HEREDITY, GENETICS AND PROTEIN SYNTHESIS
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Genes and Chromosomes and their role.
A chromosome is the threadlike microscopic material that is found in the nucleus of a cell. It is responsible for housing the hereditary material or information, and it houses them in the form of genes (Encyclopedia Britannica 2013). They are mostly found at the center of the nucleus, and they carry with them long pieces of DNA, which are the building components of the human body. According to research, chromosomes are always in pairs, and the human cell contains twenty-three pairs of chromosomes (Encyclopedia Britannica 2013). This makes a total of forty-six chromosomes. Fifty percent of the chromosomes are obtained from the father, while the other fifty are obtained from the father. The individual chromosomes are given names. Some of the most talked about are the X and y chromosomes, which are also referred to as sex chromosomes. These are the chromosomes involved in determining the sex of the child. The other chromosomes, that is, apart from the sex chromosomes are referred to as autosomes (Encyclopedia Britannica 2013). In regard to the sex chromosomes, the females have two x chromosomes, while the males have two y chromosomes. The males have one y and one x chromosome. It is vital to note that chromosomes are very small, hence are not visible in the nucleus of the cell unless when the cell is dividing.
A microscopic view of the chromosome reveals a constriction in the center that is called the centromere. This structure divides the chromosome into two parts. One part is short; the other is longer. The short arm is labelled p while the longer is labelled q.
According to the Genetic Home Reference (2014), a gene is a functional and physical component of heredity. They carry information that is involved in the process of making proteins, the building blocks of human body. It is estimated that the of a human being has between twenty thousand and twenty-five thousand genes (Genetic Home Reference 2014). Genes exist in pairs; one part inherited from a particular parent. However, about one percent of genes are different in people. The forms of the same gene that have small differences in DNA-base sequencing are referred to as alleles. The unique characteristics that each person has are brought by the small differences in the DNA sequencing.
Role of Genes and Chromosomes in heredity.
Heredity is said to be the transmission of characteristics or features from the parents to their offspring (Bateson 2007). This happens through the transmission of DNA. During conception, the father’s sperm gives the zygote twenty three chromosomes. The zygote also receives twenty-three chromosomes from the mother. This completes its pairs to make a total of forty-six chromosomes. The unique characteristics arise due to gene mixing-up during the process of cell division. A look at cell division can better explain this.
Cell division occurs in two types, one that is called mitosis, and one that is called meiosis. The difference between the two is that, mitosis gives rise to identical cells, while meiosis gives rise to reproductive cells, ova and sperm cells (Genetic Home Reference 2014). In addition, mitosis results in two cells, while meiosis gives rise to four cells. The critical process of mitosis is regulated by genes. Health problems occur if a defect occurs in the process of mitosis. On the other hand, the role of meiotic division is to ensure that human beings have an equal number of chromosomes in all their generations. The two step processes thus decrease the number of chromosomes by half, from forty six to twenty-three. In addition, the process of meiosis enables genetic variation through DNA shuffling in a process called crossing over.
Five distinct stages of cell division have been identified. The first one is referred as prophase (Cheeseman and Desai 2008). During the first stage, the chromosomes condense, centrioles move to the far opposite sides of the cell, and the microtubules begin to polymerize. This continues to prometaphase. At this stage, the nuclear membrane starts to fragment in preparation for dividing into daughter cells. At metaphase, all the chromosomes are at the acme of condensation. The centromeres are aligned together at the equator of the spindle. At anaphase, the sister chromatids separate. The final stage is the telophase, when the cell breaks into two, to form the daughter cells.
Gregor Mendel and Mendelian Principles of Inheritance.
Gregor Mendel who was an Augustinian monk and he lived between 1822 and 1884. He studied heredity by cross-breeding pea plants which had different characteristics (Bateson 2007). It is his results that led to the laws of heredity. The first Mendelian law is the principle of segregation. It posits that the two members of a gene pair do segregate against one another in the process of gamete formation. The one half of the gametes carries one allele, and the other half of the gametes carries the other allele. In other words, this allows one parent to give only one allele to the child. The other parent also gives only one allele. This means that the offspring will have two alleles just like the parents. However, since the offspring has combined the allele from the father and the mother, they will be unique.
The second Mendelian law is called the principle of independent assortment (Bateson 2007). This means that the genes for a particular trait are inherited differently with the genes for the other traits. The laws are followed with some concepts. The concept of dominance posits that only one allele of a gene is required to express outer traits. Recessive concept posits that two genes must be present for the trait to be expressed. Heterozygous concept explains that the alleles of a gene are non-identical. The opposite is homozygous, which means that the alleles are very identical. The Mendelian principles of inheritance are shown through the diagrams below.
Diagram 1.1. Retrieved from google diagrams. It shows the offspring of heterozygous parents.
Diagram 1.2. Retrieved from google diagrams. It shows the results of Mendel’s first cross. The diagram shows that that the results of homozygous dominant parents and a homozygous dominant parent will be heterozygous. All the offspring express the traits of the dominant parent.
GENE AND CHROMOSOME MUTATIONS
This section discusses gene and chromosome mutations and the differences between them. A mutation refers to an unpredictable change that occurs in the genetic material or component of an organism (Pinon 2002). In can occur in a section of a chromosome, where it will be called a chromosome mutation, or involve a gene. In the case where it involves a gene it is called a gene mutation. In other words, gene mutations are also said to be small-scale mutations. Causes of chromosome mutations vary from unknown causes to changes that occur in chromosomes during meiosis, as well as, chemical causes. The result of chromosome mutation is a change in the chromosome number in a cell, or a change in the form of the chromosome (Pinon 2002). While gene mutations change a strand of DNA or a single gene, chromosome mutations change the whole chromosomal structure.
Chromosome mutations occur in the form of breakages or duplications that are capable of changing the chromosome structure (Miller and Therman 2000). Since this changes the gene structural arrangement on the chromosome, protein production is affected. As a result, developmental difficulties or even death can result. However, some mutations may not bring harm to an individual. There are several types of chromosome mutations. Translocation is the type of chromosomal mutation where a section of the chromosome breaks and joins another chromosome that is not homologous to the mother chromosome (Miller and Therman 2000). Deletion is the type where part of chromosome breaks from a chromosome. This means that the chromosome is left without some genetic material. Duplication involves production of extra genes in the chromosome. Inversion is type of mutation where a portion breaks, reverses, and gets REPLACEed back to the chromosome. This changes the order of gene arrangement. Isochromosome is the type of mutation caused by inappropriate division of the centromere. An example of a chromosomal mutation is non-disjunction during meiosis, which leads to Klinifelters or Tuners syndrome. Klinfelters syndrome is a syndrome in which a male has an extra sex chromosome, while inn turner syndrome, a female is born with only one sex chromosome.
A gene mutation, on the other hand, results from the alteration of the original sequencing of nucleotide in the DNA (Genetic Home Reference 2014). They are mainly caused by environmental factors like radiation, chemicals and ultra-violet light. A gene mutation can also be caused by errors in meiosis and mitosis. It can affect a large segment, or a single nucleotide. A very common type is the point mutations, categorized as follows. Silent mutations are the simplest form, because they do not affect the production of proteins. This is aided by the fact that an amino acid can be encoded be encoded by multiple amino acids. Missense mutations change the sequencing of amino acids and lead to change in the production of differing amino acids. Nonsense mutation changes the sequencing of the genetic codons, and causes a stop codon be coded for, instead of an amino acid. An example of gene mutations is in sickle cell anemia where codon GAG changes to GTG leading to a change of amino acid glutamine to valine (Genetic Home Reference 2014).
The transcription process of protein synthesis is the step that involves the transcribing of the genetic information from the DNA molecule to the RNA molecule (Moldave 2012). Proteins are then produced from the transcribed DNA message or rather the RNA molecule. The DNA refers to molecules stored in the nucleus of a cell with a role of coding for protein production. The transcription process ensures that the information contained in the DNA is not transferred to proteins directly, but is first transcribed into RNA. The advantage of this is to ensure that this information is not tainted. The DNA transcription process occurs in three major steps.
The first step of transcription occurs in the form of binding of RNA polymerase to DNA (). This is an enzyme that transcribes DNA. It starts at a very specific point and ends at a particular point as per the nucleotide sequences. The specific area of the DNA where the RNA polymerase attaches is called the promoter region. The second step is elongation (Moldave 2012). It is aided by some proteins called transcription factors. They unwind the DNA strand allowing the transcription of the RNA polymer to only a single strand of the DNA. This leads to the production of the messenger RNA. The last stage is termination. This is realized at the point where the RNA strand reaches the terminator sequence. At this point, it detaches from the DNA and releases to RNA polymer. It is important to note that a difference exists between the RNA polymer and the DNA. This is because the RNA contains nucleotide base uracil, which is not contained in DNA.
3.2 Translation process
This is the process that accomplishes the process of protein synthesis. It involves the coding of the message contained in the messenger RNA into amino acid sequences to make a protein (Moldave 2012).The site for translation is the nucleus. For the mRNA to be translated, it undergoes several modifications after living the nucleus. During the modification process, there are sections of the mRNA that cannot code for amino acids. These sections are removed during the process. On one side of the mRNA, a poly-A tail is added(Nierhaus and Wilson 2009). This contains several bases of adenine. The other end of the mRNA is added a guanosine triphosphate cap. The purpose of the modifications is to remove the unnecessary sections of the mRNA and also to ensure that the ends of the mRNA are protected. The modifications thus ensure that the mRNA molecule is prepared for translation.
The process involves the work of messenger RNA, transfer RNA and the ribosomes. The transfer RNA is the molecule that performs the actual role of translating the message contained in the MRNA sequence nucleotide to the specific sequence of the amino acid (Nierhaus and Wilson 2009). Amino acids are then formed by joining the sequences. To facilitate this process, the transfer RNA has three loops. On one end, it has an attachment site for amino acids. It also contains a special site on the middle loop that is called an anti-codon site. The role of the anticodon site is to recognize an area in the mRNA that is called a codon.
3.3 Amino acid assembly
Amino acid assembly of protein synthesis is aided by the transfer RNA as the adaptor molecules. The transfer RNA reads the information in the messenger RNA and delivers the sequences to the ribosomes (Spedding 1990). Every transfer RNA is specifically attached to one of the twenty amino acids, and the three bases help to recognize the complementary codon that is in the messenger RNA. Then as one transfer RNA binds and releases, amino acids on the other adjacent transfer RNA are joined forming a continuous chain of amino acids.
Every codon in the messenger RNA molecule is read at a time. For every codon that is read, the transfer RNA binds temporarily to the messenger RNA with the assistance of the complementary anticodon (Spedding 1990). When the amino acids are added, the transfer RNA disengages itself from the messenger RNA allowing for other messages to be read. The sequence then undergoes folding to form a functional amino acid.
Bateson, W. 2007. Mendel’s principles of heredity. New York, NY: Cosimo Books.
Cheeseman, M. I., &Desai, A. 2008, Molecular architecture of the kinetochore microtubule interface. Nature Reviews Molecular Cell Biology, Vol 9, pp. 33-46.
Encyclopedia Britannica. 2013. Chromosome. Retrieved online from http://www.britannica.com/EBchecked/topic/116055/chromosome
Genetic Home Referencing. 2014. What is a gene? Retrieved online from http://ghr.nlm.nih.gov/handbook/basics/gene
Miller, J. O., & Therman, E. 2002. Human chromosomes. New York, NY: Springer.
Moldave, K. 2012. RNA and protein synthesis. New York, NY: Elsevier.
Nierhaus, H. K., & Wilson, D. 2009. Protein synthesis and ribosome structure: Translating the genome. Hoboken, NJ: John Wiley & Sons.
Pinon, R. 2002. Biology of human reproduction. Sausalito, CA: University Science Books.
Spedding, G. 1990. Ribosomes and protein synthesis: A practical approach. Oxford: Oxford University Press.