Download Chromosomes - George Weller

Chromosomes DNA, genes and alleles DNA is a molecule that stores information in living things. Most of the DNA in living things consists of two strands wound around each other in a double­helix structure. The double­helix is only about 2nm wide but it can be several centimeters long. The two strands within a DNA double­helix are held together by hydrogen bonds between bases on the two strands. The sequence of base­pairs in a DNA molecule stores instructions for the order in which to join amino acids in order to produce proteins. A gene is a section of a DNA molecule that contains the instructions for how to make one protein. The number of genes in a human is currently estimated at about 20 000­25 000. Grapes are estimated to have about 30 000 genes and fruit flies are estimated to have about 15 000. In some cases, more than one version of a gene exists. These different versions of the gene code for different versions of the same protein. In other words, the amino acid sequences that they code for have some differences, but are similar enough that the proteins can carry out the same function. Each version of a gene is called an allele of that gene. Chromosomes In eukaryotic cells, most of the DNA is found in the nucleus (a very small amount is found in chloroplasts and mitochondria). The DNA in the nucleus is separated into structures called chromosomes. In humans, each chromosome contains a few hundred to a few thousand genes. Since a gene is an instruction for how to make a protein, it follows that a chromosome is a collection of instructions, a bit like an instruction manual that contains instructions on how to make many different things. The chromosomes in a nucleus all float around independently of each other. They are like a set of instruction manuals which between them contain the instructions (genes) for how to make every protein that the organism can make. In most eukaryotic organisms, the chromosomes come in pairs called homologous pairs. The number of chromosomes varies widely between different organisms. Mosquitos have 3 pairs of chromosomes, fruit flies have 4 pairs, humans have 23 pairs, pineapples have 25 pairs, donkeys have 31 pairs, and the adders­tongue fern has 630 pairs. DNA replication and chromatids Copyright George Weller 2016 For more notes and resources, visit www.georgeweller.net When a cell has just been produced through mitosis, each chromosome in the nucleus is simply one very long DNA molecule ­ a long double­helix containing hundreds or thousands of genes. There is a special region within each chromosome called the centromere. During interphase, which is the longest stage of the cell cycle, the DNA is replicated. Every single chromosome is replicated to produce any exact copy. The two copies of each chromosome remain joined together. These two DNA molecules are connected together by their centromeres. Although this structure is made up of two DNA molecules, it is still referred to as one chromosome. The two identical DNA molecules within a chromosome are called chromatids. Two identical chromatids that are joined in this way are called a pair of sister chromatids. Condensation When the cell enters mitosis or meiosis, one of the first things that happens is that the chromosomes condense. This means that both of the DNA molecules (chromatids) within each chromosome become highly coiled (supercoiled) so that the chromosome becomes much shorter and thicker. The chromosome can now bind a stain and be seen under the light microscope. Fertilisation and homologous pairs of chromosomes Fertilisation is when two gametes (sex cells, for example a sperm cell and an egg cell) fuse with each other to form a single cell called a zygote. This zygote is a new individual. In multicellular organisms, the zygote then undergoes many mitotic cell divisions to form the adult organism. Each gamete contains a full set of chromosomes. For example, a human sperm cell contains 23 chromosomes and a human egg cell contains 23 chromosomes. These 23 human chromosomes contain between them every single human gene. When the gametes fuse, their chromosomes are combined into one nucleus which now contains two sets of chromosomes. For example, in humans the set of 23 chromosomes from the sperm cell combines with the set of 23 chromosomes from the egg cell to form two sets of chromosomes (46 chromosomes). The zygote then divides many times by mitosis to produce the adult human. Since mitosis produces cells that are genetically identically to their parent cell, all of the cells in the body of the adult human (apart from the sperm and egg cells, which are produced by meiosis) contains two sets of chromosomes (46 chromosomes). This means that every somatic cell (cell that is not a sex cell) contains two copies of each chromosome. It has two copies of chromosome 1 (called chromosome 1A and chromosome Copyright George Weller 2016 For more notes and resources, visit www.georgeweller.net 1B), two copies of chromosome 2 (called chromosome 2A and chromosome 2B), etc. These pairs of chromosomes are called homologous pairs of chromosomes. A homologous pair of chromosomes has the following features: ● One of the chromosomes is maternal (comes from the mother) and the other is paternal (comes from the father). ● The two chromosomes are the same length. ● The two chromosomes contain the same genes in the same locations. ● The chromosomes may contain different alleles of the genes. ● The two chromosomes have their centromeres in the same location. Ploidy The term ploidy refers to the number of sets of chromosomes within a nucleus. A cell that contains one set of chromosomes (e.g. a gamete) is described as haploid. A cell that contains two sets of chromosomes is described as diploid. The letter n is often used to represent the number of chromosomes in a set. Therefore, the number of chromosomes in a haploid nucleus is n and the number of chromosomes in a diploid nucleus is 2n. In humans n = 23 and 2n = 46. In fruit flies n = 4 and 2n = 8. There are some organisms that have more than two sets of chromosomes. Durum wheat (the wheat from which pasta is made) has four sets of chromosomes ­ it is tetraploid (4n). This is because is was formed by a hybridisation of two diploid (2n) grasses in which all of their chromosomes were combined in one nucleus. Bread wheat is hexaploid (6n). It was formed by a hybridisation of durum wheat (4n) with another diploid (2n) grass. The cell cycle The cell cycle is the process by which a cell grows and then divides into two daughter cells. It is called the cell ​
cycle​
because it repeats. The cell cycle consists of two phases: interphase and the mitotic phase. Interphase takes up most of the cell cycle. It is when the cell grows and the DNA is replicated. The mitotic phase takes place once the DNA is replicated and the cell is large enough to divide. It consists of mitosis, during which the sister chromatids of each chromosome are pulled apart into two separate nuclei, followed by cytokinesis, during which the cytoplasm is separated into two cells, each one containing one of the new nuclei. Copyright George Weller 2016 For more notes and resources, visit www.georgeweller.net The cell cycle consists of interphase and mitotic phase. Interphase is made up of gap phase I, during which the cell grows, synthesis phase, during which the DNA is replicated, and gap phase II, during which the cell grows and the replicated DNA is checked for errors. The mitotic phase consists of mitosis and cytokinesis. Mitosis Mitosis is a form of cell division which results in two daughter cells which are genetically identical to each other and to the parent cell. During mitosis, the number of chromosomes does not change. If mitosis starts with a diploid cell, then it produces two diploid daughter cells. At the start of mitosis, each chromosome consists of two identical sister chromatids joined at the centromere. This is because each chromosome has previously been replicated during the S phase of interphase. Mitosis consists of four stages called prophase, metaphase, anaphase and telophase. During prophase, the chromosomes condense, the nuclear envelope breaks down and the spindle forms. During metaphase, the chromosomes line up on the equator of the cell and the chromosomes become attached to the spindle by their centromeres. During anaphase, the spindle pulls the sister chromatids within each chromosome apart and pulls them to opposite poles of the cell. During telophase, nuclear envelopes form around the chromosomes at the two ends of the cell. The cell then divides in two by cytokinesis to produce two genetically identical daughter cells. Mitosis and Cytokinesis in an organism with two pairs of chromosomes (2n=4) Copyright George Weller 2016 For more notes and resources, visit www.georgeweller.net Meiosis Meiosis is a form of cell division which results in four daughter cells which are genetically different to each other and to the parent cell. During meiosis, the number of chromosomes halves. If meiosis starts with a diploid cell, then it produces four haploid daughter cells. Meiosis is the process by which gametes are made, meaning that it is crucial to sexual reproduction. Sexual reproduction is a major source of variation between living organisms. When organisms reproduce asexually (using mitosis), the only source of variation is mutations. Copyright George Weller 2016 For more notes and resources, visit www.georgeweller.net During sexual reproduction (using meiosis) alleles are mixed and shuffled, which leads to a much larger amount of variation. Meiosis consists of two divisions. In the first division (meiosis I) the chromosomes line up in homologous pairs, and one chromosome from each homologous pair goes into each daughter cell. In the second division (meiosis II) the chromosomes line up on their own (just like in mitosis) and one sister chromatid from each chromosome goes into each daughter cell (just like in mitosis). Inheritance When a diploid cell undergoes meiosis to produce haploid gametes, each gamete will receive one of the two copies of each chromosome. For example, each gamete will either receive chromosome 1A or chromosome 1B. Which version of each chromosome the gamete receives is decided by random chance. Consider the example of eye colour. The allele of the eye colour gene which produces brown eyes (represented by the symbol B) is dominant. The allele of the eye colour gene which produces blue eyes (represented by the symbol b) is recessive. Each person has two copies of the eye colour gene, because they have two copies of the chromosome that contains it. This means that each person can either have two copies of the blue eye allele (bb) in which case they would have blue eyes, or one copy of each allele (Bb) in which case they would have brown eyes, or two copies of the brown eye allele (BB) in which case they would have brown eyes. Take for example a man who has one copy of each allele (Bb) and therefore has brown eyes. When cells within his body undergo meiosis to produce sperm cells, each sperm cell will receive only one copy of the chromosome that contains the eye colour gene. This means that roughly half of his sperm cells will receive a chromosome containing the blue eye allele (b) and roughly half of his sperm cells will receive a chromosome containing the brown eye allele (B). Consider what happens if this man reproduces with a woman who also has one copy of each allele (Bb). Half of the man’s sperm cells contain the blue eye allele (b) and half of his sperm cells contain the brown eye allele (B). Half of the woman’s egg cells contain the blue eye allele (b) and half of her egg cells contain the brown eye allele (B). This means that depending on which sperm cell fertilises which egg cell, there are a number of different things that could happen. These possibilities are shown in the genetic diagram below: Copyright George Weller 2016 For more notes and resources, visit www.georgeweller.net Mother Father Phenotype Brown eyes Brown eyes Genotype Bb Bb Gametes B or b B or b Possible zygotes formed: X B b B BB Bb b Bb bb Possible offspring produced: Offspring Genotypes BB, Bb, bb Ratio of genotypes Phenotypes Ratio of phenotypes 1:2:1 Brown eyes, Blue eyes 3:1 The diagrams above show that: ● There is a one in four chance that a sperm containing the b allele will fuse with an egg containing the b allele to produce a blue eyed child. ● There is a one in four chance that a sperm containing the b allele will fuse with an egg containing the B allele to produce a brown eyed child. ● There is a one in four chance that a sperm containing the B allele will fuse with an egg containing the b allele to produce a brown eyed child. ● There is a one in four chance that a sperm containing the B allele will fuse with an egg containing the B allele to produce a brown eyed child. Overall then, there is a one in four chance that they will have a blue eyed child and a three in four chance that they will have a brown eyed child. Copyright George Weller 2016 For more notes and resources, visit www.georgeweller.net