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What is DNA?

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DNA DNA, otherwise known as deoxyribonucleic acid has changed our lives, including our perception of life, by both scientists and the average person. Science has developed dramatically due to the discovery of DNA. DNA was first discovered by the Swiss physician Friedrich Miescher and Gregor Mendel who, in 1869, discovered a microscopic substance in the pus of discarded surgical bandages. As it resided in the nuclei of cells, he called it "nuclein". In 1919 this discovery was followed by Phoebus Levene's identification of the base, sugar and phosphate nucleotide unit. Levene suggested that DNA consisted of a string of nucleotide units linked together through the phosphate groups. However, Levene thought the chain was short and the bases repeated in a fixed order. In 1937 William Astbury produced the first X-ray diffraction patterns that showed that DNA had a regular structure. In 1943, Oswald Theodore Avery discovered that traits of the "smooth" form of the Pneumococcus could be transferred to the "rough" form of the same bacteria by mixing killed "smooth" bacteria with the live "rough" form. Avery, along with coworkers Colin MacLeod and Maclyn McCarty, identified DNA as this transforming principle.DNA's role in heredity was confirmed in 1953, when Alfred Hershey and Martha Chase in the Hershey-Chase experiment showed that DNA is the genetic material of the T2 phage. In 1953, based on X-ray diffraction images taken by Rosalind Franklin and the information that the bases were paired, James D. ...read more.


Also, there are a fixed number of chromosomes in each species. Human body cells contain 46 chromosomes, mouse cells contain 40 and garden pea cells contain 14. The number if chromosomes in a species is the same in all of its body cells. There are 46 chromosomes in each of our liver cells, in every nerve cell skin cell and much more. Also, the chromosomes have different shapes and sizes and can be recognised by a trained observer. Also, the chromosomes are always in pairs, as our 46 chromosomes consist of 23 from our mother and 23 from our father. The number of each chromosome in each body cell of a plant or animal is called the diploid number. Because chromosomes are always found in pairs, it is always an even number. The chromosomes of each pair are called homologous chromosomes. Gamete production and chromosomes The genes on the chromosomes carry the 'instructions' which turn a single cell zygote into an organism. The zygote is formed at fertilisation, when a male gamete fuses with a female gamete. Each gamete brings a set of chromosomes to the zygote. The gametes must then each contain only half the diploid number of chromosomes, otherwise the chromosome number would double each time an organism reproduced sexually. Each human sperm cell contains 23 chromosomes and each human ovum contains 23 chromosomes. When the sperm and ovum fuse at fertilisation, the diploid number of 46 chromosomes is produced. ...read more.


This is shown in the diagram below: Mutations A mutation is a spontaneous change in a gene or a chromosome. In a gene mutation it may be that one or more genes are not replicated correctly. A chromosome mutation may result from damage to or loss of part of a chromosome during mitosis or meiosis, or even the gain of an extra chromosome as in Down's syndrome An abrupt change in a gene or chromosome is likely to result in a defective enzyme and will usually disrupt the complex reactions in the cells. Most mutations, therefore, are harmful to the organism. Also, only about 3% of human DNA consists of genes. The rest consists of repeated sequences of nucleotides that do not code for proteins, and is sometimes called 'junk DNA', but that term only means that we do not know its function. If mutations occur in these non-coding sequences, they are unlikely to have any effect on the organism and are, therefore, described a neutral. Rarely a gene or chromosome mutation produces a beneficial effect and this may contribute to the success of the organism. All of which I have explained in a detailed manner really does show us that DNA is a rather amazing thing, and it controls our whole lives, as well as the lives of plants, and other animals. Mitosis; meiosis; protein synthesis; replication; gamete production; mutations and the structure of DNA itself are altogether rather extraordinary. ?? ?? ?? ?? 10B Tofeek Jumaily 01/11/07 ...read more.

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Here's what a star student thought of this essay

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Response to the question

Overall, this is a very good piece, with clear understanding of the topic, with lots of facts and explanations showing understanding of what you are talking about. There is a good explanation and history of DNA, with appropriate facts, as ...

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Response to the question

Overall, this is a very good piece, with clear understanding of the topic, with lots of facts and explanations showing understanding of what you are talking about. There is a good explanation and history of DNA, with appropriate facts, as well as a good expansion on the topic with information on replication with meiosis and mitosis as well, which helps demonstrate your knowledge of the entire syllabus, but is not out of place on the topic of DNA, as it adds more information over and above what is expected. Images also help accompany the detailed explanation and are well placed within the piece.

Level of analysis

Technical language is used extensively throughout the piece, but is sparse enough to make the piece readable. Pictures and facts are used to support structured points which are developed throughout, as well as explained to demonstrate your understanding. I would either remove or rewrite the final conclusion, as it seems very informal, in contrast to the rest of the piece and seems to lower the tone slightly, which is a shame as this is an intelligent piece, as you have shown you understand the topic, as well as related information in the syllabus, such as meiosis and mitosis.

Quality of writing

My only negative comments on this piece are on the level of English portrayed in the piece. You clearly have shown your intelligence, but the short sentences and lack of flow between some sentences lets you down. If you restructure your sentences a bit more, and rebuild a sentence rather than tacking an extra bit on the end, it will read a lot better and make the piece look far more sophisticated.

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Reviewed by pratstercs 11/02/2012

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