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Explain the structure & function of DNA and elaborate on gene expression
(Top new, 2011)
1.1 - Explain in detail regarding the structure and functions of DNA, Gene and Chromosomes.
The structure and functions of DNA are as - Deoxyribo Nucleic Acid is a string of molecules that contains biological information, codes and instructions that is needed for reproduction, repair and growth of all living organisms. DNA is found in the nucleus of the cell. The DNA is structured by two strands running in opposite direction anti-parallel to each other with hydrogen bonds between them, this is called a double helix.
DNA is made up of chemical building blocks called nucleotides or bases. The building blocks are further divided into three groups, phosphate group, sugar group and one of four types of nitrogen bases. The four types of nitrogen bases are found in nucleotides which are Adenine (A), Thymine (T), Guanine (G) and Cytosine (C) (NIH, 2014). The sequence of pairing of these bases will determine what biological information is passed on to the new development. Base A will only be paired with T, base T will only be paired with A , base G will only be paired with C and base C will only be paired with G.
The function of DNA is to instruct the organism to repair, grow and reproduce. To carry out these functions the DNA sequence with the instruction need to be converted into messages which produce protein known as gene (NIH, 2014)
(shutterstock, 2015) The structure and function of Genes are as – A gene is the basic unit of inherited genetic information. A human has two copies of genes, one inherited from each if the parent which determines the inherited characteristics. Genes are made from and are a segment of DNA, they are the instruction that produce molecules called proteins. The function of genes is to guide protein synthesis to perform many functions within the body, to regulate the expression of other genes, to form ribosomes these are structures in the cell that are important for making proteins, and to transport amino acids the building block for proteins to ribosomes to create proteins (Structure and function of Gene, 2001, Genetics home reference, 2015).
The structure and functions of Chromosomes are as – Chromosomes carry genes they make organisms grow, repair and reproduce. Chromosomes are found in the nucleus of the cell and are the basic storage units of genes. In each chromosome DNA is tightly packed and coiled many times around proteins called histones which maintain its shape. All chromosomes have a centre point called centromere the location of centromere gives the chromosome its characteristic shape. The centromere divides the chromosome into two parts the shorter part is called ‘p arm’ and the longer part is called ‘q arm’ (Genetic home reference, 2015). A chromosome is not visible within the nucleus, it is only in a visible state when it is going through the phase of cell division, this phase is called metaphase. Humans have 23 pairs or 46 chromosomes in each cell, of which are 1 pair or 2 chromosome sex chromosomes and 22 pairs or 44 chromosomes are named autosomes
1.2 – Describe the mechanism and importance of DNA replication.
DNA carry information for making the protein for the cell, these proteins put into place all of the functions and characteristics of the organism. It is very important for a cell to divide and continue life; the important characteristic of DNA is that is can make lots of copies of itself. When the cell reproduces it is very important that it passes on all the information to the daughter cell. Before the cell can reproduce it needs to replicate and or copy its DNA. The structure of DNA makes it easy for it to copy itself, as the double helix can ‘unzip’ down the middle of the DNA. The double helix has two strands that run anti parallel to each other, and this serves as a pattern for replication for the other side. This is known as semi conservative replication. The stages for semi conservative replication are as follows:-
- An enzyme called gyrase marks a section in the double helix to separate the section
- An enzyme called helicase unzips the double stranded DNA structure
- Proteins called single strand binding proteins (SSB), temporarily binds to both side to keep them separated
- An enzyme complex called DNA polymerase does down the DNA strand and adds new nucleotides (bases) to each strand. The bases pair with their partners i.e A with T, A with A, T with A, C with G and G with C.
- A section of DNA polymerase proofreads the new DNA
- An enzyme called DNA ligase glue up the fragments to make it one long continues strand
- The new copies that are made wind back up again automatically (how stuff works, 2015)
(How genes work, 2014)
2.1 – Explain the events of mitosis and meiosis
Mitosis is the process in which the cell divides into tow cells. Within mitosis the nucleus of the cell divides which results in two identical chromosomes. This process is followed by a process called cytokinesis, where the rest of the cell divides into two spate cells called daughter cells. Mitosis has four stages the prophase, the metaphase, the anaphase and the telophase.
Mitosis include the following stages:-
Interphase – Within the process of DNA replication in the nucleus, the DNA has already been duplicated, therefore by the time prophase starts there is already two sets of identical DNA within the nucleus. This stage is known as the interphase.
Prophase - At the starting stage of prophase the chromatin start to condense into an ‘X’ shape held together in the middle by DNA called centromere each half of the X is a duplicated half of DNA. When the chromatin is coiled together into a X they are called mitotic chromosomes. Towards the end of the prophase stage the materials that enclose the nucleus breaks down and after the DNA has formed into an X material called centiroles pull to opposite ends of the cell, also spindle like fibres then emerge from the centromere. The chromosomes devlope structures in the middle called kinectochores.
Metaphase – At the metaphase stage a string called microtubules attach to the kinectochores from both sides of the chromosomes, in order that they can be pulled apart at a later stage. The chromosomes line up with the spindle like fibres which are situated vertically around the cell. The chromosomes are then positioned on the metaphase plate, which is the middle of the parent cell. At the end of the metaphase each chromosome has microtubules attach to both half of itself, and the chromosomes are straight lined down the middle of the cell.
Anaphase – At the anaphase stage, as soon as the chromosomes are lined up correctly the spindle fibres pull down the two identical DNA halves apart from each other, and move them to opposite sides of the cell. The two sets of chromosomes will become the nuclei of two new cells which are identical daughter cells and identical to the parent cell.
Telophase – At the telophase stage, the chromosomes are at the end of the cell, they then start to uncoil and spread out like they were before they became X’s at the prophase stage. The spindle fibres break down and the nucleus material around the nucleus generates again to enclose the chromosomes and the nucleus.
Cytokinesis – At the cytokinesis stage, the middle of the cell the metaphasal plate pinch together, this separates the cell into two cells. This forms two identical functioning cells.
(Daniels biology blog, 2011)
Meiosis is the process in which one cell divides into four different cells. It is a two part cell division process meiosis I and meiosis II in organisms that sexually reproduce. Meiosis produces gametes, these are eggs and sperm which contain half the number of chromosomes 23 (haploids) than the parent cells which have 46 chromosomes (diploids) (About education, 2015)
Interphase - This is the process of DNA replication, as explained above
Prophase I –The DNA is condensed and at this stage the chromosomes are matched gene by gene and are first visible as tetrads at prophase stage. The material around the nucleus breaks down and the spindle fibres start to form. Crossing over happens when chromatides exchange genetic information, this results in new alleles on the chromosomes.
Metaphase I – In this stage the centromere of each chromosomes are attached to the spindle fibres, this pull the tetrads to the centre of the spindle. The chromosomes are then straight lined as tetrads.
Anaphase I – At this stage the chromosomes separate and move to opposite side of the cells, however the centromeres do not separate which make sure that each new cells will only receive one chromosomes from each chromosome pair.
Telophase I – In this stage the spindle fibres break down and the chromosomes uncoil within the cell. The cytoplasm in the cell divides to make two new cells, each cell will receive half the genetic information as it only has one chromosome of each pair of chromosome.
Prophase II – In this stage a spindle of fibre are forming in each of the two new cells and attach to the chromosomes. The chromosomes with two chromatids are now visible as they condense.
Metaphase II – In this stage the chromosomes with two chromatids are pulled to the centre of the cell as the spindle of fibre in now fully formed, lining up in any order.
Anaphase II – In this stage the centormere spilt into two and the chromosome with one chromatid separate and move to opposite side of the cell.
Telophase II – In this stage the nuclei reform and chromosomes with one chromatid uncoil within the nucleus and the spindle fibres break down. The four daughter cells are haploids and have the right amount of DNA
(Step by step meiosis, 2007)
(D1 Genetics wikiB, 2011)
2.2 Compare the products of meiosis to those of mitosis
Mitosis and meiosis are both types of cell division, however there are differences between them. Meiosis only happens in sex cells called gametes, the egg and sperm. The DNA from each involved cell is muddled up with section of DNA from different cells to form the condensed chromosome the X. In mitosis the two half’s of the X are identical. In meiosis the result is of four different cells that are each unique genetically, whereas in mitosis the end result is of two identically same cells (Wisegeek, 2015).
2.3 Explain the importance of meiosis in generating variation
Meiosis is the process of cell division in sexually reproductive organisms. It is very important as it creates a genetically diverse population. This happens because within meiosis each gametes include a mixture of different DNA from each parent chromosomes, the result of meiosis is that it produces offspring that have genetic material of two different persons. The characteristics of parent chromosomes are mixed with characteristics of offspring chromosome which result in a new and unique chromosome. The chromosomes contain the basic DNA which sets out the physical and genetic characteristics of the offspring, this enables organisms to produce physically and genetically unique offspring due to meiosis a high level of genetically diverse population is kept. In some people how are favoured by natural selection have a better fitness to other because of their basic pairing of genes. A poplutaion can survive environmental charges due to genetic variation in meiosis. Meiosis not only produces offspring that are physically and genetically different but also fit to survive the environment as this is a result of genetic variation (Buzzel, 2009).
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