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How Genes Work

Proteins, cell growth, and cell division

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Please choose from the following list of questions for information about genes and their functions within cells.

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What are proteins and what do they do?

Proteins are large, complex molecules that play many critical roles in the body. They do most of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs.

Proteins are made up of hundreds or thousands of smaller units called amino acids, which are attached to one another in long chains. There are 20 different types of amino acids that can be combined to make a protein. The sequence of amino acids determines each protein’s unique 3-dimensional structure and its specific function.

Proteins can be described according to their large range of functions in the body, listed in alphabetical order:

Examples of protein functions
Function Description Example
Antibody Antibodies bind to specific foreign particles, such as viruses and bacteria, to help protect the body. Immunoglobulin G (IgG) (illustration)
Enzyme Enzymes carry out almost all of the thousands of chemical reactions that take place in cells. They also assist with the formation of new molecules by reading the genetic information stored in DNA. Phenylalanine hydroxylase (illustration)
Messenger Messenger proteins, such as some types of hormones, transmit signals to coordinate biological processes between different cells, tissues, and organs. Growth hormone (illustration)
Structural component These proteins provide structure and support for cells. On a larger scale, they also allow the body to move. Actin (illustration)
Transport/storage These proteins bind and carry atoms and small molecules within cells and throughout the body. Ferritin (illustration)

For more information about proteins and their functions:

KidsHealth from Nemours offers a basic overview of proteinsThis link leads to a site outside Genetics Home Reference. and what they do.

Nature Education’s Scitable offers information on the function of proteins.This link leads to a site outside Genetics Home Reference.

Arizona State University’s “Ask a Biologist” discusses the different kinds of proteinsThis link leads to a site outside Genetics Home Reference. and what they do.


How do genes direct the production of proteins?

Most genes contain the information needed to make functional molecules called proteins. (A few genes produce other molecules that help the cell assemble proteins.) The journey from gene to protein is complex and tightly controlled within each cell. It consists of two major steps: transcription and translation. Together, transcription and translation are known as gene expression.

During the process of transcription, the information stored in a gene’s DNA is transferred to a similar molecule called RNA (ribonucleic acid) in the cell nucleus. Both RNA and DNA are made up of a chain of nucleotide bases, but they have slightly different chemical properties. The type of RNA that contains the information for making a protein is called messenger RNA (mRNA) because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm.

Translation, the second step in getting from a gene to a protein, takes place in the cytoplasm. The mRNA interacts with a specialized complex called a ribosome, which “reads” the sequence of mRNA bases. Each sequence of three bases, called a codon, usually codes for one particular amino acid. (Amino acids are the building blocks of proteins.) A type of RNA called transfer RNA (tRNA) assembles the protein, one amino acid at a time. Protein assembly continues until the ribosome encounters a “stop” codon (a sequence of three bases that does not code for an amino acid).

The flow of information from DNA to RNA to proteins is one of the fundamental principles of molecular biology. It is so important that it is sometimes called the “central dogma.”

Through the processes of transcription and translation, information from genes is used to make proteins.

Through the processes of transcription and translation, information from genes is used to make proteins.

For more information about making proteins:

The Genetic Science Learning Center at the University of Utah offers an interactive introduction to transcription and translationThis link leads to a site outside Genetics Home Reference..

Information about RNAThis link leads to a site outside Genetics Home Reference., transcriptionThis link leads to a site outside Genetics Home Reference., and translationThis link leads to a site outside Genetics Home Reference. is available from GeneEd.

North Dakota State University’s Virtual Cell Animation Collection offers videos that illustrate the processes of transcriptionThis link leads to a site outside Genetics Home Reference. and translationThis link leads to a site outside Genetics Home Reference..

The New Genetics, a publication of the National Institute of General Medical Sciences, includes discussions of transcriptionThis link leads to a site outside Genetics Home Reference. and translationThis link leads to a site outside Genetics Home Reference..


Can genes be turned on and off in cells?

Each cell expresses, or turns on, only a fraction of its genes. The rest of the genes are repressed, or turned off. The process of turning genes on and off is known as gene regulation. Gene regulation is an important part of normal development. Genes are turned on and off in different patterns during development to make a brain cell look and act different from a liver cell or a muscle cell, for example. Gene regulation also allows cells to react quickly to changes in their environments. Although we know that the regulation of genes is critical for life, this complex process is not yet fully understood.

Gene regulation can occur at any point during gene expression, but most commonly occurs at the level of transcription (when the information in a gene’s DNA is transferred to mRNA). Signals from the environment or from other cells activate proteins called transcription factors. These proteins bind to regulatory regions of a gene and increase or decrease the level of transcription. By controlling the level of transcription, this process can determine the amount of protein product that is made by a gene at any given time.

For more information about gene regulation:

The Genetic Science Learning Center at the University of Utah offers an explanation of gene expression as it relates to disease riskThis link leads to a site outside Genetics Home Reference..

Education Portal provides information about gene regulation in Regulation of Gene Expression: Transcriptional Repression and InductionThis link leads to a site outside Genetics Home Reference..

Additional information about gene expressionThis link leads to a site outside Genetics Home Reference. is available from the Wellcome Trust.


What is the epigenome?

DNA modifications that do not change the DNA sequence can affect gene activity. Chemical compounds that are added to single genes can regulate their activity; these modifications are known as epigenetic changes. The epigenome comprises all of the chemical compounds that have been added to the entirety of one’s DNA (genome) as a way to regulate the activity (expression) of all the genes within the genome. The chemical compounds of the epigenome are not part of the DNA sequence, but are on or attached to DNA (“epi-“ means above in Greek). Epigenomic modifications remain as cells divide and in some cases can be inherited through the generations. Environmental influences, such as a person’s diet and exposure to pollutants, can also impact the epigenome.

Epigenetic changes can help determine whether genes are turned on or off and can influence the production of proteins in certain cells, ensuring that only necessary proteins are produced. For example, proteins that promote bone growth are not produced in muscle cells. Patterns of epigenome modification vary among individuals, different tissues within an individual, and even different cells.

A common type of epigenomic modification is called methylation. Methylation involves attaching small molecules called methyl groups, each consisting of one carbon atom and three hydrogen atoms, to segments of DNA. When methyl groups are added to a particular gene, that gene is turned off or silenced, and no protein is produced from that gene.

Because errors in the epigenetic process, such as modifying the wrong gene or failing to add a compound to a gene, can lead to abnormal gene activity or inactivity, they can cause genetic disorders. Conditions including cancers, metabolic disorders, and degenerative disorders have all been found to be related to epigenetic errors.

Scientists continue to explore the relationship between the genome and the chemical compounds that modify it. In particular, they are studying what effect the modifications have on gene function, protein production, and human health.

For more information about the epigenome:

The National Institutes of Health (NIH) offers the NIH Roadmap Epigenomics ProjectThis link leads to a site outside Genetics Home Reference., which provides epigenome maps of a variety of cells to begin to assess the relationship between epigenomics and human disease.

The National Center for Biotechnology Information (NCBI) provides the NCBI EpigenomicsThis link leads to a site outside Genetics Home Reference. database of maps of the epigenomes of various species and many cell types.  

Human Epigenome AtlasThis link leads to a site outside Genetics Home Reference. from Baylor College of Medicine allows for comparison of the epigenomes of many species and cell types.  

Ongoing research is being done with the Human Epigenome ProjectThis link leads to a site outside Genetics Home Reference.

The University of Utah provides an interactive epigenetics tutorialThis link leads to a site outside Genetics Home Reference..

The National Human Genome Research Institute provides a fact sheetThis link leads to a site outside Genetics Home Reference. on Epigenomics.

Many tools for understanding epigenomics are available through the NIH Common Fund Epigenomics ProjectThis link leads to a site outside Genetics Home Reference..


How do cells divide?

There are two types of cell division: mitosis and meiosis. Most of the time when people refer to “cell division,” they mean mitosis, the process of making new body cells. Meiosis is the type of cell division that creates egg and sperm cells.

Mitosis is a fundamental process for life. During mitosis, a cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells. Because this process is so critical, the steps of mitosis are carefully controlled by a number of genes. When mitosis is not regulated correctly, health problems such as cancer can result.

The other type of cell division, meiosis, ensures that humans have the same number of chromosomes in each generation. It is a two-step process that reduces the chromosome number by half—from 46 to 23—to form sperm and egg cells. When the sperm and egg cells unite at conception, each contributes 23 chromosomes so the resulting embryo will have the usual 46. Meiosis also allows genetic variation through a process of DNA shuffling while the cells are dividing.

Mitosis and meiosis, the two types of cell division.

Mitosis and meiosis, the two types of cell division.

For more information about cell division:

Information about mitosisThis link leads to a site outside Genetics Home Reference. and meiosisThis link leads to a site outside Genetics Home Reference. is available from GeneEd.

The University of Illinois at Chicago offers an outline of meiosis, mitosis, and regulation of the cell cycleThis link leads to a site outside Genetics Home Reference..

North Dakota State University’s Virtual Cell Animation Collection offers videos that illustrate the processes of mitosisThis link leads to a site outside Genetics Home Reference. and meiosisThis link leads to a site outside Genetics Home Reference..

Nature Education’s Scitable explains how DNA is copied and distributed during the stages of meiosisThis link leads to a site outside Genetics Home Reference..


How do genes control the growth and division of cells?

A variety of genes are involved in the control of cell growth and division. The cell cycle is the cell’s way of replicating itself in an organized, step-by-step fashion. Tight regulation of this process ensures that a dividing cell’s DNA is copied properly, any errors in the DNA are repaired, and each daughter cell receives a full set of chromosomes. The cycle has checkpoints (also called restriction points), which allow certain genes to check for mistakes and halt the cycle for repairs if something goes wrong.

If a cell has an error in its DNA that cannot be repaired, it may undergo programmed cell death (apoptosis) (illustration). Apoptosis is a common process throughout life that helps the body get rid of cells it doesn’t need. Cells that undergo apoptosis break apart and are recycled by a type of white blood cell called a macrophage (illustration). Apoptosis protects the body by removing genetically damaged cells that could lead to cancer, and it plays an important role in the development of the embryo and the maintenance of adult tissues.

Cancer results from a disruption of the normal regulation of the cell cycle. When the cycle proceeds without control, cells can divide without order and accumulate genetic defects that can lead to a cancerous tumor (illustration).

For more information about cell growth and division:

The National Institutes of Health’s Apoptosis Interest GroupThis link leads to a site outside Genetics Home Reference. provides an introduction to programmed cell death.

The National Cancer Institute’s fact sheet What is Cancer?This link leads to a site outside Genetics Home Reference. explains the growth of cancerous tumors.

Nature Education’s Scitable explans how cells become cancerousThis link leads to a site outside Genetics Home Reference..


How do geneticists indicate the location of a gene?

Geneticists use maps to describe the location of a particular gene on a chromosome. One type of map uses the cytogenetic location to describe a gene’s position. The cytogenetic location is based on a distinctive pattern of bands created when chromosomes are stained with certain chemicals. Another type of map uses the molecular location, a precise description of a gene’s position on a chromosome. The molecular location is based on the sequence of DNA building blocks (base pairs) that make up the chromosome.

Cytogenetic location

Geneticists use a standardized way of describing a gene’s cytogenetic location. In most cases, the location describes the position of a particular band on a stained chromosome:

17q12

It can also be written as a range of bands, if less is known about the exact location:

17q12-q21

The combination of numbers and letters provide a gene’s “address” on a chromosome. This address is made up of several parts:

  • The chromosome on which the gene can be found. The first number or letter used to describe a gene’s location represents the chromosome. Chromosomes 1 through 22 (the autosomes) are designated by their chromosome number. The sex chromosomes are designated by X or Y.

  • The arm of the chromosome. Each chromosome is divided into two sections (arms) based on the location of a narrowing (constriction) called the centromere. By convention, the shorter arm is called p, and the longer arm is called q. The chromosome arm is the second part of the gene’s address. For example, 5q is the long arm of chromosome 5, and Xp is the short arm of the X chromosome.

  • The position of the gene on the p or q arm. The position of a gene is based on a distinctive pattern of light and dark bands that appear when the chromosome is stained in a certain way. The position is usually designated by two digits (representing a region and a band), which are sometimes followed by a decimal point and one or more additional digits (representing sub-bands within a light or dark area). The number indicating the gene position increases with distance from the centromere. For example: 14q21 represents position 21 on the long arm of chromosome 14. 14q21 is closer to the centromere than 14q22.

Sometimes, the abbreviations “cen” or “ter” are also used to describe a gene’s cytogenetic location. “Cen” indicates that the gene is very close to the centromere. For example, 16pcen refers to the short arm of chromosome 16 near the centromere. “Ter” stands for terminus, which indicates that the gene is very close to the end of the p or q arm. For example, 14qter refers to the tip of the long arm of chromosome 14. (“Tel” is also sometimes used to describe a gene’s location. “Tel” stands for telomeres, which are at the ends of each chromosome. The abbreviations “tel” and “ter” refer to the same location.)

The  gene is located on the long arm of  at position 7q31.2.

The CFTR gene is located on the long arm of chromosome 7 at position 7q31.2.

Molecular location

The Human Genome Project, an international research effort completed in 2003, determined the sequence of base pairs for each human chromosome. This sequence information allows researchers to provide a more specific address than the cytogenetic location for many genes. A gene’s molecular address pinpoints the location of that gene in terms of base pairs. It describes the gene’s precise position on a chromosome and indicates the size of the gene. Knowing the molecular location also allows researchers to determine exactly how far a gene is from other genes on the same chromosome.

Different groups of researchers often present slightly different values for a gene’s molecular location. Researchers interpret the sequence of the human genome using a variety of methods, which can result in small differences in a gene’s molecular address. Genetics Home Reference presents data from NCBIThis link leads to a site outside Genetics Home Reference. for the molecular location of genes.

For more information on genetic mapping:

The National Human Genome Research Institute explains how researchers create a genetic mapThis link leads to a site outside Genetics Home Reference..

The University of Washington provides a Cytogenetics GalleryThis link leads to a site outside Genetics Home Reference. that includes a description of chromosome banding patternsThis link leads to a site outside Genetics Home Reference..

Information about assembling and annotating the genomeThis link leads to a site outside Genetics Home Reference. is available from NCBI.


What are gene families?

A gene family is a group of genes that share important characteristics. In many cases, genes in a family share a similar sequence of DNA building blocks (nucleotides). These genes provide instructions for making products (such as proteins) that have a similar structure or function. In other cases, dissimilar genes are grouped together in a family because proteins produced from these genes work together as a unit or participate in the same process.

Classifying individual genes into families helps researchers describe how genes are related to each other. Researchers can use gene families to predict the function of newly identified genes based on their similarity to known genes. Similarities among genes in a family can also be used to predict where and when a specific gene is active (expressed). Additionally, gene families may provide clues for identifying genes that are involved in particular diseases.

Sometimes not enough is known about a gene to assign it to an established family. In other cases, genes may fit into more than one family. No formal guidelines define the criteria for grouping genes together. Classification systems for genes continue to evolve as scientists learn more about the structure and function of genes and the relationships between them.

For more information about gene families

Genetics Home Reference provides information about gene families including a brief description of each gene family and a list of the genes included in the family.

The HUGO Gene Nomenclature CommitteeThis link leads to a site outside Genetics Home Reference. (HGNC) has classified many human genes into families. Each grouping is given a name and symbol, and contains a table of the genes in that family.

The textbook Human Molecular Genetics (second edition, 1999) provides background information on human gene familiesThis link leads to a site outside Genetics Home Reference..

The Gene OntologyThis link leads to a site outside Genetics Home Reference. database lists the protein products of genes by their location within the cell (cellular component), biological process, and molecular function.

The ReactomeThis link leads to a site outside Genetics Home Reference. database classifies the protein products of genes based on their participation in specific biological pathways. For example, this resource provides tables of genes involved in controlled cell death (apoptosis), cell division, and DNA repair.


Next: Mutations and Health

 
Published: December 22, 2014