CIS Biology Team Intro. In order to understand DNA, Chromosomes and Genes (and how they are related),one must understand a little bit about a cell and its nucleus. When the cell is not dividing, its nucleus contains chromatin, a combination of protein and DNA. When the cell is dividing into two new cells, the DNA contained within the nucleus is duplicated via mitosis, which ensures that each new cell contains a complete copy of all genetic material found within the parent cell. Mitosis is a process by which a eukaryotic cell (a cell containing complex structures) separates the chromosomes within its nucleus into two identical sets of nuclei. During the division of a cell, the chromatin organizes itself into chromosomes. Each chromosome contains a DNA molecule, and each molecule is comprised of many genes, each containing instructions needed to direct the synthesis of a protein for a specific function. DNA DNA, or Deoxyribonucleic acid, is a nucleic acid that contains the genetic instructions that are used in the development and functioning of all living organisms, with the exception of an RNA virus. The role of DNA molecules is the storage of information for the long-term. The instructions it contains are needed to construct other components of cells, such as RNA molecules and proteins, much like a blueprint. The segments in a DNA strand that contain this information are called genes, but other sequences of DNA have structural purposes, or are for regulating the use of the information. DNA consists of two long polymers made from repeating units called nucleotides, with sugars and phosphate groups joined by ester bonds. These strands run in opposite directions, and are therefore antiparallel. Attached to each sugar is one of four bases known as nucleobases. These four bases are Adenine, Guanine, Thymine and Cytosine. These four bases appear in the DNA, so they are known as DNA-bases. There is a fifth base called Uracil, which appears in RNA. The sequences of these nucleobases encode information that is read using the genetic code, which specifies the sequence of amino acids within proteins. The code is read by copying a stretch of DNA into the related nucleic acid RNA in a process called transcription. A nucleobase linked to a sugar is called a nucleoside, and a base linked to a sugar and one or more phosphate groups is called a nucleotide. The table below shows the Nucleobases, and when they are linked with a sugar. CIS Biology Team Nucleobase Adenine Guanine Thymine Cytosine Uracil Nucleoside Adenosine A Guanosine G Thymidine T Cytidine C Uridine U Chromosomes So what exactly are chromosomes? Chromosomes are organized structures of DNA and protein found within a cell. It is a single strand of DNA containing many genes, regulatory elements and other nucleotide sequences. A regulatory sequence is a segment of DNA where regulatory proteins (DNA-binding protein) bind in a preferential manner. These proteins bind to short stretches of DNA called regulatory regions, which are positioned a short distance from the gene being regulated. Chromosomes vary widely between different organisms. The DNA molecule may be circular or linear, and can be composed of a varying number of nucleotides. Nucleotides are molecules that make up the structural units of RNA and DNA when joined together. A typical eukaryotic cell (cells with nuclei) has linear chromosomes, while prokaryotic cells (cells without defined nuclei) have circular chromosomes, though there are exceptions to both rules. In addition to this, cells may contain more than one type of chromosomes. For example, the mitochondria present within eukaryotes have their own small chromosomes. In eukaryotes, chromosomes are packaged by proteins into a condensed structure known as chromatin. This allows the strand of DNA to fit within the nucleus. Chromosomes are also essential for cellular division, and must be replicated, divided and passed successfully to their daughter cells to ensure genetic diversity and the survival of their offspring. Compaction of the duplicated chromosomes during mitosis and meiosis generally result in the classic four armed structure. Chromosomes can be divided into two types – autosomes and sex chromosomes. Certain generic traits linked to a person’s sex are passed on through the sex chromosomes. Put simply, the autosomes contain everything else (the rest of the genetic hereditary information). All act in the same way during cell division. CIS Biology Team The images above represents the 23 paired chromosomes in an average human. For those who are struggling with the math, that’s 46 total chromosomes in a human. The image on the left shows the karyogram of a human male, as noted by the sex chromosomes (pair #23), which contains one X and one Y chromosome. This is known as heterogametic sex. The image on the right shows the karyogram of a human female, which is shown through the presence of two X chromosomes. This is known as homogametic sex. Note that the presence of solely two Y chromosomes is impossible. Females and males alike both retain one of their mother’s X chromosomes, and females receive the second X chromosome from their fathers (or rather from their paternal grandmother on her father’s side, as the father retains his X chromosome from his mother). Males therefore receive their Y chromosome from their fathers as well. While the X chromosome does contain a few thousand genes, few of those genes actually play a part in sex determination; instead they are autosomal genes, genes that code for several proteins related to non-sexual traits. In mammals, the Y chromosome contains a gene known as SRY (Sex-determining region Y), which triggers development as a male. The Y chromosomes also contain genes necessary for the production of sperm. During the embryonic development in females, one of the X chromosomes is randomly inactivated, permanently. This happens in nearly all somatic cells (a biological cell forming the body of an organism). This phenomenon is known as X-inactivation or Lyonization and results in the creation of a barr body. Since women have two X chromosomes, one of them is deactivated to ensure that the female does not have twice as many X chromosome gene products as the male, which possesses only a single copy of the X chromosome. Once the chromosome is deactivated, it will forever be deactivated. When seen through a microscope, the inactive X chromosome is seen as a granule on the nucleus. This granule is known as a Barr body. A Barr body is the inactive X chromosome in a female somatic cell. In men and women with more than one X chromosome, the number of Barr bodies visible at interphase is always one less than the total number of X chromosomes present. Note that X-inactivation isn’t generally confined to women alone, but to anyone with two X chromosomes present. In order to better understand Barr bodies, here is an example: A man with a CIS Biology Team 47, XXY karyotype1 have a single Barr body, whilst a woman with a 47, XXX karyotype have two Barr bodies. The image below points out the location of a Barr body. Besides the common XX/XY system for sex determination, there are other sex determination systems. The XX/XO system is used for many insects. The females are XX, signifying two X chromosomes, and the males are XO, signifying only one X chromosome. The O represents the lack of another X chromosome. There is also the ZW sex determination system, which is used for birds and certain fish and insects. Males are the homogametic sex in this system, using ZZ, while females are heterogametic, using ZW. The Z chromosome is larger and contains more genes, much like the X chromosome in the XX/XY system. Genes The image above is an illustration of the placement of genes within a chromosome. A gene can be defined as a region of DNA that controls one, and only one hereditary characteristic. It usually corresponds to a sequence used during the production of a specific protein or of RNA. Living organisms depend on genes to function, as they hold the information needed to build and maintain 1 Karyotype is the number and appearance of chromosomes in the nucleus of a eukaryotic cell. It can also refer to the complete set of chromosomes in a species, or an individual organism. CIS Biology Team the cells within an organism and to pass on the cells traits to offspring. Genes can code for various traits, such as eye color, blood type, number of eyes etc. When a gene is copied and transmitted from the cell to its offspring, everything is copied. This includes the coding DNA sequences, noncoding regulatory DNA sequences and introns. An intron is a nucleotide sequence within a gene that is removed via RNA splicing to generate the final product of a gene. The total complement of genes in an organism or cell is known as its genome. In prokaryotes, the majority of genes are located on a chromosome of circular DNA, while eurkaryotes usually possess multiple linear DNA helices packed into complexes called chromosomes. Many species can carry more than one copy of their genome within each somatic cell. Cells with only one copy are called haploid, while those with two are called diploid. Those with three or more are called polypoids. CIS Biology Team Bibliography Bailey, Â Regina. "Nucleic Acids." Biology. N.p., n.d. Web. 15 Aug. 2011. <http://biology.about.com/od/molecularbiology/a/nucleicacids.htm>. "Cells and DNA - Genetics Home Reference." Genetics Home Reference - Your guide to understanding genetic conditions. N.p., n.d. Web. 15 Aug. 2011. <http://ghr.nlm.nih.gov/handbook/basics>. Copasetic. "XY Chromosomes and XX What about YY?." Unexplained Mysteries - Paranormal Phenomena and the World's Greatest Unexplained Mysteries. N.p., n.d. Web. 15 Aug. 2011. <http://www.unexplained-mysteries.com/forum/index.php?showtopic=148559>. Kimball, John. "Nucleotides." Kimball's Biology Pages. N.p., 24 Feb. 2011. Web. 15 Aug. 2011. <http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/Nucleotides.html>. Petty, Yvette . "DNA, Chromosomes and Genes." DNA Tutorial. N.p., n.d. Web. 15 Aug. 2011. <http://www.dnatutorial.com/DNAChromosomes.shtml>. "Sex Determination and Sex-linked Inheritance." Biology Questions. N.p., n.d. Web. 15 Aug. 2011. <http://www.biology-questions-and-answers.com/sex-chromosomes.html>.