Genetics: chromosome structure and function

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Genetics

TAQ 1:

Genes are located on the rodlike structures called chromosomes and are found in the nucleus of every cell in the body. Genes are coded instructions for making everything that the body needs, especially proteins. And because proteins determine the structure and function for each cell in the body, essentially genes are responsible for all the characteristics humans inherit. Genes vary in size depending on the sizes of the proteins for which they code. Genes are the basic unit of genetics and human beings have between 20,000 and 25,000 genes. Genes only count for about 3% of our DNA.

Genes are packed together in bundles called chromosomes. A chromosome is made up of a very long strand of DNA and contains many genes that are arranged into a particular sequence on each chromosome. Each gene also has a particular location on a chromosome called its locus. Human beings have 23 pairs of chromosomes, a total of 46. One of these pairs is the sex chromosome and determines the sex of a person and the other 22 are autosomal chromosomes and determine the rest of the bodies makeup. Chromosomes also contain other chemical components that influence gene function, not just DNA.

TAQ 2:

  1. Gregor Mendel, born in Austria in 1822, discovered the basic principles of heredity through experiments in his monasteries garden. He was considered a pioneer in the field of genetics and his experiments showed that the inheritance of certain traits in pea plants followed particular patterns. He chose peas due to their many distinct varieties and easily produced offspring. In 1854 Mendel began researching the transmission of heredity traits in plant hybrids at around the time it was generally accepted that the heredity traits of the offspring of any species were the diluted blending of whatever was present in the parents. Mendel cross fertilised pea plants that clearly had opposite characteristics for example tall with short and smooth with wrinkled and after analysing the results reached two of his most important conclusions. The law of segregation; which established that there are dominant and recessive traits passed on randomly from parents to offspring. This provided an alternative at the time as blending inheritance was seen as the dominant theory. And the law of independent assortment; which found that traits were passed on independently of other traits from parents to offspring. Although Mendel carried out his experiments on pea plants, he stood by the theory that all living things have such traits.

2. If one parent was a homozygous non tongue-roller (rr) and one was a heterozygous roller (Rr) then the chances of the child being a tongue roller would be 50% as the rolling gene is also the dominant gene. This can be worked out using the punnet diagram below.

r

r

R

Rr

Rr

r

rr

rr

TAQ 3:

1.

Genetic linkage is the tendency of genes that are located close together on a chromosome to be inherited together during meiosis. Genes which are located closer to each other are less likely to be separated onto different chromatids during chromosomal crossover and are therefore said to be genetically linked. These are also more likely to be inherited together. The further apart two genes are on a chromosome the more chance there is that they will split up and become separated during a process called homologous recombination. Genes that are on separate chromosomes are never linked.

A chromosome is a single piece of DNA and genes are segments of DNA arranged along a chromosome. A single chromosome can have thousands of genes. Humans have 23 chromosome pairs, 46 i n all. Homologous chromosomes have the same genes arranged in the same order but have slightly different DNA sequences. And different versions of the same genes are called alleles. Homologous chromosomes often contain different alleles. Alleles are important because they account for the differences in inherited characteristics from one individual to another. For example different alleles of the same genes can make our eyes blue, brown, or green.

2.

Human cells contain 23 pairs of chromosomes and a total of 46. There are 22 pairs of autosomes and one pair of sex chromosomes. The sex chromosomes are the X and the Y chromosome. A female has two X chromosomes and a male has an X and a Y chromosome. During fertilisation, the new gamete always inherits one of the mothers X chromosomes and either an X or a Y chromosome from the father, depending on which chromosome the fertilising sperm cell happened to inherit. So if a sperm cell containing an X chromosome fertilises the egg the resulting zygote will be XX and therefore female and if a sperm cell containing a Y chromosome fertilises the egg the resulting zygote will be XY and therefore male. A zygote is the initial cell that is formed when two gamete cells are joined by means of sexual reproduction. It could be said then that the father, or his sperm at least determines the sex of the baby however it is not always the first sperm to reach the egg that necessarily fertilises it. Human eggs are rather choosy.

3.

Chromosomal crossover or crossing over is the exchange of genetic material between homologous chromosomes (a set of one maternal chromosome and one parental chromosome that pair up with each other during meiosis) that results in recombinant chromosomes. It is one of the final phases in genetic recombination which occurs during phase I of meiosis during a process called synapsis. At this stage each chromosome has replicated into two strands called sister chromatids. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.

Crossing over is when homologous chromosomes pair up with each other and swap different segments of their genetic material. Crossing over also accounts for genetic variation because due to the swapping of genetic material, the chromatids held together by the centromere are no longer identical. So when the chromosomes then go onto meiosis II and separate, some of the daughter cells receive daughter chromosomes with recombined alleles. Due to this genetic recombination the offspring will have a different set of alleles and genes than their parents do. Crossing over is also important because it results in new combinations of genes that are different from either parent, contributing to genetic diversity.

TAQ 4:

Variation is all the differences which exist between members of the same species. There are two types of variation, continuous and discontinuous.

Continuous variation is the type that has no limit on the value that can occur within a population. For any species, a characteristic that changes gradually over a range of values shoes continuous variation. Some examples of this are height, weight, foot length and heart rate. A line graph is usually used to represent continuous variation.

Discontinuous variation is the type of variation that has distinct groups for organisms to belong to. It is a characteristic of any species with only a limited number of possible values and none in between. A bar graph is usually used to represent discontinuous variation. Some examples of discontinuous variation are gender (male or female), blood group (A, B, AB, O), eye colour and finger prints.

TAQ 5:

  1. Mutations are random changes that can occur in genes. They can be caused by background radiation and chemicals that we come into contact with like cigarette smoke. This causes an alteration to the base pair sequence in the genetic code.

As a result of these changes, the cell can sometimes die if severe enough, it could divide uncontrollably and become cancerous or the changes could be so small that the cell survives. In very rare instances the changes/mutations can be beneficial to humans and produce new and even useful characteristics.

2. A de novo mutation is a new mutation that has occurred due to an error in the copying of genetic material or an error in cell division. This may result in a family member being the first in a family to have a genetic condition as a result of a mutation in the egg or sperm of one of the parents or in the fertilised egg itself. A new mutation that occurs in a somatic cell can cause cancer. It is often impossible to tell exactly when a de novo mutation has happened. De novo mutations may explain genetic disorders in which an affect child has a mutation in every cell in the body but the parents do not, and there is no family history of the disorder.

3. Mosaicism is a condition in which cells within the same person have a different genetic make up. The condition can effect any type of cell including blood cells, skin cells and egg and sperm cells (gametes). Mosaicism happens very early on in the development of an unborn baby by an error in cell division. Genetic testing can diagnose mosaicism and symptoms can vary and are often very difficult to predict. If a person has both normal and abnormal cells, symptoms will not be as severe. Treatment of mosaicism depends on the type and severity of the disorder, i.e. less intense treatment for a smaller number of abnormal cells. An example of mosaicism is mosaic klinefelter syndrome. This condition is when there is the presence of an extra X chromosome in a male. It is normal for a boy to have an X and Y chromosome, however someone with mosaic klinefelter syndrome would have an extra X and this would be written XXY.

4. Polymorphism means a discontinuous genetic variation resulting in the occurrence of several different forms or types of individuals among the members of a single species. A discontinuous genetic variation divides the individuals of a population into two or more sharply distinct forms. The most obvious example of this is the separation of most higher organisms into male and female sexes. Another example is blood type in humans. In continuous variation by contrast, the individuals do not fall into sharp classes but instead are almost imperceptibly graded between wide extremes. If the frequency of two or more discontinuous forms within a species is too high to be explained by mutation, that variation and population displaying it is said to be polymorphic.A polymorphism that persists over many generations is usually maintained because no one form possesses an overall advantage or disadvantage over the other in terms of natural selection.

TAQ 6:

Protein synthesis is the process of individual cells constructing proteins. DNA produces an RNA template which then directs amino acids to be introduced into the growing protein chain in the proper sequence. A specific transfer-RNA (tRNA) attaches to each specific amino acid and brings the amino acid to the RNA for incorporation.

Transcription happens in the nucleus and is the unfolding of DNA and the production of a messenger-RNA (mRNA) strand. The DNA uncoils and provides the pattern for the formation of a single strand of mRNA. After the production of the RNA, the DNA refold into the original double helix; the mRNA is then exported to the cytoplasm for further processing. Amino acids will link with specific tRNA molecules for proper placement in the protein chain. The tRNA is a small coiled molecule that accepts an amino acid on one end and matches to an mRNA molecule on the other. The tRNA and mRNA interact to put the amino acid in the proper sequence for developing the protein. Once added the amino acid to the sequence,the tRNA is cleaved from it and recycled for further use in the process.

Amino acid assembly takes place in the ribosome. This structure consists of two subunits containing ribosomal RNA that enclose the mRNA and catalyse the formation of the amide linkages in the growing protein in a process known as translation. When protein synthesis is complete, the two subunits dissociate and release the completed protein chain.

Protein synthesis is fairly fast. Amino acids are added to the growing peptide chain at a rate of about 3-5 amino acids per second.

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