AQA GCSE B13 notes

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in tissues all over body and in asexual reproduction. Meiosis is another type of cell division that takes place in reproductive organs of animals and plants. Meiosis results in gametes which have half original number of chromosomes. Meiosis Gametes formed by meiosis in which chromosomes number reduced by half, in body cell are two sets of chromosomes (one from mother, one from father). When cell divides to form gametes: Genetic information copied (four sets of each chromosome instead of two) ● Each chromosome forms a pair of chromatids (similar to mitosis) Cell divides twice in quick succession to form four gametes each with single set of chromosomes Each gamete produced genetically different from all others, gametes contain random mixtures of original chromosomes - introduces variation. Fertilisation More variation added when fertilisation, each sex cell has single set of chromosomes. When two sex cells join, single new cell formed has full set of chromosomes. Gametes have 23 pairs in humans. When egg and sperm join, produce single new body with 46 chromosomes in 23 pairs - correct number. Combination of genes on chromosomes of fertilised egg is unique. Once fertilisation complete, unique new cell divides by mitosis to form new individual. Embryo develops and cells differentiate to form different tissues, organs, organ system. Variation Differences between asexual and sexual reproduction reflected in different types of cell division involved. ● Offspring produced are result of mitosis from parent cells Contain exactly same chromosomes and genes as parent ● No variation Asexual Advantages Disadvantages Advantages and disadvantages to asexual and sexual reproduction ● The best of both worlds Asexual Only one parent needed ● Less time and energy efficient ● ● Gametes produced by meiosis in sex organs of parents ● Introduces variation as each gamete different ● Gametes fuse together - more variation Combination of genes in new pair of chromosomes contain different forms of same genes (alleles) from each parent - produce variation in characteristic of offspring (no need for mate) Faster than sexual Advantage in favourable conditions Environment changes and one organism cannot survive, none can No variation ● Sexual ● Sexual Produces variation in offspring If environment changes, variation gives survival advantage (some offspring able to survive and reproduce) Natural selection Takes time and energy to find mate and spread gametes Slower than asexual reproduction Reproduction in fungi Asexual is most common. Fungi made up of mass of thing threads (hyphae) that form structure we can see. In asexual reproduction, fungal spores produced by mitosis and are genetically identical to parent. Some fungi reproduce sexually when conditions not good. Two hyphae from different fungi join and nuclei fuse New hyphae has two set of chromosome ● Undergoes meiosis to make haploid spores (only one set of chromosomes) Different to original hyphae Some of spores may reproduce fungi better adapted to survive adverse conditions Reproduction in plants Flowers contain organs of sexual reproduction. Gametes (pollen and egg cells) produced using meiosis. Pollen from one flower reach female parts of another flower - pollination (mating). Flowers adapted to attract animal pollinators to make easy for their pollen to be carried by wind and caught by another flower. Once pollen fused with egg cell, seeds formed. Sexual reproduction introduces variation and enables plants to survive as conditions change through natural selection. Plants can reproduce asexually. New plant grows using mitosis. Asexual reproduction means new plants formed even if flowers destroyed. Main disadvantage is new plants are identical to parents and no variation introduced. Reproduction in malaria parasites Malarial parasites pend part of life cycle in body of female mosquito and part of life cycle in blood and organs of human beings. Asexual and sexual reproduction part of life cycle. Malarial parasites reproduce asexually in human liver and blood cells. When mosquito takes blood, drop in temperature between human body and mosquito triggers sexual reproduction inside red blood cells. These burst out of blood cells and meet to form zygotes with two sets of chromosomes. Zygotes undergo meiosis to produce new asexual parasites that will infect new human host. Lots of variation. DNA and the genome Humans have 46 chromosomes in two pairs, half inherited from mother and half from father. Different organisms have different number of chromosomes. DNA-molecule of inheritance Inside nuclei of cell, chromosomes made up of long molecules of DNA a polymer (long molecule made up of many repeating units. DNA strands twist and spiral to form double helix structure. Genes are small sections of DNA where genetic information (coded information that determines characteristics) is stored. Each gene codes for a particular sequence of amino acids to make a specific protein which include enzymes that control cell chemistry. Shows relat between genes and organism builds up. Genes control proteins that control make up of different specialised cells that form tissues which form organs and organ systems. 2003 scientists managed to sequence human genome, working in teams all around world. Done by using technology to chop up DNA and read all base sequences had improved fast during life of project. The human genome Genome is entire genetic material of organism, includes all chromosomes and genetic material found in mitochondria which contain own DNA. Always inherit mitochondrial DNA from mother as comes from mitochondria in egg. Human genome has 3 billion base pairs and 21000 genes that code for proteins. Why the genome matters Understanding genome helps to understand inherited disorders, able to understand more about disease allows more chance we have overcoming them either through medicines or by repairing faulty genes. There are genes linked to increased risk of developing many diseases. Understanding human genome makes more likely able to predict risk for each individual so can make lifestyle choices to help reduce risks includes changes that happen in genome as cancer develops. Analysing genomes of cancer, hope to better at choosing best treatment for individual. Also helps understand human evolution and history, scientists able to trace human migration patterns from ancient history and linked to early members of human family tree. DNA structure and proteins synthesis DNA found in nucleus of cells and controls protein synthesis but proteins synthesised in cytoplasm in ribosomes. DNa made of alternation sugar and phosphate sections (make up backbone of molecule). Attached to each sugar is one of four different compounds (bases) represented by A, C, G and T. Combination of sugar, phosphate and base is nucleotide. DNA polymer is made of repeating nucleotide units. Nucleotide units grouped into threes which code for amino acid. Order of bases controls order in which amino acids are assembled to produce particular protein for use in body cells. Each gene codes for particular combination of amino acids which made specific protein. Change or mutation in single group of bases enough to change or disrupt whole protein structure. More about DNA Key to structure and functioning of DNA is way bases join up. In complementary strands of DNA, C always links to G on opposite strand and T always links to A. Holds structure of DNA double helix together. Making proteins Protein synthesis in cell controlled by DNA in nucleus in sophisticated series of steps. Genes in DNA produce tem for proteins, tem reflex sequence of bases in DNA but small nucleus through pores in nuclear membrane. to leave Double helix unwinds and unzips ● RNA makes copy of one strand of DNA section ● ● RNA moves out of nucleus through tiny pores RNA attaches self to ribosome Ribosome read cods on RNA ribosome 3 bases/code = 1 amino acid correct/corresponding amino acid bought to ribosome Amino acids form long chain - proteins Cytoplasm contains carrier molecules each attacked to specific amino acid, carrier molecule attach selves to template in the order given by DNA. Amino acid joined together to form specific protein. Carrier molecules bring keep bringing specific amino acid to add to growing protein chain in correct order until template completed. Protein detaches from carrier molecules and carrier molecules detach from template and return to cytoplasm to pick up more amino acids. Once protein complete molecule folds to form unique shape that enable to carry out functions. Any change in order of bases in DNA structure of gene alter template made, different template may result in different sequence of amino acids joining together so result in change in protein synthesised by gene. Gene expression and mutation Gene expression Gene code for protein, once protein made gene has been expressed. Most DNA don't code for proteins. Non-coding parts of DNA involved in switching genes on or off, explains how can synthesise so many different chemicals with so few genes. Each gene control synthesis lots of different proteins may depend on how much of gene switched on or off or other genes. Variation in non-coding areas affect how genes are expressed. So affects phenotype (physical appearance of organism) Mutation New forms of genes result from changes in existing genes and these changes are mutations, often tiny changes in sequence of bases in strand of DNA. Mutations occur as result of mistake made in copying the DNA for new cells as reproduce. When mutation takes place in coding DNA, do not alter protein formed. Few mutations code for change in amino acid so altered protein that folds to give different shape so active site of enzyme no longer fit substrate or lose its strength. Changes caused by mutation give an advantage e.g produce more efficient enzyme or stronger structural protein. When mutation in non-coding DNA, does not directly affect phenotype but variants in non-coding DNA can affect which genes switched on or off. Changing genes that are expressed, changes in non-coding DNA can have big effect on phenotype of organism. Inheritance in action How inheritance works Chromosomes inherit carry genetic information in form of genes, many of genes have different forms (alleles). Each allele codes for different protein, combination of alleles inherit determine characteristic, can make biological models help predict outcome of any genetic cross. Homozygote - individual with two identical alleles for characteristics Heterozygote - individual with different alleles for characteristic Genotype - describes alleles present or genetic makeup of individual regarding particular characteristic Phenotype - physical appearance of individual regarding particular characteristic. Allele is different version of same gene. Alleles present in individual (genotype) control proteins made which result in characteristic. Dominant - expressed in phenotype when only present in one chromosome (expressed using capital letter) Recessive - only control development of characteristic if are present on both chromosomes (expressed using lower case) Genetic crosses Genetic cross when consider potential offspring that might result from two known parents, need to look at both possible genotypes and possible phenotypes of offspring. Using genetic diagrams Use punnett square to predict outcome of different genetic crosses, gives: ● Alleles for characteristic carried by parents Possible gametes that can be formed from these How these many combine to form characteristic in offspring, possible genotypes allow to work out possible phenotypes Inheriting different alleles result in development of different phenotypes, genetic diagrams help to explain what is happening and predict possible offspring. Give probability that particular genotype of phenotype will be inherited in given genetic cross. More about genetics Sex determination One feature of phenotype is inherited. Humans have 23 chromosomes. In 22, each chromosome in pair is similar shape, each has genes carrying information about many different characteristics of body. One pair of chromosomes different - sex chromosomes, determine sex of offspring. Females - XX Males - XY When cells undergo meiosis to form gametes, one sex chromosome goes into each gamete so egg cell contain X chromosome, half of sperm contain X chromosome and other half contain Y chromosome. Family trees Trace genetic characteristics through family by using family trees can show inherited diseases, family likeness, different alleles people have inherited. Family trees used to work out if individual likely to be homozygous or heterozygous for particular alleles. Inherited disorders Disorders result of change in bases or coding of genes can be passed from parent to child. Types of diseases known as inherited disorders. Polydactyly Extra fingers of toes caused by dominant allele. Inherited from one parent who has the condition. If have polydactyly and heterozygous have 50% chance of passing disorder onto child - half of gametes contain faulty dominant allele, if homozygous children definitely have condition. Cystic fibrosis Genetic disorder which affects organs of body especially lungs, digestive system, reproductive system. Disorder of cell membranes, prevents movement of certain substances as mucus made by cells becomes thick and sticky. Organs clogged by mucus - stop working properly. Pancreas cannot make and secrete enzymes properly as tubes blocked with mucus, reproductive system affected (infertile). Treatment includes physiotherapy, antibiotics to keep lungs clear of mucus and infections. Enzymes used to replace ones pancreas cannot produce and to thin mucus but there is no cure. Caused by recessive allele so inherited from both parents. Children affected born to parents who do not suffer from disorder - parents have dominant healthy allele but carrying recessive cystic fibrosis allele, no symptoms, don't know they are carrier 1 in 25 carry cystic fibrosis allele - obvious if have child with partner who carries allele, every time egg and sperm fuse, chance alleles combine. Curing genetic disorders No way of curing genetic disorders, hope genetic engineering (replacing faulty alleles with health ones) develop, currently working on gene replacement for cystic fibrosis beginning to make progress in halting disease and improving lung function. So for not managed to cure anyone with inherited genetic disorder but remain hopeful possibility. Screening for genetic disorders Genetic tests available to show if carry faulty allele, allows to make choices as to have family or not. Possible to screen embryos and foetuses during pregnancy for alleles that cause inherited disorders. Embryos screened before implants in mother during IVF. Screening embryos Screen embryo need to harvest cells from developing individual. Two main methods - amniocentesis and chorionic villus sampling. Amniocentesis - 15-16 weeks, taking fluid from around developing fetus, contains fetal cells used for genetic screening Chorionic villus sampling - 10-12 weeks, taking small sample of tissue from developing placenta, provides fetal cells to screen Both tests associated risk of causing miscarriage but currently main methods used to obtain fetal cells for screening. New methods depend on analysing fetal cells found in blood of mother offer promise as less invasive. Carrying out the screening Once cells collected from embryo need to be screened. Whatever potential genetic problem, screening process similar, DNA isolated from embryo cells and tested for specific disorders. If screening shows fetus affected and parents have choice. May decide to keep baby or decide to abort or not process with implantation. This prevents birth of child with serious problems, parents can try again to have healthy baby, may choose to have pre-implantation embryo screening using IVF to avoid having another affected pregnancy. Concerns about embryo screening Advantages Becoming more reliable and accurate Able to know complications Know if baby has inherited genetic disorders Disadvantages ● Process used to collect cells increase risk of miscarriage Can give false positive or negative result (lead to termination of healthy pregnancy) Have to make decision which is not easy • • • Expensive 'Designer babies' - parents choose desirable characteristics (less variation)