Unit 3 Biological Change PDF

Unit 3 - Biological Change D4.1 Natural Selection describes how organisms better adapted to their environment tend to survive and produce more offspring. -​ It can change the traits of a population over time. Natural Selection involves the following ➔​ More offspring produced than can sur...

Unit 3 - Biological Change D4.1 Natural Selection describes how organisms better adapted to their environment tend to survive and produce more offspring. -​ It can change the traits of a population over time. Natural Selection involves the following ➔​ More offspring produced than can survive ➔​ Variation of traits within the population ➔​ Limited resources so competition occurs between individuals ➔​ Individuals that are better adapted to the environment survive ➔​ Survivors reproduce In order for natural selection to occur, Darwin concluded that there must be variation amongst the members of a population. -​ If all members were identical, then some individuals would not be favoured more than others. -​ Genetic variation can results from several processes:​ - mutation​ - meiosis​ - sexual reproduction Mutations are changes in an organism’s DNA sequence. New alleles are created, which can change the traits of a species member. Meiosis contributed to ovulation by producing new combinations of alleles. Because of crossing over and independent orientation, no two gametes produced are likely to be the same. During sexual reproduction, male and female gametes fuse to form a diploid cell. As a result of this fusion, the offspring contains alleles from two individuals. This allows mutations from different members to come together, creating new combinations of alleles. -​ Species vary in the number of offspring Remember from c4.1 that density independent factors are factors that limit population size and are not impacted by population density. For example temperature, water availability, ocean acidity. Changes in these factors can cause selection pressure that favours specific variations within a population. In the struggle for existence, members within the same specifies compete with each other for: -​ Territory -​ Mates -​ Food Thoose better adapted for obtaining these tend to survive and produce more offspring. These individuals are said to have more fitness. Over time, the frequency of beneficial traits increases in a population as the better adapted members reproduce. aS a result, the characteristics of the population will gradually change. Survival value is a way to describe how useful a particular trait is. If it helps and the organism survives, it has value. Reproductive potential refers to the maximum numbers of offspring that could survive without factors causing them to die. In reality this maximum size is never achieved due to the limiting factors. Species members that reproduce can pass their traits to their offspring, However, this does not include traits that they obtained during their lifetime. So acquired traits are not significant in the process of evolution. Sexual selection is when an individual has characteristics or traits that make it more successful at producing offspring than others. Drastic differences in physical appearance between male and female members of a species (or sexual dimorphism) are commonly the result of secular selection in bird species. Topic D3.2 - Inheritance Inheritance and Gametes Most cells have complete pairs of homologous chromosomes in their nuclei. This is referred to as being diploid. Haploid cells contain one chromosome from each pair in their nuclei. Gametes are haploid and muse fuse in order to form a diploid cell. Meiosis, a reductive division, is the process of producing four haploid nuclei from a single diploid cell. In diagrams, the state of a cell's chromosomes is represented as a factor of ‘n’. -​ Diploid cells = 2n -​ Haploid cells = n Haploid cells are formed from diploid cells during meiosis, which is further discussed in Topic D2.1. The process of meiosis is made up of two cell divisions which produce haploid cells (gametes) ; these cells are able to fuse to create a diploid zygote. Haploid gametes, which only contain half the chromosomes and one allele of each gene. A zygote results when two gametes fuse together, iT is able to grow and develop into an adult organism. During gamete production, the two alleles for a gene segregate into different daughter nuclei. The overall combination of alleles in the resulting cells is random, promoting variation. When gametes fuse, the resulting diploid zygote has two alleles for every genne, these alleles may be the same or different. Heterozygous - the two alleles for a gene are different. Homozygous - the two alleles for a gene are the same. Gregor Mendel was an Austrian monk who is known as the father of genetics. He noted the pea plants in his garden had characteristics that were sometimes passed onto offspring. To study how pea plants pass on traits, he created plants that were purebred for seven traits. Those traits included: Then he crossed a large number of plants with different characteristics to determine which traits would appear in offspring. His observations allowed him to discover the principles of inheritance which are discussed in this topic. Flowers are reproductive structures found in flowering plants. Their function is to effect reproduction by providing a mechanism for the fertilisation of eggs by sperm. Pollination occurs when a pollen reaches the stigma. The pollen can come from the same plant (self fertilisation) or a different one. In fertilisation, a pollen grain on the stigma grows down the style and reaches the ovary. eAch fertilised ovule is diploid and develops into a seed. The ovary develops into a fruit, which protects and provides nutrients for seeds. Monohybrid Cross A gene is a section of DNA that determines or influences traits. They are heritable, which means that they can be passed from parent to offspring. Variations of the gene for a single traits are called alleles. For example, there can be alleles for genes that determine: -​ Hair colour -​ Blood type -​ Ear shape In plants, there can be alleles or: -​ Flower colour -​ Seed colour -​ Height Eukaryotic chromosomes exist in sets called homologous chromosomes. Each member of a pair contains the same genes for the same loci. However, the allele for each gene may not be the same. When discussing inherited traits, two important terms are used: Genotype: the combination of alleles an organism has, typically using pairs of letters. Phenotype: the observable physical characteristics Some alleles are able to mask the effects of others when they are present. These are called dominant alleles and are represented using upper case letters (A=purple). Recessive alleles are those that are masked by dominant alleles. They are represented using lower case letters (a=white). Genotype is the combination of alleles inherited from the parent generation for a single trait. Phenotype is the observable trait. In most cases there are only 2 possible phenotypes; -​ Dominant -​ Recessive In pea plants Purple (P) - dominant White (p) - recessive Punnett grids are a tool that can be used to predict the outcomes of monohybrid crosses. These are crosses that involve the study of a single trait. Phenotypic plasticity is the ability to alter gene expression based on environmental conditions. Phenylketonuria (PKU) is a genetic disease caused by a mutation in the gene that creates an enzyme (phenylalanine hydroxylase) that metabolises tyrosine from phenylalanine. Excess phenylalanine and reduced tyrosine levels occur in affected individuals. PKU is a recessive genetic disorder requiring 2 recessive alleles to result in an affected individual. Symptoms include: -​ Reduced development of the head or brain -​ Mental retardation or learning disabilities -​ Hyperactivity -​ Seizures Although alleles are able to cause variations fo a trait, their DNA sequences are oftenly only different by one or few bases. Small changes in DNA can change the amino acid sequences of protein. This change can have subtle or sometimes drastic effects on characteristic. Human blood types are determined by the presence of proteins on the surface of red blood cells. The blood types of parents offspring can be predicted using monohybrid Punnett squares. Blood Type Genotypes There are four blood types in humans: A, B, AB and O. The genotype for each is shown in the table below. Each allele is represented as an I with superscript or i (absence of protein). Mismatches between donor blood and recipient can result in an immune response. The ABO blood group system describes the antigens. To prevent mismatch, blood typing can be done. This involves the use of antibodies to test for which antigens are present in samples. Specific antigens are present if agglutination is observed when blood and antibodies are mixed. Determining blood type - example 1 Punnett grids for blood types are used the same way as other monohybrid crosses. For example, This grid shows the results of crossing two AB individuals. Co-dominance occurs when co-dominant alleles are present. These have joint effects and so are both seen in the phenotype. I.e. both phenotypes are present (AB blood type) or the flowers below. Incomplete dominance occurs when alleles have joint effects and so are both seen blended in the phenotype. In the example here, red and white alleles mix to form a pink flower. Diseases and Sex Linkage Within a human’s karyotpe, there are two types of chromosomes, defined by whether or not thry affect the sex of the individual. -​ Sex chromosomes determine gender (pair 23) XX = female XY = male -​ Autosomal chromosomes do not determine sex (pair 1-22) Genetic diseases are disorders that are caused by errors in the genome. MMost result from recessive alleles on the autosomal chromosomes (1-22), though some are the results of dominance. Many diseases have been found in humans, but most are very rare. Improving genetic techniques are allowing scientists to identify and study more. Other genetic diseases can be linked to sex, which means the assoccciated allelels ae locateddd on the sex chromsoomes (23). As a result, the sex of offpsring can determinne the linklihood of inheriting these desaes. Two common examples include: -​ Red-green colour blindness -​ Hemophilia Punnett grids can be used to determine the inheritance of sex-linked traits. The sex chromosomes of the parents are used instead of autosomal alleles (male = XY, female = XX). the general setup is shown here: Red-green colorblindness is a sex-linked condition caused by a recessive allele on the XX chromosome. Since males only have one X, they are more likely to be affected than women, who have two alleles for each X linked gene. Those with the disease have mutated genes for red or green colour receptors in the retina, as a result, they are unable to properly distinguish the two colours. Sex linked inheritance example 1 Haemophilia is another genetic disease caused by a recessive allele on the X-chromosome. The disease prevents the ability to form Factor VIII, which is vital for blood clotting. It is life threatening. Individuals with this disorder typically have a life expectancy of about 10 years if left untreated. Treatment involves infusing fActor VIII isolated from donor blood sources. Pedigrees Patterns observed in pedigrees can be used to determine the type of disease/trait that is being studied. The typical possibilities are: Variation Members of the same species have slightly different sets of characteristics. Some of these differences are inherited from their parents and others are the result of environment. The differences between individuals of the same species are called variations. Such differences can be categorised as one two types, discontinuous and continuous. Discontinuous variation is variation that has distinct groups for organisms to belong to. Either an organism has a trait or it doesn't. These traits tend to be qualitative and can be represented using a bar graph. Examples include blood groups. Continuous variation is a variation that has no limit on the value that can occur within a population. They also tend to be polygenic. Every organism in a species shows the characteristics, but to a different extent. Examples include height, weight and heart rate. Polygenic traits are those that are influenced by two monroe traits. Because of this, continuous variation is observed. Patterns of inheritance food these traits statistically differ from predicted ratios. For example, human skin colour is determined by at least 3 independent genes (6 alleles), resulting in continuous variation. As shown here, there is a defined curve in skin colour phenotypes. Polygenic traits, such as human height, can be influenced by environmental factors such as diet and exercise. These influences result in a range of values in the species and so display continuity. Even with the same genetic information, identical twins can display different variations of a traits based on the environments they are exposed to. Cell and Nuclear Division -​ All living things rely on some form of cell division to create new life. In eukaryotes this means mitosis or meiosis. Mitosis is the process that cells use to create genetically identical daughter cells (same chromosomes, genes, alleles, etc.). It occurs in four primary phases: -​ Prophase -​ Metaphase -​ Anaphase -​ Telophase The details of each will be covered shortly. At the beginning of mitosis, there are two copies of each chromosome that are connected at a point in the centromere and are referred to as sister chromatids. Meiosis is a vital aspect of sexual reproduction as it produces gametes (sperm and ovum). Since gametes have half the chromosomes, they are able to fuse to create a diploid cell. The process of meiosis is made up of two cell divisions, which produces haploid cells. These stages are similar to those of meiosis. Mitosis produces two genetically identical “daughter” cells from a single “parent” cell, whereas meiosis produces cells that are genetically unique from the parent and contain only half as much DNA. DNA is the form of chromosomes that must be copied in both mitosis and meiosis. The way they separate and the end goal, however, is different. Within eukaryotic cells, DNA associates with histone proteins to form nucleosomes. Each is made up of a core of eight histones with DNA wrapped around twice. The DNA histone association in nucleosomes contributes to a pattern known as supercoiling. This allows great lengths of DNA to be stored in a very compact space with the nucleus. This is an adaptation that allows large genomes to be packaged. Mitosis Prophase is the first stage of mitosis and consists of the following events: -​ Chromosomes supercoil and become visible -​ Centrioles move to opposite poles -​ Microtubule spindles form between the centrioles -​ Nucleolus becomes invisible -​ Nuclear membrane disappears as prophase ends Metaphase begins after the nuclear membrane has broken down and is no longer visible. -​ Spindle microtubules fully develop and attach to the centromeres -​ Sister chromatid pairs align along the equator of the cell -​ The orientation of each pair is random After the sister chromatid pairs have aligned at the equator, anaphase begins. -​ Microtubes contract and split the sister chromatids at the centromere -​ Nuclear membranes begin reform the chromosomes -​ Microtubules pull the chromatids to the opposite poles of the cell Telophase is the final stage of mitosis: -​ Chromosomes arrive at each of the poles microtubules break down -​ Nuclear membranes begin to reform around the chromosomes -​ Chromosomes uncoil and become invisible through light microscope Mitosis ends with the events of telophase. Cytokinesis begins during the end of telophase but is not part of mitosis. It is the process that physically divides the cell in two and occurs differently in plants and animals. Cytokinesis in Animals Since animal cells do not have a cell wall, their cytokinesis process is simpler than that of plants. -​ Plasma membrane pinches the two identical nuclei become separated. Two new cells are formed. -​ Each cell has two complete sets of identical chromosomes. -​ They also have new cytoplasm, organelles and centriole. Cytokinesis is Plants -​ A cell from along the center of the cell and joint to the cell membrane -​ Vacuoles release cellulose needed to form a cell wall -​ Plasma membrane forms on either side of the new cell wall -​ When the membrane joins with the cell membrane, two new cells have formed Meiosis Most cells have complete pairs of homologous chromosomes in their nuclei. This is referred to as being diploid. Haploid cells contain one chromosome from each pair in their nuclei. Gametes are haploid and must fuse in order to form a diploid cell. Meiosis is the process of producing four haploid nuclei from a single diploid cell. In diagrams, the state of a cell’s chromosomes is represented as a factor of ‘n’. -​ Diploid cells = 2n -​ Haploid cells = n Hapoloid cells are formed from diploid cells during meiosis. In meiosis, chromosomes are replicated in interphase before the process begins. The replication results in two pairs of sister chromatids that are bound at the centromere. Chromatids in a pair are identical and so have the same gene. Each pair is considered a single chromosome. The DNA in each sister chromatid is identical and condensed. By the end of meiosis, they will separate and become unique chromosomes. Prophase 1 -​ Chromosomes condense and become visible under a microscope -​ Nucleolus breaks down & spindle fibers begin to form  -​ Homologous chromosomes pair up -​ Sections of DNA can be exchanged between homologous chromosomes in a process called crossing over. Metaphase 1 -​ Spindle fibers attach to the kinetochores on each sister chromatid (connected at centromere) -​ The pairs of homologous chromosomes align at the cell's equator The orientation of the random chromosomes is random. Members of each pair can face either pole. Anaphase 1 -​ Spindle fibers contract and separate the homologous chromosomes pairs This halves the chromosome count of each cell Telophase -​ Homologous chromosome pairs each opposite poles -​ Nuclear envelopes reform around each haploid set -​ A cleavage furrows forms, which results in two distinct copes Prophase 2 -​ Nuclear membrane breaks down and the spindle apparatus forms in each cell -​ Centrioles move to the poles of the haploid cells -​ Spindle fibers attach to each set of sister chromatids at their kinetochores Metaphase 2 -​ Chromosomes align along the equator of the cell. -​ Which pole each sister chromatid face is random. Anaphase 2 -​ Spindle fibers contract and separate the sister chromatids at the centromere -​ Chromatids (now chromosomes) are pulled to opposite poles Telophase 2 Nuclear membranes begin to form around the four haploid nuclei. Cleavage furrows form which result in four distinct haploids. The following diagram shows the behavior of homologous chromosomes during meiosis I and meiosis II. At the end of meiosis I, homologous chromosomes pairs split into different cells. At this point, each chromosome still consists of sister chromatids, however they may not be identical. At the end of meiosis II, the sister chromatids separate each other's daughter into haploid cells. As in meiosis I , the pairs are randomly oriented and separate independently of each other. During gametogenesis, the process of making sperm and eggs both rely on meiosis. The process is similar in both but the key difference is the amount of cytoplasm in the finished cells. Cytoplasm -​ Requirements for growth must be present in the egg. -​ As a result, during meiosis I, one large cell and one very small cell are produced. -​ The small cells degrade while the large cell continues in meiosis and becomes a mature ovum. Sperm only contain the resources necessary to move and penetrate the egg. During differentiation, most cytoplasm is eliminated. As a result, the size of egg cytoplasm is significantly larger. Variation & Non-Disjunction Genetic variation is necessary for species to change and evolve over time. During meiosis, there are two stages that promote this. The first is crossing over during prophase in which sections of chromosomes are swapped. This results in new allele combination. Chiasmata Diagram - When visualizing chiasmata, homologous chromosome pairs can be drawn using two colours. A ‘X’ structure between chromatids can be used to show the stages of crossing over. Random orientation during metaphase I and metaphase II results in nearly infinite possible combinations of chromosomes. Without crossing over, there are 2^23 possibilities. Sperm and ovum cells are the haploid products of meiosis in humans. The fusion of sperm and egg cells is random, which further promotes genetic variation of the resulting diploid zygote. Non-disjunction occurs in meiosis when chromosomes fail to separate corrrectly. This results in daughter cells with too many or too few chromosomes. Down syndrome is caused by non-disjunction when a zygote receives an extra copy of chromosome #21. This can be determined with a karyogram. D1.3 During translation, ribosomes interpret the mRNA sequence and synthesize polypeptide chains. The resulting proteins are typically released into the cytoplasm or rough ER. The sequence of amino acids in a polypeptide chain is determined by the sequence of the mRNA nucleotides (A, U, C, G). Every three bases of mRNA make up a codon. Each codon corresponds to an amino acid determined by the genetic code. The genetuc code is typically shown as a chart like the one below. The bases of the codon correspond to an amino acid or a stop signal. Some AA’s have multiple codons, which others only have one. As a ribosome reads a mRNA strand, the AA’s on the tRNA’s are bound via condensation to form a polypeptide chain. Although alleles are able to cause variations of a trait, their DNA sequences are oftenly only different by one or a few bases. Small changes in DNA can change the amino acid sequence of a protein. This change can have subtle or sometimes drastic effects on a characteristic. Mutations are changes in DNA that can occur randomly or as the result of external factors. If mutations affect the structure or characteristic of a protein, they can result in a new allele.