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QUESTIONS: 2014; 1a DNA Structure Base Phosphate Sugar -‐ Deoxyribose • • • • The bases are the source of genetic variation and are what make different strands of DNA unique • One of these monomers is pictured above and is called a ‘nucleotide’. A nucleotide is made up of a sugar (deoxyribose), and a base There are four different bases: Adenine, Thymine, Guanine and Cytosine • The sugar and phosphate are the same across all nucleotides and link together to make a strong ‘backbone’ for the bases to attach to These bases adhere to a strict set of rules known as the ‘complementary base pairing’ rules that help ensure processes such as DNA replication can occur • These rules state that Adenine can only bind to Thymine and Cytosine can only bind to Guanine DNA is classified as a ‘polymer’. This means it is made up of thousands of repeating units known as ‘monomer’s © StudyTime 2015 Practice Question Use the unlabeled diagram below to discuss the structure of a strand of DNA d DNA is made up of thousands of repeated units. These units are arranged in pairs in a ‘double helix’ structure that resembles a ladder of bases. Each repeaBng unit is known as a nucleoBde and has an idenBcal sugar and phosphate group. The nucleoBdes can however differ in which base is aGached to them. If a nucleoBde contains the base ‘Adenine’, its neighboring nucleoBde must contain the base ‘Thymine’, and vice versa. Similarly, if a nucleoBde contains the base ‘Guanine’, its neighboring nucleoBde must contain ‘Cytosine’. This is according to the complementary base pairing rules and defines the geneBc diversity between differing strands of DNA. © StudyTime 2015 QUESTIONS: 2014; 1b 2013; 2a 2012; 1a, b Genes, Alleles, Chromosomes Gene Allele Chromosome • An organism contains thousands of nucleotides arranged in a very long chain of DNA • The location of a particular gene is the same across all chromosomes • In order for all of the DNA to fit into the nucleus, this DNA is arranged into smaller pieces of wound up DNA known as ‘chromosomes’. Humans contain 46 chromosomes each consisting of tightly wound DNA • Organisms of the same species can have different versions of these genes. Different forms of a particular trait (i.e ‘green eyes’) are known as different ‘alleles’ • Chromosomes are further divided into sections known as ‘genes’. Each gene is responsible for the ‘coding’ of a particular trait (i.e ‘eye colour’) • Therefore, chromosomes are made up of DNA, which is made up of genes. These genes have different expressions known as alleles. © StudyTime 2015 Practice Question Use the words alleles, DNA, chromosomes, genes and nucleus to complete the following sentence: d ______ reside in the ________ and are made up of wound_______. This is arranged in sections called ________ which have different versions known as _____ Chromosomes reside in the nucleus and are made up of wound DNA. This is arranged in sections called genes which have different versions known as alleles. © StudyTime 2015 QUESTIONS: 2013; 2b Alleles Bb BB Heterozygous • Alleles are different versions of a particular trait, such as different eye colours • Alleles are determined by the unique sequence of bases found in a particular gene in an organism • Organisms such as humans which form as a result of sexual reproduction contain two separate alleles for each trait. These alleles determine how the trait is expressed on the organism Homozygous Dominant • Dominant alleles are denoted using a capital letter and mask the effects of the second allele. This means that a dominant allele will always be expressed • Recessive alleles are denoted by a lower case letter and are able to be masked by a more dominant allele. This means that a recessive allele will only be expressed if both alleles present are of the recessive type • An organism with two identical alleles for a particular gene is known as homozygous (i.e BB). An organism with two differing alleles is heterozygous (Bb) © StudyTime 2015 Practice Question Define the following as heterozygous or heterozygous and state which allele will be expressed d B = Black hair b = Blonde hair Alleles Present Heterozygous/Homozygous Allele Expressed BB Homozygous dominant Black hair bb Homozygous recessive Blonde hair Bb Heterozygous Black hair © StudyTime 2015 Mutations Change in base sequence • Mutations are the only way of creating totally new alleles in a species • Mutations originate from a change to one or more bases in the intended base sequence of an organism’s DNA • The change in base sequence means that the amino acid sequence and hence the protein created by the DNA will differ to the one intended • Mutations can be either harmful or beneficial, although harmful ones are more common • It is easier for a beneficial mutation to become established in a population. Once it does, it becomes known as an allele • Although mutations can occur in any cell of an organism, only those occurring on gametes (sperm or egg) can be passed on to offspring during reproduction © StudyTime 2015 Practice Question Too much exposure to direct sunlight can lead to a mutation causing skin cancer. Explain what this mutation causes at the DNA level and discuss why skin cancer isn’t directly inheritable. d Too much exposure to direct sunlight can cause a base to change in the DNA of a cell. Because DNA codes for specific proteins, any change to the DNA sequence can directly effect what the organism is able to create. A cell that is effected by the mutaBon has had its DNA sequence altered to the point where it becomes dangerous to itself. Skin cancer isn’t directly inheritable as the mutaBon occurs on a skin cell. As skin cells are not involved in reproducBon, a mutaBon confined to non-‐sex cells is not passed on to offspring. © StudyTime 2015 QUESTIONS: 2014; 1a, b, c 2013; 3a Sexual Reproduction 23 23 • • • All organisms perform either asexual or sexual reproduction, with some able to perform both. Both types or reproduction have their own advantages and disadvantages • Humans perform sexual reproduction. This means that human offspring have two parents and genetics that are a combination from both Sexual reproduction is advantageous as it means offspring are never genetically identical to their parents. This is useful as it adds genetic diversity to a population and hence increases survival rates • During sexual reproduction, offspring inherit a set of alleles for each gene from each parent • Because of this, gametes have half the number of chromosomes usually found in an individual (23) Sexual reproduction can be disadvantageous as it carries a high energy cost, and requires a reproducing individual to find a mate © StudyTime 2015 Practice Question State whether the following concepts are characteristic to sexual or asexual reproduction Concept High energy cost Sexual/Asexual t Sexual Gametes with half the number of intended chromosomes Sexual Necessity to find mate Sexual Offspring geneBcally idenBcal to parent Asexual Faster Asexual Offspring not idenBcal to parent Sexual © StudyTime 2015 QUESTIONS: 2014; 1d, 2a 2013; 1c, d 2012; 2b Genotypes/Phenotypes Bb Black Hair Genotype • The allele combination of an organism for a particular gene can be described in two different ways: using either the genotype or the phenotype • The genotype describes the actual alleles present, whilst the phenotype represents the result of the allele combination • The ‘genotype’ is represented as two letters, each describing an allele from either parent. Dominant alleles are in capital letters while recessive alleles are lower case Phenotype • The phenotype is represented as words and describes the physical outcome of the genotype • Genotype and Phenotype ratios can describe the potential outcomes for offspring calculated using a punnett square. They describe the likelihood of each potential genotype and phenotype across the offspring of two individuals © StudyTime 2015 Practice Question For each allele combination, give the phenotype and genotype. Then give the overall Genotype and phenotype ratios for the offspring Allele 1 Allele 2 Genotype Phenotype Black Hair (B) Black Hair (B) BB Black Hair Blonde Hair (b) Black Hair (B) Bb Black Hair Blonde Hair (b) Bb Black Hair Black Hair (B) Blonde Hair (b) Blonde Hair (b) bb t Blonde Hair Genotype raBo = 1 : 2 : 1 Phenotype raBo = 3 : 1 © StudyTime 2015 QUESTIONS: 2014; 1c 2013; 3c, 4a, b • • Genetic Variation/ Adaptations Genetic variation is a concept highly important to the survival of a species. It means that not all members of a population are susceptible to the same diseases/ predators and hence are unlikely to become extinct if the environment changes Genetic variation arises due to the processes involved in Meiosis, and the nature of sexual reproduction combining two sets of alleles • During an environmental change, those with beneficial genetics will live to reproduce and pass on their alleles • Those that lack beneficial alleles will not survive to reproduce. The harmful alleles will therefore die out. This process is called ‘natural selection’ • The beneficial alleles which go on to establish in the population go on to be called ‘adaptations’ and aid organisms in that particular environment © StudyTime 2015 Practice Question A new predator is introduced to a population. Those with blue coloring are able to blend into their surroundings to avoid being seen, whilst the orange organisms are not. Discuss what will happen to the amount of blue and orange individuals in the population over time. d v The organisms with blue coloring have a phenotype which is favorable to their environment. This means they each have an allele combination which contributes towards their survival. Because they have a greater survival rate, organisms with blue coloring have a much greater chance of reproducing. This means that more blue color alleles will be passed on to future generations. Because of this, the number of blue individuals in the population will increase over time while the number of orange will decrease. © StudyTime 2015 QUESTIONS: 2014; 4c 2012; 2a Punnett Squares B b B BB Bb b Bb bb • • • Punnett squares are a way of working out the likelihood of different genotypes arising from a set of parents By counting the relative numbers of each genotype and phenotype represented, punnett squares are a convenient way of calculating genotype and phenotype ratios The outer cells of the square represent the alleles present in each of the parents. (It doesn’t matter where you put each parent, as long as you are consistent) • The inner cells of the square are then filled in by combining the two adjacent alleles together • Remember that a capital letter represents a dominant allele and a lower case letter represents an allele that is recessive. When filling in the punnett square, always write the capital letter first © StudyTime 2015 Practice Question Two individuals heterozygous for hair colour (Bb) reproduce to create a series of offspring. Draw a Punnett square for this cross and use it to form an expected phenotype and genotype ratio for the offspring 1. Maternal alleles = B, b 2. Fill in punnett square: Paternal alleles = B, b B B b B BB Bb b b Bb bb B b 3. Genotype ratio = 1 : 2 : 1 Phenotype ratio = 3 : 1 © StudyTime 2015 QUESTIONS: 2014; 4a, b 2013; 1a, b Pedigree Charts Unaffected Male Affected Male Unaffected Female Affected Female • Pedigree charts are special family trees that give information on the presence of a specific trait across multiple generations • Different shapes are used to represent males and females. This makes it easy to tell whether a specific trait is more likely to affect males or females • They are often used to track the presence of a disease or a specific allele through a family • • A key is provided with most pedigree charts. This details how to read whether an individual is affected or not Each shape on the pedigree chart represents an individual, with horizontal lines showing breeding partners and vertical lines leading to offspring • By looking at offspring, pedigree charts can be used to ‘work backwards’ to figure out the genotypes of parents © StudyTime 2015 Practice Question Use the pedigree chart and key below to identify the genotype of each parent for a particular disease. P = affected, p = unaffected d Unaffected Male Affected Male Unaffected Female Affected Female 1. The mother has the unaffected phenotype 2. As the unaffected phenotype is recessive, she must not have a dominant allele 3. The mother’s genotype is ‘pp’ 4. The father has the affected phenotype 5. As the affected phenotype is dominant, he could have either the ‘Pp’ genotype, or ‘PP’ 6. The father has unaffected offspring. In order for this to occur, he must pass on a recessive allele. The father therefore has the genotype ‘Pp’ © StudyTime 2015 QUESTIONS: 2014; 3b Pure Breeding B b b Bb bb Test cross for a heterozygous (not pure breeding) individual b Bb bb • • • Because a dominant allele will lead to an individual expressing the dominant phenotype regardless of the other allele, it can be hard to tell whether a dominant individual has the heterozygous or homozygous dominant genotype Knowing the actual genotype is useful to breeders and farmers as an individual with the homozygous dominant genotype is ‘pure bred’ This terms comes from the idea that two pure-bred individuals can only reproduce to create individuals with the dominant phenotype • The best way to work out the genotype of an individual is to run a ‘test cross’ using them. This involves breeding the unknown individual with a homozygous recessive individual • If any of the offspring from the cross exhibit the recessive phenotype, the unknown individual must be heterozygous • If all of the offspring exhibit the dominant phenotype, it can be indicated that the unknown individual has the homozygous dominant genotype. The more offspring there are created by the cross, the more sure we can be of this fact © StudyTime 2015 Practice Question A farmer has cows with both the recessive allele for black colour (B), or the dominant allele for brown colour (b). He wants to know whether one of his black cows has the heterozygous or homozygous dominant genotype. Explain, with the aid of punnett squares, how he could do this. 1. 2. 3. 4. In order to work out the unknown genotype, the farmer should use a test cross This involves crossing the unknown individual with a homozygous recessive individual If any of the offspring are brown coloured, the unknown individual is heterozygous If all of the offspring are black, there is a chance the unknown individual is Pure breeding. The more crosses are done, the more sure we can be. B b b Bb bb b Bb bb B B b Bb Bb b Bb Bb Heterozygous test cross Homozygous Dominant test cross © StudyTime 2015 QUESTIONS: 2013; 3b 2012; 3a Cell Division 4n Mitosis 2n n • • • 2n n Cells do not grow in size. Instead, they replicate to form identical cells in the processes of tissue growth and repair Mitosis is the cell replication responsible for growth and repair. It involves a single parent cell splitting into two identical ‘daughter’ cells Before Mitosis occurs, all of the DNA inside the parent cell is replicated in order for a copy to go into the daughter cell n Meiosis n • Meiosis is the cell replication responsible for the creation of gametes (sperm and egg cells) • The process of Meiosis begins the same way as Mitosis, but involves an extra cell division with no DNA replication • This results in four daughter cells with half the number of chromosomes as the original parent cell • We say that regular cells contain ‘2n’ chromosomes, whilst sex cells contain ‘n’ chromosomes – with ‘n’ representing the number of chromosomes unique to that species © StudyTime 2015 Practice Question Describe what type of cell division an egg cell would undergo, and use the diagram below to discuss the number of chromosomes the daughter cells will have As egg cells are a type of gamete, they will undergo Meiosis. This is because they are only involved in reproduction, and have no need to perform growth or repair. Meiosis involves two cell divisions but only one DNA replication. This results in daughter cells with half the number of chromosomes present in the parent cell. It is beneficial for gametes to contain half the number or chromosomes present in a regular cell as they are designed to combine with each other to form a zygote. The zygote contains the regular number of chromosomes and goes on to form an individual during sexual reproduction. © StudyTime 2015