Molecules: Gene Mutations PDF

Molecules D1.3.1 Gene mutations as structural changes to genes at the molecular level The DNA from which genes are made is very stable and its base sequences...

Molecules D1.3.1 Gene mutations as structural changes to genes at the molecular level The DNA from which genes are made is very stable and its base sequences c an be copied with great accuracy. Cells have methods of correcting errors in DNA replic ation, but changes do sometimes occur in DNA molecules. If a change occurs to the base sequence of a gene, it is a gene mutation. Gene mutations are random and should be distinguished from deliberate changes made by molecular biologists when editing genes. ▴ Figure 3 Eleven players are allowed in There are three main types of gene mutation. a soccer team during a match. Substitutions Substitution—one base in the coding sequence of a gene is replaced by of one player for another are allowed. The a different base. For example, adenine present at a particular point in the referee may send a player o for a foul base sequence could be substituted by cytosine, guanine or thymine. Base or misconduct but does not allow extra substitutions c an happen by chemic al changes to bases or by mispairing players. In a very rare example, Moscow during DNA replic ation. The commonest example of mispairing is G to T. If D ynamo elded 12 players for the second half in a match against Glasgow R angers in this is not corrected before replic ation next occurs, the result is a G to A or T 1945. In soccer, deletions are common but to C base substitution on the coding strand. insertions are extremely rare! Insertion—a nucleotide is inserted, so there is an extra base in the sequence of the gene. This is a more major change as it requires a break to be made in the sugar–phosphate backbone of the DNA molecule. Activity: How common Deletion—a nucleotide is removed, so there is one base less in the sequence of the gene. This requires two breaks in the sugar–phosphate backbone. are mutations? Multiple insertions and deletions c an occur where two or more consecutive Recent research into mutations nucleotides are added or removed. involved nding the base sequence of all genes in parents and their ospring. It showed that D1.3.2 Consequences of base substitutions there was one base mutation per 8 1.2 × 10 bases. C alculate how The consequences of base substitution mutations are mostly neutral or many new alleles a child is likely to deleterious and in some c ases they are lethal. have as a result of mutations in their parents. Assume that there are In the non-coding DNA between genes on chromosomes, base substitutions 25,000 human genes and these are unlikely to have any effect. Only changes to the coding sequences of genes are 2,000 bases long on genes c an affect the amino acid sequences of polypeptides. average. S ame-sense mutations are base substitutions that change one codon for an amino acid into another codon for the same amino acid. They are possible bec ause of the redundancy of the genetic code. For example, a change from AGC to AGT still codes for the amino acid serine. S ame-sense mutations do not affect the phenotype, though they may make it possible for a second mutation to change the codon into one for a different amino acid. Nonsense mutations change a codon that codes for an amino acid into a stop codon (ATT, ATC or ACT). Translation is therefore terminated before a polypeptide has been completed. Usually, the resulting protein does not ▴ Figure 4 Abraham Lincoln’s features function properly. The effects of this depend on what the protein’s function is. resemble Marfan syndrome, caused by a mutation to the brillin-1 (FBN1) gene on Mis-sense mutations alter one amino acid in the sequence of amino acids in a chromosome 15. However, recent evidence polypeptide. They may not have much effect if the new amino acid has a similar suggests that he suered from a dierent structure and chemical properties to the original one (a synonymous substitution) genetic disease, multiple endocrine or if it is positioned in part of a protein that is not critical in terms of function. But neoplasia (MEN2B). This condition is due a mis-sense mutations can also have severe and even lethal effects. Many genetic mutation in the RET gene on chromosome 10 diseases are due to mis-sense mutations, for example sickle cell disease. 593  Continuity and change A very small proportion of mis-sense mutations improve the functioning of a protein and increase an individual’s chances of survival. Benecial mutations are massively outnumbered by deleterious mutations but they are arguably more signicant because of evolution and adaptation. When a base substitution mutation happens in one individual in a population and is inherited by ospring, a new allele of one gene has been produced and the genetic diversity of the population increases very slightly. In evolutionary terms, this is benecial for the population as a whole. When the DNA from individual humans is sequenced, large numbers of base substitutions are found that have happened at some time in the past. These are known as single-nucleotide polymorphisms, frequently abbreviated to SNPs and pronounced “snips”. SNPs c an occur in noncoding regions of DNA. The presence of some SNPs is associated with certain diseases. These correlations allow scientists to look for SNPs to determine an individual’s genetic predisposition to develop a disease. D1.3.3 Consequences of insertions and deletions Insertions and deletions are less likely than substitutions to have benecial consequences. M ajor insertions or deletions of nucleotides in a gene almost always result in the coded-for polypeptide ceasing to function. Minor insertions and deletions of one or two nucleotides can also result in total loss of function in a polypeptide. This is because they are frameshift mutations. They change the reading frame for every codon from the mutation onwards in the direction of transcription and translation. This is illustrated in Figure 5. Original sequence C G A T A C A T G T T G T A T G C G alanine methionine tyrosine asparagine isoleucine arginine After a base substitution C G A C A C A T G T T G T A T G C G alanine valine tyrosine asparagine isoleucine arginine After a deletion C G A T A A T G T T G T A T G C G alanine isoleucine threonine threonine tyrosine alanine After an insertion ▸ Figure 5 Deletions and insertions c ause more changes than substitutions in the C G A T A C A T G T C T G T A T G C G amino acid sequence translated from the base sequence of a gene, as this example of alanine methionine tyrosine arginine histidine threonine ? a small part of a gene shows 594  Molecules Insertions and deletions of a multiple of three nucleotides are not frameshift mutations but c an still have severe consequences bec ause there will be one or more amino acids more or less in the polypeptide expressed from a mutated gene. This may c ause radic al changes to the structure of the protein and therefore affect whether or not it c an c arry out its functions. BRCA1—an example of gene mutation The BRCA1 gene (pronounced “bracker-one”) codes for the BRCA1 protein in humans. BRCA1 is referred to as a tumour suppressor gene, but its actual function is DNA repair. It c an mend double strand breaks and help to correct mismatches in base pairing. If this gene mutates and the BRCA1 protein c annot c arry out its function, the consequence is an increased risk of other mutations due to the lack of DNA repair. In body cells, the most obvious sign of this is an increased risk of tumour formation and c ancer, particularly breast, ovarian and prostate c ancer. O ver 20,000 variants (alleles) of BRCA1 have been identied, including base substitutions, deletions and insertions. These c an be viewed online in the BRCA Exchange database. The numbering system for variants is explained there, using one example each of a substitution, deletion and insertion. this is a base substitution from adenine to guanine a base substitution changes the amino acid 356 from glycine to arginine, but it is a benign mutation that does not increase the risk of cancer c.1067A>G the letter c tells a deletion of nucleotide 73 changing us the details amino acid 5 from asaragine to c.73del folloing are for threonine but this is a frameshift mutation the coding so all subseuent amino acids are also seuence of the changed—this increases the risk of cancer gene c.10161017in s an insertion of cytosine beteen nucleotides 1016 and 1017 changing amino acid 33 from lysine to asaragine and all subseuent amino acids as this is a numbering indicates frameshift—the cancer risk is increased hich nucleotides are affected by the mutation ▴ Figure 6 Not all variants of the BRCA1 gene increase the risk of c ancer S ame-sense base substitutions are benign, as are some substitutions that change the amino acid in the most important functional domains in the BRCA1 protein. Of the variants that are non-benign, there is a range of increased risk of c ancer but some of the variants increase the lifetime risk of breast c ancer in women by as much as 80%. 595  Continuity and change Data-based questions: BRCA1 mutations BRCA1 is a tumour suppressor gene. There are over Jewish non-Hispanic Whites. Splice mutations aect the 500 variants of the gene that increase the risk of breast editing out of introns, which are base sequences in genes and ovarian c ancer in women and the risk of some that are transcribed but not translated. other c ancers in both sexes. Most of these variants are Source: Bougie O, Weberpals, JI. Clinical Considerations extremely rare in human populations. Table 1 shows the of BRCA1- and BRCA2-Mutation Carriers: A Review. frequency of types of base substitution mutation among International Journal of Surgical Oncology, vol. 2011, Article breast c ancer patients in ve groups among the US ID 374012, 2011. https://doi.org/10.1155/2011/374012 population. The NAJ+H White group were non-Ashkenazi Mutation type Number (%) Hispanics Afric an Americ ans Asian Americ ans Ashkenazi Jewish NAJ + H White (n = 21) (n = 8) (n = 3) (n = 8) (n = 14) Frameshi 15 (71) 2 (25) 0 8 (100) 5 (36) Mis-sense 3 (14) 3 (38) 0 0 2 (14) Nonsense 3 (14) 1 (13) 3 (100) 0 4 (29) Splice 0 2 (25) 0 0 3 (21) ▴ Table 1 Frequency of types of base substitution mutation among ve groups of breast c ancer patients in the US 1. More frameshi mutations of the BRCA1 gene were b. Mutations are random changes to the base found in this research than other types of mutation. sequence. There are far more possible mis-sense Explain the reasons for the high frequency of mutations than nonsense mutations. Suggest frameshi mutations in patients with breast c ancer. reasons for the relatively high percentage of patients with nonsense mutations. 2. a. 15% of the breast c ancer patients in the combined groups had a mis-sense mutation. 3. a. Compare and contrast the data for Afric an C alculate the percentage that had a nonsense Americ an patients with the data for Ashkenazi mutation. Jewish patients. b. Suggest reasons for the dierences. Using a database The Online Mendelian Inheritance in M an (OMIM) 1. Use the OMIM database to determine which D atabase provides information on many mutations found chromosome is aected by CCR5-Δ32 and what is its in humans. The Allele Frequency D atabase (ALFRED) loc ation on the chromosome. provides information about how these mutations are 2. Determine how many variants of the mutation there are. distributed across populations around the world. An allele 3. Use the database to identify geographic regions with is a variety of a gene. A mutation c auses a new allele to be relatively high frequencies of the allele. formed. An interesting human mutation is the CCR5-Δ32 mutation. The 4. One hypothesis for the distribution of the allele CCR5-Δ32 allele is notable for its recent origin, unexpectedly is that it originated from Viking populations that high frequency, and distinct geographic distribution. Together, migrated into Europe. Use the ALFRED to contrast the these features suggest that the CCR5-Δ32 allele: distribution of the allele in populations in Northern Europe and Southern Europe. arose from a single mutation 5. Another hypothesis was that the CCR5-Δ32 mutation was historic ally subject to positive selection. enabled survival from the plague (infection by the The mutation is responsible for the two types of resistance bacterium Yersinia pestis). Compare the areas where to HIV. CCR5-Δ32 hampers the ability of HIV to infect there is a high frequency of the allele with areas that immune cells. This is bec ause the mutation c auses the were signic antly aected by historic al outbreaks of CCR5 receptor (normally on the outside of cells) to be the plague. reduced in size and to no longer sit outside the cell. 596  Molecules D1.3.4 C auses of gene mutation Gene mutations c an happen at any time, but the chance is normally very low as DNA is resistant to chemic al change. There is an increased risk during DNA replic ation, when base-pairing errors are sometimes made and not corrected by DNA repair. The frequency of mutation is increased by external agents known as mutagens. There are two types of mutagen: radiation and chemic als. 1. R adiation increases the mutation rate if it has enough energy to c ause chemic al changes in DNA. Gamma rays, X-rays and alpha particles from radioactive elements such as radon are mutagenic. Short-wave ultraviolet radiation in sunlight is also mutagenic. 400 nm 500 nm 600 nm 700 nm elbisiv cosmic gamma X- ultra infra rays rays rays violet red energy / eV 12 9 6  10 10 10 10 1 ◂ Figure 7 The energy of avelengt electromagnetic radiation is / m inversely proportional to its 1 15 12 9 6 10 10 10 10 10 wavelength 2. Some chemic al substances c ause chemic al changes in DNA, so are mutagenic. Examples are polycyclic aromatic hydroc arbons and nitrosamines found in tobacco smoke. Mustard gas is also mutagenic and was used as a chemic al weapon in the First World War. D1.3.5 R andomness in mutation Mutations are described as random changes. Whether or not anything c an be truly random is a debatable question, but mutations certainly are unpredictable and c annot be directed by living organisms to achieve an intended outcome. The consequences of a mutation have no inuence on the probability that this ▴ Figure 8 The mainstream smoke drawn mutation will or will not occur. No natural mechanism for changing a particular through the lter tip of a cigarette is an base with the intention of making a benecial change to a trait has been 10 aerosol containing about 10 particulates discovered or is likely ever to be discovered in living organisms. At most, some per millilitre and 4,800 chemic al organisms seem to have limited control over their overall mutation rate. compounds, at least 60 of which are mutagenic Mutations c an occur anywhere in the base sequences of a genome although some bases have a higher probability of mutating than others. This is bec ause a mutation is a chemic al change, and some chemic al changes happen more easily than others. The position of a base within the genome also aects the chance of mutation. This is bec ause the dierences in how coding and non-coding DNA sequences are used aects the likelihood of mutation. 597  Continuity and change A consequence of randomness is that a mutation is unlikely to be benecial. Genes have developed by evolution over long periods of time, in some c ases over hundreds of millions of years. So, a random change will usually be neutral or harmful. To the best of our knowledge, there is no mechanism for testing out mutations acquired during the lifetime of an organism so that only the benecial ones are passed to ospring. If a mutation happens in a somatic cell (body cell) its eects may be tested when the gene is expressed, but the mutation is eliminated when the individual dies. The situation is reversed for mutations in germ-line cells— for example, cells in the testes and ovaries of humans. These mutations c an be passed to ospring in gametes but are not tested by gene expression in body cells. Traits that are due to mutations acquired during an individual’s lifetime and that prove to be benecial c annot be inherited by ospring. This helps to explain how evolutionary change occurs. ▴ Figure 9 Is the outcome of shaking ve dice in a dice cup random? D1.3.6 Consequences of mutation in germ cells and somatic cells The consequences of mutation depend on whether it occurs in a germ cell or a somatic cell. Germ cells give rise to gametes, so genes in germ cells c an be passed to ospring. A new allele, produced by mutation in a germ cell c an therefore be inherited. In rare c ases a new allele may confer an advantage on ospring, but in far more c ases the mutation c auses a genetic disease. It is therefore particularly important to minimize the number of mutations in gamete- producing cells within the gonads (ovaries and testes). Mutations in somatic cells (body cells) are eliminated when the individual dies, so they mostly have limited consequences. A cell may die, but it c an generally be replaced. The consequences of mutations in one group of genes c an be much greater. These are the genes that have roles in control of the cell cycle and cell division. They are known as proto-oncogenes bec ause mutations c an change them into oncogenes, which are c ancer-c ausing genes. C ancer is due to loss of control of the cell cycle, resulting in uncontrolled cell division and therefore tumour formation. The cell cycle, cell division and c ancer are described in Topic D2.1 D1.3.7 Mutation as a source of genetic variation An allele is a variant of a gene, diering in one or more bases from other alleles of the gene. Mutation changes the base sequence of a gene, so it changes one allele into another. Mutation increases the number of dierent alleles of genes in a population, so it increases genetic variation. Meiosis and sexual reproduction c an increase variation by generating new combinations of alleles, but mutation is the original source of all genetic variation. Most mutations are either harmful or neutral for an individual organism but nonetheless mutation is needed in all species. This is bec ause natural selection requires genetic variation, so species c annot evolve without it. Especially during times of rapid environmental change, populations must adapt to new conditions. 598  Continuity and change D2.1.1 Generation of new cells in living organisms by cell division All organisms need to produce new cells, for growth, maintenance and reproduction. They do this by cell division. One cell divides into two. The cell that divides is c alled the mother cell and those produced from it are daughter cells. The mother cell disappears as an entity in the process, unlike reproduction by animal parents. There is strong evidence for the theory that new cells are only ever produced by division of a pre-existing cell. The implic ations of this theory are profound. If we consider the trillions of cells in our bodies, each one was formed when a pre-existing cell divided in two. We c an trace this back to the original cell—the zygote that was the start of our individual lives, produced by the fusion of a sperm and an egg. Sperm and egg cells were produced by cell division in our parents. The origins of all cells in our parents’ bodies goes back to the zygote from which they developed and then on through all previous generations of human ancestors. If we accept that humans evolved from pre-existing ancestral species, we c an trace the origins of cells back through hundreds of millions of years to the earliest cells on E arth. This means there is a continuity of life from its beginnings to the cells in our bodies today. ▴ Figure 3 Attempts are being made by BaSyC, a research group in the Netherlands, to build a new living cell from individual lifeless components. This ying-cell artwork represents the challenges of the still unachieved endeavour 610  Cells D2.1.2 Cytokinesis as splitting of cytoplasm in a parent cell between daughter cells In cytokinesis, the cytoplasm of a cell is divided between two daughter cells. It is a part of cell division, along with nuclear division by mitosis or meiosis. The process of cytokinesis c an begin as soon as chromosomes have separated and are far enough apart to ensure that none of them ends up in the wrong cell. All the cytoplasm and its contents of the mother cell are shared out between the daughter cells. Plant and animal cells c arry out cytokinesis dierently. In animal cells, the plasma membrane is pulled inwards around the equator of the cell to form a cleavage furrow. This is accomplished using a ring of contractile proteins immediately inside the plasma membrane, usually at the equator. The proteins are actin and myosin and are similar to those that cause contraction in muscle. When the cleavage furrow reaches the centre, the cell is pinched apart into two daughter cells. In plant cells, microtubules are built into a sc aold straddling the equator, which is used to assemble a layer of vesicles. The vesicles fuse together to form plate- ▴ Figure 4 A starsh zygote has just shaped structures. With the fusion of more vesicles, two complete layers of divided for the rst time, to produce a two- membrane are formed across the whole of the equator of the cell. They become cell embryo the plasma membranes of the two daughter cells adjacent to the new dividing walls. They are connected to the existing plasma membranes at the sides of the cell, completing the division of the cytoplasm. bundles of microtubules scaffold the formation of a new cell wall across the equator with new plasma membrane on either side ◂ Figure 5 Construction of a new plant cell walls across the equator of a diving cell new cell wall completed across the equator vesicles vesicles complete aligned fuse to double on the form membrane plasmodesmata equator plates forming formed 611  Continuity and change The next stage in plants is for pectins and other substances to be brought in vesicles and deposited by exocytosis between the two new membranes. This forms the middle lamella that will link the new cell walls. Both daughter cells then bring cellulose to the equator and deposit it by exocytosis adjacent to the middle lamella. As a result, each cell builds its own cell wall across the equator. ▸ Figure 6 These cells are from a growing onion root. Cell A is large enough to divide but is in the early stages of mitosis so c annot yet divide Cell B has nearly completed mitosis and is already constructing a wall across the equator, with the microtubule sc aolding visible L abel C points to where the previous cell division occurred to produce cells A and B from a mother cell A C B D2.1.3 Equal and unequal cytokinesis In many c ases, cytokinesis divides the cytoplasm of the mother cell into equal halves. This happens in a growing root tip. Root growth is due to enlargement and division of cells arranged in columns. The cells in a column all dierentiate in the same way, so cytoplasm is apportioned equally when they divide. Cy t o p l a s m is sometimes divided u n e qu a l l y. Sm a l l cells pro du c e d by unequal di v i s i o n c an s u r vi ve and grow if t he y re c e i ve a nucleus and at le ast one of e ach o rga n e l l e th a t c annot be a ss e m bl e d f ro m c o mp o n e n t s in t he cell Fo r exa mp l e, m i t o c ho n d r i a c an only be pro d u c e d by di vi s i o n of a pre - exi s ti n g mi t o c ho n d r i o n , so t he re mu s t be at le ast one m i t o c ho n d r i o n in a d a u gh te r cell fo r it to be v i a bl e. Tw o ex a m pl e s of u n e qu a l division a re b u d di n g in ye a s t and oogenesis in h u ma n s. X ▸ Figure 7 Nuclei in these onion root cells have been stained red. New cell walls have mostly divided cytoplasm equally, as at X; but in some cases, the division was unequal, as at Y Y 612  Cells ◂ Figure 8 Occ asionally a plant cell divides and its chloroplasts all pass into one of the two daughter cells. The other cell with no chloroplasts c an never regain them and it produces more cells without chloroplasts when it divides. These leaves were collected from one plant and show areas of cells without chloroplasts Budding in yeast Yeast cells reproduce asexually in a process c alled budding. The nucleus divides by mitosis. A small outgrowth of the mother cell is formed. It receives one of the nuclei, but only a small share of the cytoplasm. A dividing wall is constructed, separating the two cells. The small cell then splits away, leaving a sc ar where it was attached to the larger cell. Yeast cells c arry out this budding process repeat- edly and do not have to double in size between each division. Oogenesis in humans The production of both sperm and eggs in humans starts with two divisions of a mother cell. During sperm production, the cytoplasm is divided equally in the rst and second divisions, resulting in four, equally sized small cells, each of which develops into a mature sperm. Whereas large numbers of sperm are needed in humans, usually only one egg cell (oocyte) is produced at a time, with enough stored food to sustain the developing embryo. There is therefore unequal division of cytoplasm during oogenesis. The rst division produces one large cell with nearly all the cytoplasm ▴ Figure 9 There are many dierent and a small polar body which does not develop further. Only the large cell c arries species of yeast. These are Komagataella out the second division, with unequal division of the cytoplasm again resulting phai which c an use methanol as a c arbon in one large cell and one very small polar body. The large cell develops into a source if glucose is not available. Several mature oocyte that is ready for fertilization. cells are budding cytoplasm of oocyte polar body zona pellucida ▴ Figure 10 The protective jelly coat (zona pellucida) around this human oocyte is visible. The dense cytoplasm of the oocyte contains food stores. To the right is a tiny polar body that has received a nucleus but very little cytoplasm bec ause it has no role and will not survive 613  Continuity and change D2.1.4 Roles of mitosis and meiosis in eukaryotes If a cell divides without rst undergoing nuclear division, one daughter cell has the nucleus and the other one would be anucleate (without a nucleus). Anucleate cells c annot synthesize polypeptides, so they c annot grow or maintain themselves. They have limited lifespans. For example, red blood cells, which have no nucleus, survive for about 120days. To produce extra nuclei before cell division, cells undergo either mitosis or meiosis. These two types of nuclear division have dierent roles, so most organisms use both during their life cycle. Mitosis—continuity Meiosis—change Mitosis is used to produce genetic ally Meiosis is used to halve the chromosome identic al cells. 2n is the diploid number number from diploid (2 n) to haploid (n) of chromosomes. In humans, n = 23. and to generate genetic diversity n division of a cell with only 2n one nucleus produces one n nucleated and one anucleate 2n cell n 2n 2n 2n n 2n n 2n n 2n Cells produced using mitosis Cells produced using meiosis have the same number of have half as many chromosomes chromosomes as the parent cell, as the parent cell. Division so the chromosome number is of a nucleus with two sets of maintained. chromosomes (diploid) results in nuclei with only one set (haploid). Cells produced by mitosis have This is essential to produce the same genes as the parent cell, haploid gametes from diploid so mitosis maintains the genome. germ cells in sexual life cycles. This ensures that every cell in a multicellular organism has all the Pairs of genes in a diploid genes that it needs. It also ensures mother cell are dealt randomly that the cells in an individual are to daughter cells, so there are genetic ally identic al, preventing an almost limitless numbers of to produce two daughter cells, problems such as tissue rejection. possible combinations. Meiosis each with a nucleus, the mother therefore generates variation Mitosis allows a successful cell’s nucleus must first be and genetic diversity, allowing genome to be inherited without divided evolution by natural selection. changes by offspring in asexual ▴ Figure 11 Nucleated and anucleate reproduction. cells produced from cell division 614  Cells D2.1.5 DNA replic ation as a prerequisite for both mitosis and meiosis A cell that is preparing for nuclear division by mitosis or meiosis replic ates all the DNA. This ensures that each daughter cell produced receives a full complement of genes, allowing it to perform any function required. An earlier hypothesis, now falsied, was that a single centromere held the chromatids together until anaphase, when it divided, allowing the chromatids to separate. Before replic ation, the DNA within the nucleus exists as long single molecules c alled chromosomes. Aer replic ation, there are pairs of identic al DNA molecules. These identic al DNA molecules are still considered to be part of the same chromosome and they are held together by loops of a protein complex, c alled cohesin. The cohesin loops are not cut until the start of anaphase during mitosis or meiosis. ▴ Figure 13 The number and structure Early interphase Prophase Metaphase Anaphase of chromosomes in a species c an be studied by staining cells in mitosis with a pigment that binds to DNA and then by bursting the cells on a microscope slide so the chromosomes spread out. The Indian muntjac, Muntiacus muntjak (top) has the smallest number of chromosomes per body cell among mammals (2n = 6) and the Visc acha rat, Tympanoctomys barrerae (bottom) has the largest number (2n = 102) single chromatid chromosome microtubules cohesin loops have before DNA with two attached to been cut, so sister replication. During chromatids centromeres chromatids can interphase it held together pull on chromatids separate and be would be much by cohesin but cohesin holds pulled to opposite more elongated loops, as them together poles in prophase of mitosis ▴ Figure 12 Chromosomes, chromatids and cohesion loops When DNA is in an elongated state, chromosomes are too narrow to be seen with a light microscope. They gradually become shorter and fatter during the early stages of mitosis or meiosis and are then visible. Eventually each chromosome c an be seen to have two strands, c alled chromatids. E ach chromatid contains a single very long DNA molecule, produced by DNA replic ation from an original molecule. The two strands in a chromosome are therefore known as sister chromatids and they are genetic ally identic al. Strands on dierent chromosomes are non-sister chromatids and do not usually have identic al genes. Figure 13 shows chromosomes consisting of pairs of sister chromosomes in two species of mammal. 615  Continuity and change D2.1.6 Condensation and movement of chromosomes as shared features of mitosis and meiosis During mitosis and meiosis, chromosomes are moved to opposite poles of the cell, so they c an become part of separate nuclei (Figure 14). The DNA molecules in these chromosomes are immensely long. For example, the average length in human chromosomes is more than 50,000 µm and the nucleus is less than 5 µm wide. To separate and move molecules as elongated as this without knots, tangles or breaks they must be packaged into much shorter structures. This condensation of chromosomes and their subsequent movement is therefore an essential feature of mitosis and meiosis. in mitosis and the 2nd in the 1st division of meiosis division of meiosis sister homologous chromosomes chromatids separate and (each with two sister chromatids) are moved to opposite poles are moved to opposite poles opposite poles equator of cell ▴ Figure 14 Movement of chromosomes during mitosis and meiosis Chromosomes are condensed by being made shorter. An initial shortening is c arried out by wrapping the double helix of DNA around histone proteins to form nucleosomes, and linking the nucleosomes together. There are several more stages to condense the chromosomes but they are not yet fully understood—this is an active research eld. Chromosomes are moved using microtubules. A microtubule is a hollow cylinder of tubulin proteins that can be rapidly assembled or disassembled. During interphase, microtubules serve a variety of functions including acting as a cytoskeleton. Some of these microtubules are disassembled in the early stages of mitosis and are reassembled by microtubule organizing centres (MTOCs) at the poles of the cell, which link tubulin molecules together. Microtubules are assembled that reach the equator of the cell, forming a spindle-shaped array. At the same time, protein structures called kinetochores are assembled on the centromere of each chromatid. Some of the growing microtubules link up with these kinetochores and some attach to other microtubules from the opposite pole. 616  Cells there are two types of tubulin in microtubules, both arranged in a helix α-tubulin β-tubulin 24 nm microtubules are hollow and rigid like at 2 nm the DNA the next pair of steel scaffolding double helix is narrower tubulin subunits will poles than a microtubule be added here ▴ Figure 15 Microtubules c an be changed in length by adding tubulins at one end, or removing them from the other end The kinetochore acts as a microtubule motor by removing tubulin subunits from the attached ends of the microtubules. This shortens the microtubules linking the kinetochores to the poles, putting them under tension. Initially, in mitosis the chromatids do not move bec ause loops of cohesin hold them together. As soon as the cohesin has been cut, shortening of the spindle microtubules by the kinetochores c auses sister chromatids to move to opposite poles. In meiosis, homologous chromosomes are initially held together by knot-like structures c alled chiasmata, but when these have slid to the ends of the chromosomes, movement to opposite poles c an begin. D2.1.7 Phases of mitosis Mitosis requires a precisely choreographed sequence of actions. These are usually considered as four phases. Prophase—the starting phase with condensation of chromosomes (pro = before). Metaphase—the phase after condensation with chromosomes released from the nucleus (meta = after). ▴ Figure 16 Fluorescent stains have been Anaphase—a brief phase during which the chromosomes are moved up to used to reveal the position in anaphase poles from the equator (ana = up). of DNA (blue), kinetochores (red) and Telophase—the final phase in which nuclei reform and chromosomes telomeres which form the ends of the decondense (telos = finally). chromosomes (green) 617  Continuity and change The chromosomes are chromatin (dispersed Interphase cytoplasm chromosomes dispersed through the inside the nucleus) nucleus so are not individually discernible. To prepare for mitosis, all of the DNA is replic ated and each chromosome then consists of two very elongated chromatids containing identic al DNA. plasma nuclear membrane membrane Interphase—preceding mitosis The chromosomes condense Prophase by packing the DNA tightly sister kinetochore into thicker, shorter structures. chromatids attached to the This is a protracted process held together centromere that continues throughout by loops of of the prophase. cohesin chromatid Towards the end of prophase microtubules grow from structures at the poles of the cell c alled microtubule organizing centres (MTOC s) microtubules to form a spindle-shaped array organizing spindle microtubules linking the poles of the cell. centre (MTOC) At the end of prophase the Prophase nuclear membrane breaks down. Microtubules growing Metaphase from the poles attach to the chromosomes chromosomes aligned on centromere of each chromatid. fully the equator Sister chromatids within condensed each chromosome become attached to opposite poles. The spindle microtubules kinetochore are put under tension to test spindle whether the attachment is microtubules correct. If the attachment is MTOC cytoplasm not correct, the chromosomes separated from c annot yet be pulled to either nucleus by a pole due to cohesin loops. nuclear membrane At the end of metaphase, the Metaphase chromosomes are aligned on the equator of the cell. 618  Cells Cohesin loops that have Anaphase held the sister chromatids kinetochore removes tubulin subunits to shorten spindle together are now cut, so the genetically microtubule and pull identical chromatids become separate chromosome to the pole chromosomes chromosomes. (formerly sister chromatids) Microtubules link each