Chromosome Structure and Function

Nuclear Chromosome
Colorized Image of a Eukaryotic Nuclear Chromosome. Photolibrary/Getty Images

A chromosome is a long, stringy aggregate of genes that carries heredity information and is formed from condensed chromatin. Chromatin is composed of DNA and proteins that are tightly packed together to form chromatin fibers. Condensed chromatin fibers form chromosomes. Chromosomes are located within the nucleus of our cells. They are paired together (one from the mother and one from the father) and are known as homologous chromosomes. During cell division, chromosomes are replicated and distributed equally among each new daughter cell.

Key Takeaways: Chromosomes

  • Chromosomes are composed of DNA and proteins packed tightly to form long chromatin fibers. Chromosomes house genes responsible for the inheritance of traits and guidance of life processes.
  • Chromosome structure consists of a long arm region and a short arm region connected at a central region known as a centromere. The ends of a chromosome are called telomeres.
  • Duplicated or replicated chromosomes have the familiar X-shape and are composed of identical sister chromatids.
  • During cell division, sister chromatids separate and are incorporated into new daughter cells.
  • Chromosomes contain the genetic codes for protein production. Proteins regulate vital cellular processes and provide structural support for cells and tissues.
  • Chromosome mutations result in changes in chromosome structure or changes in cellular chromosome numbers. Mutations most often have harmful consequences.

Chromosome Structure

Telomeres
A telomere is a region of the DNA sequence at the end of a chromosome. Their function is to protect the ends of the chromosome from degradation. Here they are visible as highlights at the tips of the chromosomes. Credit: Science Picture Co/Subjects/Getty Images

A non-duplicated chromosome is single-stranded and consists of a centromere region that connects two arm regions. The short arm region is called the p arm and the long arm region is called the ​q arm. The end region of a chromosome is called a telomere. Telomeres consist of repeating non-coding DNA sequences that get shorter as a cell divides.

Chromosome Duplication

Chromosome duplication occurs prior to the division processes of mitosis and meiosis. DNA replication processes allow correct chromosome numbers to be preserved after the original cell divides. A duplicated chromosome is comprised of two identical chromosomes called sister chromatids that are connected at the centromere region. Sister chromatids remain together until the end of the division process where they are separated by spindle fibers and enclosed within separate cells. Once the paired chromatids separate from one another, each is known as a daughter chromosome.

Chromosomes and Cell Division

Sister Chromatids
Chromosomes are threadlike structures composed of DNA and proteins. During cell division, chromosomes consist of two arms, or chromatids, which are joined by a centromere. Joined chromatids are called sister chromatids. Credit: Adrian T Sumner/The Image Bank/Getty Images

One of the most important elements of successful cell division is the correct distribution of chromosomes. In mitosis, this means that chromosomes must be distributed between two daughter cells. In meiosis, chromosomes must be distributed among four daughter cells. The cell's spindle apparatus is responsible for moving chromosomes during cell division. This type of cell movement is due to interactions between spindle microtubules and motor proteins, which work together to manipulate and separate chromosomes.

It is vitally important that a correct number of chromosomes be preserved in dividing cells. Errors that occur during cell division may result in individuals with unbalanced chromosome numbers. Their cells may have either too many or not enough chromosomes. This type of occurrence is known as aneuploidy and may happen in autosomal chromosomes during mitosis or in sex chromosomes during meiosis. Anomalies in chromosome numbers can result in birth defects, developmental disabilities, and death.

Chromosomes and Protein Production

Reverse Transcription
DNA is transcribed and translated to produce proteins. Reverse transcription converts RNA to DNA. ttsz/iStock/Getty Images Plus 

Protein production is a vital cell process that is dependent upon chromosomes and DNA. Proteins are important molecules that are necessary for almost all cell functions. Chromosomal DNA contains segments called genes that code for proteins. During protein production, the DNA unwinds and its coding segments are transcribed into an RNA transcript. This copy of the DNA message is exported from the nucleus and then translated to form a protein. Ribosomes and another RNA molecule, called transfer RNA, work together to bind to the RNA transcript and convert the coded message into a protein.

Chromosome Mutation

Genetic Mutation
Genetic Mutation. BlackJack3D/E+/Getty Images

Chromosome mutations are changes that occur in chromosomes and are typically the result of either errors that happen during meiosis or by exposure to mutagens such as chemicals or radiation. Chromosome breakage and duplications can cause several types of chromosome structural changes that are typically harmful to the individual. These types of mutations result in chromosomes with extra genes, not enough genes, or genes that are in the wrong sequence. Mutations can also produce cells that have abnormal numbers of chromosomes. Abnormal chromosome numbers typically occur as a result of nondisjunction or the failure of homologous chromosomes to separate properly during meiosis.