Heredity, Genetics and Protein Synthesis - Essay Example

? Heredity, Genetics and Protein Synthesis - Heredity, Genetics and Protein Synthesis TAQ Write a short essay describing the role of genes and chromosomes in the transmission of human characteristics giving at least one example. In ancient times, people wondered how babies were born and why they share many characteristics of their parents. For long it was a mystery, until Gregor Mandel, an Austrian monk introduced some principles that formed the basis of modern genetics. He explained that these characteristics or heritable properties are transferred as units called genes. (Sharma 2005, p.2). With the development in science and availability of modern research techniques, it was soon confirmed that these genes are nucleotide sequence in a DNA molecule. This sequence is like an instruction manual of how characteristics will be developed in an individual. As Mandel had explained earlier, one characteristic or gene is inherited from each parent. But where are these genes located and how they are transferred from parents to offspring? This was the question that was soon answered following a year after Mandel’s death. It was initially proposed that genetic material is located in nucleus. Walter Sutton and Theodore Boveri proposed in 1903, that genes are carried in special structures called chromosomes. (Sharma 2005, p.14). Genes are set of instruction carried by special molecules called Deoxyribonucleac Acid or DNA. It is this molecule that winds around histone proteins to form a chromosome. DNA has a unique structure that enables it to preserve and transfer genetic material. DNA is a double stranded, anti parallel helical structure with a deoxyribose sugar and a phosphate backbone. Each nucleotide has either a purine or pyramidine nitrogenous base attached to it. Guacine pairs with cytosine and thymine pairs with adenine on complementary strands. This variable sequence of base pair is actually the genetic code. A specific gene is present at fixed location on a chromosome called locus. There are 23 pairs of chromosome present in a somatic cell of a human being and each parent contributes one chromosome to form that pair. How these chromosomes are transferred from parents to offspring? The answer is simple; gametes of each parent contain 23 chromosomes and when two gametes fuse these chromosomes are added together in one cell called zygote. All the subsequent cells that are formed from zygote by the process of mitosis contain identical genetic material. So any somatic cell has a pair of same chromosome, one from mother and one from father. As mentioned earlier, genes are present at fixed locations on a chromosome. If there is a pair of chromosome it means there will be two sets of same gene, one from each parent present at the same loci. The base sequence of these two sets of genes may not be same and this variation in gene is called an allele. (Kail & Cavanaugh 2007, p.44). Now important question is which of these two genes will be expressed in the offspring? Remember that genetic makeup of an individual is called genotype and its physical expression is a phenotype. In most cases, both alleles are expressed to produce a phenotype. Sickle cell disease is caused by mutation in the beta haemoglobin gene. When this gene is expressed it forms an abnormal protein which causes alteration in the shape of red blood cell from biconcave disk to sickle shaped (Bloom, 1995). As part of previous discussion, this gene also has two alleles inherited from each parent. An individual with only one mutated gene will not get this disease because the other normal gene will also be expressed and compensate for abnormal gene. This is a very good example of how genes are key determinants of human characteristics. Not all sequences in a DNA represent a gene. All the genetic material in nucleus and in mitochondria together is called a genome. Only 1% f this genome is actually genes. Thousand of these genes guide species in growth and development of other characteristics. One gene is a complete unit with all the instructions to express one characteristic and one chromosome can carry hundreds of these genes. Only a difference in one sex chromosome can alter many characteristics, can change a phenotype from male into a female even if all the other genetic instructions are same. This shows the strength of genes and chromosome in expressing a phenotype. So it can be concluded that genes are units of organized genetic instructions and are present on special locations in a chromosome. Genetic information is transferred from one generation to another by transmission of these chromosomes as explained by chromosome theory of inheritance. Therefore, the role played by genes and chromosomes in transmission of human characteristics is quiet evident. TAQ 2: In your own words describe the Medelian laws of heredity paying special attention to mono and di-hybrid inheritance. Gregor Mendel experiments with pea plants produced some major results that were further defined as laws of inheritance. He emphasized that any organism contain two alleles for single characteristic or trait inherited from each parent. When these organisms form gametes these two alleles are separated (segregated) and only one of them is present in gamete cell. This is Mendelian’s law of segregation. In his law of independant assortment, Mendel highlighted that inheritance of one trait is does not affect inheritance of other traits. (Mikulecky, Gilman & Peterson 2008, p.232). This simply means that assortment of alleles during gamete formation is not dependant on their parental combination. (Rao & Kaur 2007, p.110) All these laws were deduced from Mendel’s concept and experimentation of mono and dihybrid crossing. Monohybrid cross is when two parents with only one distinct allele or trait are crossed. On the other hand mating plants with two distinct traits such as variable height and pea smoothness is called dihybrid cross (Mikulecky, Gilman & Peterson 2008, p.232). Punnett square diagrams are used to describe both mono and dihybrid crossings. In these diagrams each trait is denoted by a letter and possible combination of these traits is crossed as shown below. TS tS Ts ts TS TTSS TtSS TTSs TtSs tS TtSS ttSS TtSs ttSs Ts TTSs TtSs TTss Ttss ts TtSs ttSs Ttss ttss (T = tall, t = small, S = smooth and s = rough) Although, in this example, there are 9 possible genotypes but there are only four possible phenotypes that are expressed at a ratio of 9:3:3:1. In a monohybrid cross this ratio is 3:1 with two possible phenotypes. TAQ 3: Explain linkage, sex determination and crossing over in chromosomes and their role in transmission. Some genes that are very close together on the same chromosomal region can be linked. Linkage means that the loci of two or more genes are so close that they are mostly inherited together. This is in direct conflict with Mendel’s law of independent assortment. During the prophase of meiosis I, homologous chromosomes align and exchange portions of their DNA, a process known as crossing over. This results in new allele combinations and is a source of genetic diversity There are 23 pairs of chromosome in a human cell. 22 pairs are called somatic chromosome whereas 1 pair is of sex chromosomes. A male has two distinct sex chromosomes, X and Y, instead of two X chromosomes as seen in females. During gamete formation, female gamete or ovum always contain one X chromosome. In males, sperm can either have an X or Y chromosome. If a sperm containing X chromosome fuse with ovum both X combine to give an XX genotype and female phenotype. The resultant zygote is a male if sperm containing Y chromosome fertilize the ovum. Some genetic diseases are sex linked meaning that the mutation causing gene is located on either sex chromosome. They are either called X-linked or Y-linked disorders depending which chromosome contains mutated gene. Fragile X syndrome is a mutation in FMR1 gene located on X chromosome. (Cold Spring Harbour Laboratory, 2002) TAQ 4: Make a list of examples of continuous and discontinuous variation in biology. Continuous Variation In continuous variation, a characteristic is evident in all members of species but the extent is variable. Therefore, there is a measurable range and these characteristics are quantitative. (Pickering 2000, p.198). Continuous variation results from various genes acting together or both genes and environment may play their part. (Toole & Toole, 2004, p. 83) List of common continuous variations: 1. Height 2. Skin color 3. Weight 4. Foot length (length of other parts of the body such as fingers etc) Discontinuous Variation In discontinuous variation there is no range or measureable value of a characteristic. An individual either have it or doesn’t have it. Environment does not play any role and genes alone are responsible for this type of variation. (Pickering 2000, p.198). List of common discontinuous variations: 1. Gender 2. Eye color 3. Blood group 4. Hair color 5. Rolling tongue TAQ 5: Give two examples with a description of the following types of gene and chromosome mutation. De novo Mutations Not all mutations are inherited from parents as some mutations can be de novo. It simply means that mutated allele was not transferred from parents in the germ cells but a healthy allele of the resulted offspring underwent a new mutation. In fact, most of the autosomal dominant diseases are a result of de novo mutations. Achondroplasia is an AD disease and 7/8 cases are caused by de novo mutation with only 1/8 being transmitted by achondroplastic parents (Jorde, et al. 2003, p.68). Autism is another example of de novo mutation. Mosaicism Mosaicism can be defined as presence of two or more cell lines in the body that are genetically distinct (Jorde, et al. 2003, p.68). Most common is the germline mosaicism in which the germ cells are mutated during embryonic development. The somatic cells may not be altered at all. Due to this, parent will not show any effect of germ line mutation but the offspring may express the disease. Unaffected parents with multiple offspring suffering from same disease are a classical picture of mosaicism. Osteogenesis imperfecta type II patients also show similar inheritance in some cases and disease is believed to be caused by this type of mutation. Mosaicism can be a cause of a disease in some percent of cases only. Other examples include Duchenne muscular dystrophy and haemophilia A (Jorde, et al. 2003, pp.68-9) Polymorphism Polymorphism can be defined as variation in DNA sequence or presence of multiple alleles that commonly co-exist in a population. It is important to understand the difference between mutation and polymorphism. Mutation is a variation in DNA sequence that is way different from normal and that variable allele is not commonly found in a population. If a locus has more than one allele and it is present in at least 1% of population then it is polymorphism but if its occurrence is below this value it is called a mutation. (Jorde, et al. 2003, p.29). Polymorphism is not a disease but is found normally in our population. ABO blood group and Rh factor are some examples of polymorphism. TAQ 6: Explain the process of protein synthesis: transcription, translation and amino acid assembly and explain the types of RNA and their and functions. Introduction Protein synthesis is a complex process as it involves various steps, from reading the genetic codes to actually assembling amino acids in a precise order. This process is especially important because it is a way organisms express their genetic information. Genetic instructions stored in the form of DNA molecules are used as a template for the synthesis of proteins. But how these proteins are able to influence to an extent that can change a phenotype? Almost, all of the enzymes that are used as machinery for synthesis of other molecules are proteins. Therefore, specific expressions of these regulatory proteins are nature’s way of giving instructions during development of an organism. Protein synthesis is not a single but a multi step process. The first step involves formation of a messenger molecule that is used as a guide to arrange amino acids in a particular sequence in the subsequent steps. RNA structure To understand steps involved in protein synthesis it is compulsory to first analyze important molecules that takes part in this process. Nucleic acids are the key molecules that form the infrastructure of both DNA and RNA. Nucleic acids are synthesized by assembling nucleotides. A nucleotide consists of a five carbons or pentose sugar, nitrogen containing base and phosphate. The only difference between a DNA and RNA is the type of pentose sugar it contains. RNA has a hydroxyl group attached at the second carbon of pentose sugar, whereas, DNA has only a hydrogen atom bonding at the similar position. RNA types and Function In both eukaryotic and prokaryotic organisms, RNAs performs various functions depending on their type. Ribosomal RNA (rRNA) is the most common or most abundant type of RNA. It forms the structural component of a ribosome along with other proteins. Nucleolus, small aggregated structure of condensed chromatin in a nucleus is the site for rRNA synthesis. After their synthesis in the nucleus they move out in the cytoplasm. Messenger RNA or mRNA is a very important molecule. It is formed as a complementary strand to DNA sequence during transcription. Basically, it is a blueprint of protein synthesis and is used exclusively for this purpose. (Starr, C, Evers, & Starr, L 2010, p.120). In cells, different proteins are required to be synthesized, therefore, many different mRNA each coding for particular protein can be found. For assembly during transcription, different amino acids are required to be aligned in a specific position in a ribosome. This critical process of transporting amino acid is performed by transfer RNA. tRNA is a very short nucleic acid with less than 100 nucleotide arranged in a cloverleaf shape. (Liljas, 2004, p.28). Nucleic Acid Polarity To explain the process of transcription, it is important to first describe the polarity of nucleic acids. Both RNA and DNA have polarity and each end is called either five prime (5’) or three prime (3’) depending on the terminal molecule. A 5’ end has a phosphate molecule attached to fifth carbon of pentose sugar and 3’ end has a hydroxyl group attached at the third carbon of pentose sugar. Transcription As mentioned earlier, DNA cannot be directly translated to form proteins and the base sequence is first copied in the form of a messenger RNA. The process of forming a complementary strand of mRNA from DNA is called transcription. There are various enzymes or polymerases that take part in the formation of messenger RNA. The process of mRNA synthesis is a bit different in a prokaryotic cell as compared to eukaryotic cell. Hence, different RNA polymerases are found in each of these cells. RNA Polymerases Prokaryotes contain only one RNA polymerase that can synthesize all different types of RNAs. On the other hand, three different types of RNA polymerases can be found in eukaryotic cell. RNA polymerase I is responsible for the synthesis of ribosomal RNA and is found in nucleolus. Messenger RNA formed during transcription is synthesized by RNA polymerase II. RNA polymerase III synthesizes transfer RNA. (Garret & Grisham 2010, p. 924). Coding and Template DNA strand DNA is a double stranded helical structure. The strand which runs in 5’-3’ direction is the coding strand. But it is the complementary template strand, 3’-5’, which is used during transcription. Messenger RNA has a same base sequence as coding strand with the exception of thymine which is replaced by uracil in mRNA. Steps in Transcription (Prokaryote) Step 1: Binding of RNA polymerase to the promoter region on the template strand is the first step in transcription. Identification of this region and then binding is facilitated by sigma factor. This sigma factor is released as soon as transcription is initiated. Step 2: As RNA polymerase moves on template strand is in 3’-5’ direction, it synthesize complementary RNA strand in 5’-3 direction. The base pairing is universal. A guanine base is paired with cytosine, whereas, adenine is paired with thymine or uracil. Step 3: RNA polymerase keeps synthesizing mRNA until it reach a termination signal. There are two ways by which this termination takes place. Rho dependent termination is when rho protein moves along the newly formed mRNA and displaces RNA polymerase at the termination site. Rho independent termination takes place when newly formed RNA form a hairpin loop by folding itself and cause dissociation of RNA polymerase. Difference between Prokaryotic and Eukaryotic Transcription The process of transcription in prokaryotic cell is already discussed. However, this process is a bit different in eukaryotes. An important concept to understand here is the fact that prokaryotic gene is continuous and does not have a non coding region. Therefore, mRNAs formed in prokaryotes do not require any post translation splicing or modification. In eukaryotes, newly formed mRNA contains both introns and exons. Introns are the non coding region and required splicing during post transcriptional modification. Another difference is the use of specific polymerases by eukaryotic cell to synthesize different types of RNA. In eukaryotes, RNA polymerase II is responsible for mRNA formation and use transcription factors called (TFIID) instead of sigma factor for initiation of transcription. (Latchman, 2008). Unlike eukaryotes, transcription and translation can occur simultaneously in prokaryotic cell. Translation Translation can be defined as a process in which amino acids are assembled together in a specific sequence to form a protein using the messenger RNA. Nucleotide sequences are read as triplet codes from the messenger RNA and corresponding amino acids defined by those triplet codes are assembled together. A peptide bond is formed between two amino acids during protein synthesis. This peptide bond is formed between carboxyl and hydroxyl group of amino acids with a release of one water molecule. Ribosomes are the structures where protein synthesis takes place. They can be found free in the cytoplasm or bound to endoplasmic reticulum. As discussed earlier, mRNA sequence is exactly same as coding strand of DNA except that mRNA contain uracil base instead of thymine. To further understand how mRNA sequence is exactly read it is important to focus on the concept of genetic codes. Genetic Code A codon is a three base pair genetic code that corresponds to a specific amino acid. There are total of 64 codons out of which 61 codes for amino acid and 3 are stop codons. Starting codon in any mRNA is AUG which codes for methionine in eukaryotes and formylmethionine in prokaryotes. More than one codon can code for same amino acid but one codon can only code for one amino acid. For example, UCU, UCA, UCG and UCC, all these codons code for serine and cannot code for any other amino acid. Due to these properties, these codes are called unambiguous and degenerate. Moreover, these codes are universal which mean that it is same in all organisms. Adaptor molecule and Amino Acid Activation Transfer RNA is responsible for transporting amino acids to ribosome for peptide bond formation. It is important because amino acids do not have direct affinity for mRNA. Transfer RNA in this case serves as an adaptor molecule. After its synthesis, tRNA binds to their appropriate amino acids in the cytoplasm. As mentioned earlier, tRNA has a cloverleaf shape with a phosphate attached at 5’ end and hydroxyl (OH) attached at 3’ end. It is this hydroxyl group to which amino acids are attached after activation. Binding of amino acid to its cognate tRNA is a two step process. In the first step, amino acid is activated by attaching an ATP molecule to form aminocyl-AMP. This activated amino acid can now bond with the hydroxyl group on the tRNA releasing the AMP. Steps of Translation There are three steps involved in translating mRNA to form a protein. These steps are; initiation, elongation and termination. Initiation: Messenger RNA formed during transcription is attached to the small ribosomal subunit. This ribosomal unit identifies specific sequence for binding and then slide mRNA till starting codon, AUG, is reached. tRNA carrying methionine (eukaryotes) or formylmethionine (prokaryotes) binds to this sequence by pairing with its anticodon. Anticodon sequence is complementary and anti parallel to mRNA codon sequence. There are two sites in a ribosome where binding takes place. Peptidyle (P) site is where initiator tRNA first bind. (Glick & Pasternak 2010, p.45). Elongation: Step 1: activated tRNA transport appropriate amino acid to aminoacyl (A) site. mRNA sequence aligned at A site determine which amino acid will be transported. Step 2: peptidyl transferase forms a peptide bond between the carboxyl group of growing peptide chain (site P) and hydroxyl group of newly arrived amino acid (site A). Step 3: in this step ribosome slide 3 nucleotide sequences on mRNA exposing new codon and emptying site A. This process is called translocation. Termination: The process of translation is terminated when a stop codon, UAA, UGA, or UAG, arrives at site A (Glick & Pasternak 2010, p.45). Peptidyl transferase along with other releasing factors detaches the completed protein. Reference BLOOM, M. (1995). Understanding sickle cell disease. Jackson, University Press of Mississippi. COLD SPRING HARBOUR LABORATORY, 2002. DNA Learning Centre. (Online) Available at: http://www.ygyh.org/fragx/whatisit.htm (Accessed 5 October 2012). GARRETT, R., & GRISHAM, C. M. (2010). Biochemistry. Belmont, CA, Brooks/Cole, Cengage Learning. GLICK, B. R., PASTERNAK, J. J., & PATTEN, C. L. (2010). 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Biology: today and tomorrow : with physiology. Belmont, Ca, Brooks/Cole Cengage Learning. TOOLE, G., & TOOLE, S. (2004). Essential A2 biology for OCR. Cheltenham, Nelson Thornes. Read More