copyright The Cell

The Genetic Material

• ---

  The nucleus is where the DNA is kept and RNA is transcribed. RNA is moved out of the nucleus through the nuclear pores. Proteins needed inside the nucleus are transported in through the nuclear pores. The nucleolus is usually visible as a dark spot in the nucleus, and is the location of ribosome formation.

  Ribosomes are where RNA is translated into protein. This process is called protein synthesis. Protein synthesis is very important to cells, therefore large numbers of ribosomes are found in cells. Ribosomes float freely in the cytoplasm, and are also bound to the endoplasmic reticulum (ER). ER bound to ribosomes is called rough ER because the ribosomes on the ER give it a rough sandpaper like look.. These organelles are very small, made up of 50 proteins and several long RNAs bound together. Ribosomes do not have a membrane. Ribosomes fall into two seperate units while not synthesizing protein.

  The endoplasmic reticulum is the transport system for molecules needed for certain changes and specific destinations, instead of molecules that float freely in the cytoplasm. There are two types of ER, rough and smooth. Rough ER has ribosomes attached to it, as mentioned before, and smooth ER does not.

  The golgi changes molecules and divides them into small membrane contained sacs called vesicles. These sacs can be sent to various locations in the cell.

  The lysosome is the digestive system in the cell. It breaks down molecules into their base components digestive enzymes. This demonstrates one of the reasons for having all parts of a cell compartmentalized, the cell couldnt use the destructive enzymes if they werent sealed off from the rest of the cell.

  The instructions for making an organism is called a genome. It is the plan for all structures and activities for the exsitance of the cell or organism. In the nucleus in every single cell, the genome within is made up of coiled threads of deoxyribonucleic acid (DNA) and protein molecules, combined in structures called chromosomes.

  DNA is only 50 trillionths of an inch wide. Every organism has DNA that is the pattern of its life.. Understanding how DNA does this requires some knowledge of its structure and organization.

  In humans and other higher organisms, DNA molecules consists of two strands that wrap around each other to resemble a twisted ladder whose sides, which are made of sugar and phosphate molecules, are connected by rungs of nitrogen- containing chemicals called bases. Each strand is a arrangement of repeating similar units called nucleotides, which are each composed of one sugar, one phosphate, and a nitrogenous base. Four different bases are present in DNA adenine (A), thymine (T), cytosine (C), and guanine (G).

  The order of the bases arranged along the sugar- phosphate backbone is called the DNA sequence; the sequence specifies the exact genetic instructions required to create a particular organism with its own unique traits.

  The two DNA strands are held together by weak bonds between the bases on each strand, forming base pairs (bp). Genome size is usually stated as the total number of base pairs; the human genome contains about 3 billion bp.

  Each time a cell divides into two new cells, its full genome is duplicated; for humans and other complex organisms, this duplication occurs in the nucleus. During the cell division, the DNA molecule unwinds and the weak bonds between the base pairs break, allowing the strands to separate. Each strand directs the synthesis of a new strand, with free nucleotides matching up with their bases on each of the separated strands. Base pairing rules are adhered to, adenine will pair only with thymine (an A - T pair) and cytosine only with guanine (a C - G pair). Each new cell receives one old and one new DNA strand. Since the cells follow the pairing rules, it ensures that the new strand is an exact copy of the old one.

  Each DNA molecule contains many genes, the basic physical and functional units of heredity. A gene is a sequence of nucleotide bases, whose sequences carry the information required for constructing proteins, which provide the structural components of cells and tissues as well as enzymes for essential biochemical reactions, such as the lysosomes digestive enzymes. The human genome is has about one hundred thousand genes.

  Human genes vary in length, often extending over thousands of bases, but only about 10% of the genome is known to include the protein- coding sequences (exons) of genes. All living organisms are composed largely of proteins; humans can synthesize at least 100,000 different kinds. Proteins are large, complex molecules made up of long chains of subunits called amino acids. Twenty different kinds of amino acids are usually found in proteins. Within the gene, each specific sequence of three DNA bases (codons) directs the cells protein synthesizing machinery to add specific amino acids. For example, the base sequence ATG codes for the amino acid methionine. Since 3 bases code for 1 amino acid, the protein coded by an average- sized gene (3000 bp) will contain 1000 amino acids. The genetic code is thus a series of codons that specify which amino acids are required to make up specific proteins.

  The protein coding instructions from the genes are transmitted indirectly through messenger ribonucleic acid (mRNA), a intermediary molecule similar to a single strand of DNA. For the information within a gene to be expressed, a complementary RNA strand is produced (a process called transcription) from the DNA template in the nucleus. This mRNA is moved from the nucleus to the cellular cytoplasm, where it serves as the template for protein synthesis. We have created a program that allows you to convert a CGAT sequence into a protein that shows this.The cells protein synthesizing machinery then translates the codons into a string of amino acids that will constitute the protein molecule for which it codes. In the laboratory, the mRNA molecule can be isolated and used as a template to synthesize a complementary DNA (cDNA) strand, which can then be used to locate the corresponding genes on a chromosome map.

  The 3 billion bp in the human genome are organized into 24 units called chromosomes. All genes are arranged linearly along the chromosomes. The nucleus of most human cells contains 2 sets of chromosomes, 1 set given by each parent. (The expection to this is the sex cells, they have half the normal, to combine and add up to the normal amount). Each set has 23 single chromosomes, 22 autosomes and an X or Y sex chromosome. (A normal female will have a pair of X chromosomes; a male will have an X and Y pair.) Chromosomes contain roughly equal parts of protein and DNA; chromosomal DNA contains an average of 150 million bases.

  Chromosomes can be seen under a light microscope and, when stained with certain dyes, reveal a pattern of light and dark bands reflecting regional variations in the amounts of A and T vs G and C. Differences in size and banding pattern allow the 24 chromosomes to be distinguished from each other, an analysis called a karyotype. A few abnormalities, including missing or extra copies of a chromosome or breaks and rejoinings, can be detected by microscopic examination; Downs syndrome, in which an individual's cells contain a third copy of chromosome 21, is diagnosed by karyotype analysis. Most changes in DNA, however, are too subtle to be detected by this technique and require molecular analysis. These subtle DNA abnormalities (mutations) are responsible for many inherited diseases such as cystic fibrosis and sickle cell anemia or may predispose an individual to cancer, major psychiatric illnesses, and other complex diseases.

  Mitochondria (singular: mitochondrion) are the sites of aerobic respiration, and generally are the major energy production center in eukaryotes. Mitochondria have two membranes, an inner and an outer, visible in this electron microscope photo of a mitochondrion. Note the reticulations, or many infoldings, of the inner membrane, This serves to increase the surface area of membrane on which membrane-bound reactions can take place. The existence of this double membrane has led many biologists to theorize that mitochondria are the descendants of some bacteria that was endocytosed by a larger cell billions of years ago, but not digested. This fascinating theory of symbiosis, which might lend an explanation to the development of eukaryotic cells, has additional supporting evidence. Mitochondria have their own DNA and their own ribosomes; and those ribosomes are more similar to bacterial ribosomes than to eukaryotic ribosomes.

  Chloroplasts are the site of photosynthesis in eukaryotic cells. They are disk-like structures composed of a single membrane surrounding a fluidcontaining stacks of membranous disks. Because of their geen color chloroplast are the only organelles that can be easily seen with a light microscope.

  There is a single membrane surrounding the chloroplast. This membrane surrounds a fluid called thestroma. Floating in the stroma are stacks of disks made up of membranes, these are the grana. The grana resemble a stack of coins, however instead of coins this stack is made up of individual hollow disks called thylakoids.

  That's it! You have made it through lesson 2, the parts of cells. If you want to proceed to the next lesson, first take the lesson 2 quiz.