Genetic Unit Study Guide DNA The structure, type and functions of a cell are determined by DNA (deoxyribonucleic acid), which is double strands of nucleotides. It is the sequence of these nucleotides that determines cellular activity. During cell division, the DNA is visible as chromosomes, and are found in the nucleus of a cell. DNA determines all the characteristics of an organism, and contains all the genetic material that makes us who we are. This information is passed on from generation to generation in a species so that the information within them can be passed on for the offspring to harness in their lifetime. Structure of DNA and Nucleotides DNA is arranged into a double helix structure where spirals of DNA are intertwined with one another continuously bending in on itself but never getting closer or further away. The following diagram illustrates a nucleotide, the building blocks of DNA There are four different types of nucleotides possible in a DNA sequence, adenine, cytosine, guanine and thymine. There are billions of these nucleotides in our genome, and with all the possible combinations; this is what makes us unique. Nucleotides are situated in adjacent pairs in the double helix nature mentioned. DNA is a double-stranded molecule twisted into a helix (think of a spiral staircase). Each spiraling strand is comprised of a sugar-phosphate backbone and attached bases, and is connected to a complementary strand by non-covalent hydrogen bonding between paired bases. The bases are adenine (A), thymine (T), cytosine (C) and guanine (G). A and T are connected by two hydrogen bonds. G and C are connected by three hydrogen bonds. DNA Replication Cells do not live forever, and in light of this, they must pass their genetic information on to new cells, and be able to replicate the DNA to be passed on to offspring. It is essential that the replication of DNA is EXACT. For this to occur, the following must be available The actual DNA to act as an exact template A pool of relevant and freely available nucleotides A supply of the relevant enzymes to stimulate reaction ATP to provide energy for these reactions When replicating, the double helix structure uncoils so that each strand of DNA can be exposed. When they uncoil, the nucleotides are exposed so that the freely available nucleotides can pair up with them. When all nucleotides are paired up with their new partners, they coil into the double helix. As there are two strands of DNA involved in replication, the first double helix produces 2 copies of itself via each strand. Protein Synthesis As mentioned, a string of nucleotides represent the genetic information that makes us unique and the blueprint of who and what we are, and how we operate. Part of this genetic information is devoted to the synthesis of proteins, which are essential to our body and used in a variety of ways. Proteins are created from templates of information in our DNA. The sequence of these nucleotides are used to determine the order of amino acids used to make a protein. mRNA Although the genetic information is found in the nucleus, protein synthesis actually occurs in ribosomes found in the cytoplasm and on rough endoplasmic reticulum. For proteins to be synthesised, the genetic information in the nucleus must be transferred to the ribosomes. This is done by mRNA (messenger ribonucleic acid) in a process called TRANSCRIPTION. RNA is very similar to DNA, but fundamentally differs in two ways A base called uracil replaces all thymine bases in mRNA. The deoxyribose sugar in DNA in is replaced by ribose sugar in mRNA. At the beginning of protein synthesis, just like DNA replication, the double helix structure of DNA uncoils in order for mRNA to replicate the genetic sequence responsible for the coding of a particular protein. This allows the mRNA to move in and transcribe (copy) the genetic information. Example: if the DNA code is: G-G-C-A-T-T, then the mRNA would be C-C-G-U-A-A (remember uracil replaces thymine). With the genetic information responsible for creating substances now available on the mRNA strand, the mRNA moves out of the nucleus and away from the DNA towards the ribosomes. mRNA and tRNA The mRNA made in the nucleus travels out to the ribosome to carry the "message" of the DNA. Here at the ribosome, that message will be translated into an amino acid sequence. This process is called TRANSLATION. The RNA strand threads through the ribsosome like a tape measure and the amino acids are assembled. The amino acids are assembled according to three nucleotide base sequences called CODONs. Each codon codes for a specific amino acid. Important to the process of translation is another type of RNA called transfer RNA, which function to carry the amino acids to the site of protein synthesis on the ribosome. tRNA contains ANTICODONs, which are also three nucleotide base sequences. This allows the tRNAs to temporarily bond with the mRNA, carrying the correct amino acids. Each type of tRNA will only carry a specific amino acid. These amino acids (peptides) can combine to form a polypeptide chain (proteins), which are used in a variety of structures such as enzymes and hormones There are twenty amino acids that can combine together to form proteins of all kinds. These are the proteins that are used in life processes. When you digest your food for instance, you are using enzymes that were originally proteins that were assembled from amino acids. Example of the mRNA and tRNA relationship: if the mRNA sequence is A-A-U-C-A-U, (codon) then the tRNA sequence is U-U-A-G-U-A (anticodon) the tRNA will carry the amino acid coded by the mRNA (A-A-U and C-A-U) Cell Division Cells divide for: growth and repair creation of gametes (sex cells) as method of reproduction in unicellular organisms Binary Fission - type of reproduction that occurs in bacterial cells, single celled organism splits and becomes two identical organisms Chromosomes and DNA Chromosomes are DNA wrapped around proteins to form an Xshaped structure. Chromosomes are found in the nucleus Chromosomes are made of DNA Chromosome Numbers Each organism has a distinct number of chromosomes, in humans, every cell contains 46 chromosomes. Other organisms have different numbers (eg: dogs have 78 chromosomes per cell). Somatic Cells - body cells, such as muscle, skin, blood ...etc. These cells contain a complete set of chromosomes (46 in humans) and are called DIPLOID (or 2n). Sex Cells - also known as gametes. These cells contain half the number of chromosomes as body cells and are called HAPLOID (or n). Chromosomes come in pairs, called Homologous Pairs (or homologs). Diploid cells have 23 homologous pairs = total of 46 Haploid cells have 23 chromosomes (that are not paired) = total of 23 Sex Determination Chromosomes determine the sex of an offspring. In humans, a pair of chromosomes called SEX CHROMOSOMES determine the sex. XX sex chromosomes – female. XY sex chromosomes - male During fertilization, sperm cells will either contain an X or a Y chromosome (in addition to 22 other chromosomes - total of 23). If a sperm containing an X chromosome fertilizes an egg, the offspring will be female. If a sperm cell containing a Y chromosome fertilizes an egg, the offspring will be male. Creation of a Zygote When two sex cells, or gametes come together, the resulting fertilized egg is called a ZYGOTE Zygotes are diploid and have the total 46 chromosomes (in humans) Karyotype A karyotype is a picture of a person's chromosomes. A karyotype is often done to determine if the offspring has the correct number of chromosomes. An incorrect number of chromosomes indicates that the child will have a condition, like Down Syndrome Compare the Karyotypes below Notice that a person with Down Syndrome has an extra chromosome #21. Instead of a pair, this person has 3 chromosomes - a condition called TRISOMY (tri = three) Trisomy results when chromosomes fail to separate NONDISJUNCTION - when sex cells are created. The resulting egg or sperm has 24 instead of the normal 23. Other conditions result from having the wrong number of chromosomes: Klinefelters Syndrome - XXY (sex chromosomes) Edward Syndrome - Trisomy of chromosome #13 Mitosis This is the type or nuclear division resulting in the formation of two identical cells. Functions in cell growth and development. Interphase: The cell is not dividing at this time period. The nucleus is composed of dark staining material called chromatin, a term that applies to all of the chromosomes collectively. At this stage the DNA is threadlike and not visible as distinct bodies. A nucleolus is clearly visible inside the nucleus. This body is composed of ribosomal RNA and is the site of protein synthesis within the cell. Prior to cell division, the DNA replicates, and two pairs of protein bodies called centrioles are present in the cytoplasm at one end of the cell. Centrioles are not typically present in plant cells. Prophase: One of the centrioles moves to the opposite end of the cell. The opposite ends of the cell are called poles. Each centriole now consists of a pair of protein bodies surrounded by radiating strands of protein called the aster. Plant cells typically do not have the aster or centrioles. The nuclear membrane disintegrates and the DNA shortens and thickens, becoming visible as chromosomes. Each chromosome is doubled and consists of two chromatids, joined by a centromere. Metaphase: The chromosome doublets become arranged in the central region of the cell known as the equator. Protein threads called the spindle connect the centromere region of each chromosome doublet with the centrioles at the poles of the cells. Anaphase: The chromatids separate from each other at the centromere and the single chromosomes move to opposite ends of the cell. When the chromatids separate from each other they are no longer called chromatids. They are now referred to as single chromosomes. The single chromosomes are actually being pulled to opposite ends of the cell as the spindle fibers shorten. Telophase: The chromosomes at each end of the cell begin to organize into separate nuclei, each surrounded by a nuclear membrane. A cleavage furrow or constriction forms in the center of the cell, gradually getting deeper and deeper until the cell is divided into two separate cells. This cytoplasmic division is referred to as cytokinesis. Cytoplasmic division (cytokinesis) in a plant cell is accomplished by a partition or cell plate rather than a cleavage furrow. Interphase: Now we are back to interphase again, but now there are two daughter cells. Each daughter cell is chromosomally identical with the original (mother) cell. They each have a nucleus that contains a nucleolus and chromatin. The centrioles have divided into four protein bodies and the aster has disappeared. The five major phases of plant mitosis. Unlike animals cells, plant cells do not have centrioles or asters. During telophase, a partition or cell plate divides the cytoplasm rather than a cleavage furrow. Meiosis The genetic information found in DNA is essential in creating all the characteristics of an organism. This remains the case when passing genetic information to offspring, that can occur via a process called meiosis where four haploid cells are created from their diploid parent cell. For a species to survive, and genetic information to be preserved and passed on, reproduction must occur. This is done by passing on the information found in the chromosomes via the gametes that are created in meiosis. Chromosome Complement Humans are diploid creatures, meaning that each of the chromosomes in our body are paired up with another. Haploid cells possess only one set of a chromosome. For example, a diploid human cell possesses 46 chromosomes and a gamete created by a human is haploid possesses 23 chromosomes. Reproduction Reproduction occurs with the fusion of two haploid cells (gametes) that create a zygote. The nuclei of both cells fuse, bringing together half the genetic information from the parents into one new cell, that is now genetically different from both its parents. This increases genetic diversity, as half of the genetic content from each of the parents brings about unique offspring, which possesses a unique genome presenting unique characteristics. The Process of Meiosis The process of meiosis involves two cycles of division, involving a germ cell (diploid cell) dividing and then dividing again to form 4 haploid cells. 1st Division (Meiosis I). Here is where the critical difference occurs between meiosis and mitosis. In the latter, all the chromosomes line up on the metaphase plate in no particular order. In Metaphase I, the homologous chromosome pairs are aligned on either side of the metaphase plate. It is during this alignment that chromatid arms may overlap and temporarily fuse, resulting in crossovers Prophase: Homologous chromosomes in the nucleus begin to pair up with one another and then split into chromatids (half of a chromosome) where crossing over can occur. Crossing over can increase genetic variation. Metaphase: Chromosomes line up at the equator of the cell, where the sequence of the chromosomes lined up is at random, through chance, increasing genetic variation via independent assortment. Anaphase: The homologous chromosomes move to opposing poles from the equator Telophase: A new nuclei forms near each pole alongside its new chromosome compliment. At this stage two haploid cells have been created from the original diploid cell of the parent. 2nd Division (Meiosis II). This set of divisions is exactly like those in mitosis. Prophase II: The nuclear membrane disappears and the second meiotic division is initiated. Metaphase II: Pairs of chromatids line up at the equator Anaphase II: Each of these chromatid pairs move away from the equator to the poles via spindle fibers Telophase II: Four new haploid gametes are created that will fuse with the gametes of the opposite sex to create a zygote. Heredity Genes are the sequence of nucleotides coding for a particular trait. The genes are present in the same location of both homologous chromosomes. There are alternative forms of genes, called ALLELES. The combination of alleles is called an organism’s GENOTYPE, and are expressed physically as the PHENOTYPE. Alleles can either be dominant, recessive, or neither. Dominant alleles will express themselves regardless of whether or not the recessive allele is present. Recessive alleles must be present on both chromosomes in order to be expressed. If alleles are neither dominant nor recessive, they can express themselves together (codominant) or as a blend (incomplete dominance). Genetic Mutations Sometimes changes can occur. This is called mutation. In most cases, mutations will result in fatal or negative consequencies in the outcome of the new ogranism. Non-Disjunction and Down's Syndrome. Spindle fibers fail to seperate the chromatids during meiosis, resulting in gametes with one extra chromosome and other gametes lacking a chromosome. Chromosome Mutations. The fundamental structure of a chromosome is subject to mutation, most likely during crossing over. These changes will normally result in significant, and often detrimental, changes to the genotype and phenotype of the organism. If the chromosome mutation effects an essential part of DNA, it is possible that the mutation will abort the offspring before it has the chance of being born. Deletion of a Gene. Genes of a chromosome are permanently lost as they become unattached to the centromere and are lost forever. The new chromosomes will lack certain genes which may prove fatal. This depends on the importance of the lost genes. Duplication of Genes. In this mutation, genes are displayed twice on the same chromosome due to duplication of these genes. This can prove to be an advantageous mutation as no genetic information is lost or altered and new genes are gained. This occurs when genes from a homologous chromosome are copied and inserted into the genetic sequence. This mutation is generally harmless. Inversion of Genes. This is where the order of a particular order of genes is reversed. It occurs when the connection between genes breaks and the sequence of these genes are reversed. The new sequence may not be viable to produce an organism, depending on which genes are reversed. Advantageous characteristics from this mutation are also possible. Translocation of Genes. This is where information from one of two homologous chromosomes breaks and binds to the other. Usually this sort of mutation is lethal Alteration of a DNA Sequence The previous examples of mutation have investigated changes at the chromosome level. The sequence of nucleotides on a DNA sequence are also susceptible to mutation. Deletion. Certain nucleotides are deleted. This affects the coding of proteins that use this DNA sequence. Eg: If a gene coded for alanine, with a genetic sequence of C-G-G, and the cytosine nucleotide was deleted, then the alanine amino acid would not be able to be created, and any other amino acids that are supposed to be coded from this DNA sequence will also be unable to be produced because each successive nucleotide after the deleted nucleotide will be out of place. Insertion.. A nucleotide is inserted into a genetic sequence, altering the chain thereafter. This alteration of a nucleotide sequence is known as Framshift Mutation. Inversion.. A nucleotide sequence is reversed. This is not as serious as the above mutations because the nucleotides that have been reversed only affect a small portion of the whole sequence. Substitution.. A nucleotide is replaced with another, affecting any amino acid to be synthesised from this sequence. If the gene is essential, such as when coding for hemoglobin, then the effects are serious. In this instance a condition called sickle cell anaemia results.