome
The X chromosome is one of the two sex chromosomes that are involved in sex determination. Humans and most other mammals have two sex chromosomes (X and Y) that in combination determine the sex of an individual. Females have two X chromosomes in their cells, while males have one X and one Y.
Y Chromosome
The Y chromosome is one of the two sex chromosomes that are involved in sex determination. Humans and most other mammals have two sex chromosomes (X and Y) that in combination determine the sex of an individual. Females have two X chromosomes in their cells, while males have one X and one Y.
