B13- Reproduction

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  • Created by: sana.aaa
  • Created on: 27-01-18 11:32

Asexual reproduction:

  • only involves one parent
  • the cells divide by mitosis
  • there is no joining (fusion) of special sex cells (gametes), and so there is no mixing of genetic information. there is no variation in the offspring
  • asexual reproduction gives rise to genetically identical offspring known as clones, their genetic material is identical both to the parent and to each other
  • the only mitosis is involved in asexual reproduction
  • very common in fungi and bacteria
  • strawberries, daffodils can reproduce asexually
  • cells within the human bodies reproduce asexually, to replace worn out cells
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Sexual reproduction:

  • involves a male sex cell and a female sex cell from 2 parents, these 2 special sex cells fuse together to form a zygote, which goes on to develop into a new individual
  • gametes are formed in a special form of cell division know as MEIOSIS
  • the chromosome number of the original cell is halved, so that when the gametes join together, the new cell has the right number of chromosomes.
  • the offspring results from inheriting genetic information from both parents- characteristics from both parents, but the offspring won't be identical to either of them. This introduces VARIATION
  • sexual reproduction is RISKY because it relies on 2 sex cells, often from 2 individuals, meeting and fusing. it introduces variations which are key to the long-term survival of a species
  • in plants, the gametes are the egg cells and pollen
  • in animals, the gametes are the eggs cells and sperm
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Meiosis:

  • in animals, the female gametes (egg cells) are made in the ovaries
  • the male gametes (sperm cells) are made in the testes
  • the gametes are formed in meiosis
  • in meiosis, the chromosome number is reduced by half
  • in a body cell, there are 2 sets of each chromosome, one inherited from the mother and one from the father
  • when a cell divides to form gametes, the genetic information is copied so there are four sets of each chromosome instead of the normal 2 sets. 
  • each chromosome forms a pair of chromatids
  • the cell then divides twice in quick succession to form 4 gametes, each with a single set of chromosomes
  • each gamete that is produced is genetically different from all the others
  • gametes contain random mixtures of the original chromosomes, this introduces VARIATION
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Fertilisation :

  • more variation is added when fertilisation takes place
  • each sex cell has a single set of chromosomes- when 2 sex cells join during fertilisation, the single new cell formed has a full set of chromosomes
  • in humans, the egg cell has 23 chromosomes and so does the sperm.- when the sex cells fuse together, the join to form a new cell with 46 chromosomes in 23 pairs
  • the combination of genes on the chromosomes of every fertilised egg is unique
  • once fertilisation is complete, the unique new cell begins to divide by mitosis to form a new individual
  • the number of cells increases rapidly
  • as the embryo develops, the cells differentiate to form different tissues, organs, and organ systems
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Variation:

  • the differences between asexual and sexual reproduction are reflected in the different types of cell division involved
  • in asexual reproduction, the offspring are produced as a result of MITOSIS from the parent cell. they contain exactly the same chromosomes and the same genes as their parents. there is NO VARIATION in the genetic material
  • in sexual reproduction, the gametes are produced by MEIOSIS in the sex organ of the parents- this introduces variation as each gamete is different. when the gametes fuse, one of each pair of chromosomes, and so one of each pair of genes, comes from each parent, adding more variation
  • the combination of genes in the new pair of chromosomes will contain different forms of the same genes (alleles) from each parent- this also helps to produce variation in the characteristics of the offspring
  • MITosis- making identical two
  • MEiosis- making eggs
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Pros and cons:

  • in asexual reproduction- only one parent is needed, this is time and energy efficient as there is no need to find a mate/ spread gametes
  • it is also often faster than sexual reproduction, rapidly producing a large number of identical offspring- it can be an advantage, but if the environment changes it is a disadvantage- if one organism cannot survive- no one can
  • in sexual reproduction- 2 parents or 2 gametes are needed, and it takes time and energy to find a mate or spread gametes
  • it is often slower than asexual reproduction- however, it produces a variation in the offspring. therefore if the environment changes, the variation gives a survival advantage as some offspring will be able to survive and reproduce. this is NATURAL SELECTION
  • NATURAL SELECTION- can be sped up by humans in selective breeding to increase food production
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Reproduction in fungi:

  • the most common form of reproduction in fungi is asexual
  • the moulds that rot our food is also reproduced asexually
  • in asexual reproduction, the fungal spores are produced by mitosis and they are genetically identical to the parent
  • some fungi also reproduces sexually when the conditions are not good- when it is dry for example. two hyphae from different fungi join and the nuclei fuse so the new hypha has 2 sets of chromosomes, it undergoes meiosis to make haploid spores, each with only one set of chromosomes which are different from the original hyphae.some of the spores may produce fungi better adapted to survive the adverse condition
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Reprodcution in plants:

  • in plants, the flower contains the organ of sexual reproduction. the gametes- the pollen and the egg cells- are produced using meiosis. the pollen from one flower must reach the female parts of another flower in a process called pollination- the plant equivalent of matin. flowers are adapted either to attract animal pollinators such as insects, bird, or bats or to make it easy for their pollen to be carried by the wind and caught by another flower. once the pollen has fused with the egg cell, seeds are formed. sexual reproduction introduces variation and enables plants to survive as conditions change through natural selection.
  • many plants also reproduce asexually. a new plant grows as a result of specially directed mitosis (example include the tiny new plants that form on the end of specialised stems called RUNNERS), and the division of bulbs in plants such as DAFFODILS. asexual reproduction means that new plants are formed even if the flowers are destroyed by frost, eaten, or fail to be pollinated. the main disadvantage is that the new plants are identical to their parents and no variation is introduced
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Reproduction in malaria parasites:

  • the parasites that cause malaria reproduce differently in different stages of their life cycle. malarial parasites spend part of their life cycle in the body of a female mosquito and part of their life cycle in the blood and orgaqns of human beings. both asexaul and sexual reproduction are part of the life cycle- asexual reproduction is not an alternative only if conditions are bad. malarial parasites reproduce asexually in human liver and blood cells. when the mosquito takes her blood meal, the drop in temperature between the human body and the mosquito, triggers sexual reproduction in some of the parasites inside the red blood cells. there is a 20 minute window when sexual forms develop, burst out of the blood cells, and meet to form zygotes with 2 sets of chromosomes. these zygotes then undergo meiosis to produce new asexual parasites that with infect a new human host. the parasite show a lot of variation
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DNA- the molecule of inheritance

  • inside of the nuclei of all your cells, your chromosomes are made up of long molecules of a chemical known as DEOXYRIBONUCLEIC ACID (DNA)
  • DNA is a polymer, long molecules made up of many repeating units. these very LONG strands of DNA twist and spiral to form a DOUBLE HELIX STRUCTURE
  • your genes are small sections of this DNA- this is where the genetic information- the coded information that determines inherited characteristics- is actually stored
  • each of your chromosomes contains thousands of genes joined together. each GENE CODES for a particular sequence of amino acids to make a specific protein. these proteins include the enzymes that control your cell chemistry, this is how the relationship between the genes and the whole organism builds up
  • the genes control the proteins, which control the make up of the different specialized cells that form tissues. - these tissues then form organ and organ systems that make up the whole body
  • in 2003, scientists announced that they had managed to sequence the human genome, the HUMAN GENOME PROJECT finished 2 years early, and UNDER BUDGET
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The human genome:

  • the genome of an organism is the entire genetic material of the organism, that includes all of the chromosomes, and the genetic material found in the mitochondria. Mitochondria contain their own DNA you always inherit your MITOCHONDRIAL DNA from your mother because it comes from the mitochondria in the egg
  • the human genome contains over 3 billion base pairs and almost 21 000 genes that for proteins
  • rice has 36 000 coding genes
  • the human genome has the ability to make many different proteins from the same gene by using it in different ways, or by switching part of a gene on or off
  • the human genome project also looked at genomes for other organisms such as bacteria- this allows us to understand the causes of communicable diseases caused by bacteria, this allows us to better our methods of TREATMENT
  • the human genome project allowed scientists to study the relationship between different types of organisms. it is changing the way we classify living things
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Genome:

  • understanding the human genome helps us to understand inherited disorders - such as CYSTIC FIBROSIS and SICKLE CELL DISEASE
  • the more we understand what goes wrong in these diseases, the more chance we have of overcoming them either through medicines or by repairing the faulty genes
  • there are genes that are linked to an increased risk of developing many diseases, from heart disease to type 2 diabetes
  • understanding the human genome is playing a massive part in the search for genes linked to different types of diseases- the more we understand about the genome, the more likely we are to predict the risk for each individual, so they can make lifestyle choices to help reduce the risks. this includes the changes that happen in the genome when cancer develops
  • by analyzing the genomes of cancer cells, scientists and doctors hope to become even better at choosing the best treatment for each individual
  • understanding the human genome helps us understand human evolution and history 
  • people all over the world can be linked by patterns in their DNA, allowing scientists to trace human migration patterns from our ancient history
  • we can also be linked to early members of the human family tree
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DNA structure and PROTEIN SYNTHESIS:

  • DNA is found in the nucleus of your cells and controls protein synthesis, but proteins are synthesised in the cytoplasm of the cell on the RIBOSOMES.
  • the long strands of your DNA are made up of alternating sugar and phosphate sections, these make up the backbone of the molecule
  • attached to each sugar is one of 4 different compounds called bases- these bases are represented by the letters A, C, G and T. the combination of a sugar, a phosphate and a base is called a NUCLEOTIDE. 
  • the DNA polymer is made up of repeating nucleotide UNITS
  • the nucleotide units are grouped into 3s, and each group of 3 bases for a particular amino acid.
  • each gene is made up of hundreds or thousands of these bases, the order of the bases controls the order in which the amino acids are assembled to produce a particular protein for use in your body cells
  • each gene codes for a particular combination of amino acids, which make a specific protein
  • a change of mutation in a single group of bases can be enough to change or disrupt the whole protein structure and the way it works
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DNA:

  • the key to the structure and functioning of the DNA molecule is the way these bases join up
  • in the complementary strands of the DNA molecule, a C is always linked to a G on the opposite strand. similarly, T is always linked to A. this holds the structure of the DNA double helix together. it is also key in the way the information from the genes on the DNA is translated into proteins in the cell
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Protein Synthesis:

  • protein synthesis in the cell is controlled by the DNA in the nucleus in a sophisticated series of steps
  • genes in the DNA produces a template for the protein- the template reflects the sequence of bases in the DNA, but it is small enough to leave the nucleus through the pores in the nuclear membrane
  • the template leaves the nucleus and binds to the surface of a ribosome
  • the cytoplasm contains carrier molecules, each attached to a specific amino acid. the carrier molecules attach themselves to the template in the order give by the DNA
  • the amino acids are joined together to form a specific protein
  • carrier molecules keep bringing specific amino acids to add to the growing protein chain in the correct order until the template is completed
  • the protein detaches from the carrier molecules and the carrier molecules detach from the template and return to the cytoplasm to pick up more amino acids
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Protein Synthesis: 2:

  • once the protein chain is complete the molecule folds up to form a unique shape that will enable it to carry out its functions in the cell
  • if the protein is going to act as an enzyme, it will fold to produce the active site
  • if it is a structural protein it will form FIBROUS structures such as COLLAGEN
  • if it is going to act as a hormone, or a clotting factor in the blood, or a muscle, or part of the structure of the Cell Membrane, the protein will fold so it can carry out its specific job in the cell
  • any change in the order of the bases in the DNA structure of a gene will alter the template that is made
  • a different template may result in a different sequence of amino acids joining together and so result in a change in the protein that is synthesised by a gene
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Gene expression:

  • when a gene codes for a protein that is synthesised in the cell, the gene is said to be expressed. most of the DNA does not actually code for proteins- there are only 21000 genes bu 3 billion pairs of bases!
  • scientists are still discovering what the non-coding part of the DNA does
  • each gene can control the synthesis of lots of different proteins- this may depend on how much of a gene is switched on or off, or which other genes are on or off at the same time
  • PHENOTYPE- is the physical appearance resulting from the inherited information
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Mutation:

  • new forms of genes result from changes in existing genes and these changes are what we call mutations- they are often tiny changes in the sequence of bases in a strand of DNA
  • mutations occur all the time, often as a result of mistakes made in copying the DNA for new cells as they reproduce.
  • when mutations take place in the coding DNA, most do not alter the protein formed or alter it so slightly that its appearance and function is not changed
  • a few mutations code for a change in the amino acids that results in an altered protein that folds to give a different shape. as a result, the active site of an enzyme may no longer fit the substance, or a structural protein may lose its strength,
  • on the other hand, the changes caused by a mutation may give an advantage e.g. producing a more efficient enzyme or a structural protein
  • when mutations take place in the no coding DNA, it does not directly affect the phenotype. however, variants in the no-coding DNA can affect which genes are switched on or switched off
  • by changing the genees that are expressed, changes in the non-coding DNA can have a big effect on the phenotype of the organism
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Inheritance

  • most of your characteristics are controlled by several different genes interacting
  • the chromosomes you inherit carry your genetic information in the form of genes. many of these have different forms, or ALLELES (sometimes called VARIANTS)
  • each allele codes for a different protein. the combination of alleles you inherit will determine your characteristics
  • HOMOZYGOUS- an individual with 2 identical alleles for a characteristic, BB or bb
  • HETEROZYGOUS- an individual with different alleles for characteristics, Bb
  • GENOTYPE- this describes the alleles present or genetic makeup of an individual regarding a particular characteristic, Bb or bb
  • PHENOTYPE- this describes the physical appearance of an individual regarding a particular characteristic, black fur or brown fur in a mouse
  • The alleles present in an individual, known as the genotype, work at the level of the DNA molecules to control the proteins made- these proteins result in characteristics- such as coat colour or the presence of dimples- that are expressed as the phenotype of the organism
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Inheritance 2

  • an allele is the particular form of information in that position on an individual chromosome
  • some alleles are expressed in the phenotype even when they are only present on one of the chromosomes- the phenotype coded for by these alleles is DOMINANT
  • a capital letter is used to represent the alleles for dominant phenotypes
  • some alleles only control the development of a characteristic if they are present on both chromosomes- when no dominant allele is present. these phenotypes are RECESSIVE- a lowercase letter is used to represent recessive alleles. they are only expressed if the organism is HOMOZYGOUS
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Genetic crosses

  • a genetic cross is when you consider the potential offspring that might result from 2 known parents
  • genetic crosses are modelledusing a GENETIC DIAGRAM such as a PUNNETT SQUARE
  • a genetic diagram gives us :
    • the alleles for a characteristic carried by the parents (the genotype of the parents)
    • the possible gametes that can be formed from these
    • how these may combine to form the characteristics in their offspring. the possible genotypes of the offspring allow you to work out the possible phenotypes too
  • inheriting different alleles can result in the development of different phenotypes
  • genetic diagrams help to explain what is happening and predict what the possible offspring might be like- they give you the probabiltiy that a particular genotype or phenotype will be inherited in a given genetic cross
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Polydactyly

  • some disorders are the result of a change in the bases or coding of our genes and can be passed on from parent to child- these types of diseases are known as inherited disorders
  • sometimes babies are born with extra fingers of toes- this is called POLYDACTYLY
  • the most common form of polydactyly is caused by a DOMINANT ALLELE, it can be inherited from one parent who has the condition
  • people often have their extra digit removed
  • if you have polydactyly, and are  heterozygous, you have a 50% of passing on the disorder to any child you have- that's because your gametes will contain the faulty dominant allele
  • if you are homozygous, your children will definetely have the condition
  • some dominant genetic disorder have much more widespread effect on the way the body works than polydactyly
  • Huntington disease is a dominant genetic disorder, in which symptoms develop in middle age- it affects the nervous system and eventually leads to death
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Cystic fibrosis

  • it is a genetic disorder that affects many organs in the body, particularly the lungs, the digestive system, and the reproductive system
  • over 8500 in the UK have CF
  • cystic fibrosis is a disorder of the cell membranes that prevents the movement of certain substances from one side to the other
  • the mucus made by the cells in many areas of the body becomes very thick and sticky
  • organs, especially the lungs, can become clogged up with the thick, sticky mucus, which stops them from working properly
  • the pancreas cannot make and secrete enzymes properly because the tubes through which the enzymes are released into the small intestine are blocked with mucus.
  • the reproductive system is also affected, so many people with cystic fibrosis are infertile
  • treatment for cystic fibrosis includes physiotherapy and antibiotics to help keep the lungs clear of mucus and infections. enzymes are used to replace the ones the pancreas cannot produce and to thin the mucus. however, although treatments are getting better all the time, there is still no cure for the disorder
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Cystic fibrosis 2

  • cystic fibrosis is caused by a recessive allele, so it must be inherited from both parents. children affected by cystic fibrosis are usually born to parents who do not suffer from the disorder. the parents have a dominant healthy allele, which means their bodies work normally. however, they also carry the recessive cystic fibrosis allele, because it gives them no symptoms, they have no idea it is there- they are known as CARRIERS
  • in the UK, 1 in 25 carries the cystic fibrosis allele- most of them will never be aware of it
  • the only situation when it may become obvious is if they have children with a partner who also carries the allele
  • so far scientists have no way of curing a genetic disorder. they hope genetic engineering will be the answer, allowing them to replace faulty alleles with health ones
  • currently, scientists are working on gene replacement for cystic fibrosis and are beginning to make progress in halting the disease and improving lung function
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Screening embryos

  • genetic tests are now available that can show people if they carry a faulty allele- this allows them to make choices such as whether or not to have a family. it is possible to screen embryos and fetuses during pregnancy for the alleles that cause inherited disorders. 
  • to screen an embryo or fetus, you first need to harvest some cells from the developing individual
    • AMNIOCENTESIS- is carried out at around 15-16 weeks of pregnancy. it involves taking some of the fluid from around the developing fetus. this fluid contains fetal cells, which can then be used for genetic screening
    • CHORIONIC VILLUS SAMPLING OF EMBRYONIC CELLS- is done at an earlier stage of pregnancy- between 10-12 weeks- by taking a small sample of tissue from the developing placenta. this again provides fetal cells to screen
    • another alternative taken by some couples with an inherited disorder in the family is for the embryos produced by IVF to be tested before they are implanted in the mother, so only the babies without that disorder are born
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Screening

  • once cells have been collected from an embryo or fetus- either before implantation or by techniques such as AMNIOCENTESIS AND CHORIONIC VILLUS SAMPLING- they need to be screened. whatever the potential genetic problem, the screening process is similar. DNA is isolated from the embryo cells and tested for specific disorders.
  • if the screening shows that a fetus is affected, the parents have a choice. they may decide to keep the baby, knowing that it will have a genetic disorder when it is born. on the other hand, they may decide to have an abortion or not proceed withif the screening shows that a fetus is affected, the parents have a choice. they may decide to keep the baby, knowing that it will have a genetic disorder when it is born. on the other hand, they may decide to have an abortion or not proceed with implantation. this prevents the birth of a child with serious problems. then the parents can try again to have a healthy baby. they may choose to have pre-implantation embryo screening using IVF to avoid having another affected pregnancy implantation. this prevents the birth of a child with serious problems. then the parents can try again to have a healthy baby. they may choose to have pre-implantation embryo screening using IVF to avoid having another affected pregnancy
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Concerns with embryo screening

  • the procces used to collect cells from a developing fetus increase the risk of miscarriage- so, in some cases, a healthy fetus will be miscarried as a result of a test to see whether it has a genetic abnormality, which is obviously very distressing for the patient.
  • the screening procedures are becoming more reliable and accurate all the time, but sometimes they can still give false positive or a false negative result. this can, very occasionally, lead to a termination of a healthy pregnancy or the unexpected birth of a child with a genetic disorder
  • embryo screening means that people have to make decisions about whether to terminate a pregnancy- this is not an easy decision
  • there are economic considerations too- screening is expensive. it is usually offered to people with a family history of genetic disorders and older parents who are more at risk of having a child with genetic problems
  • sometimes people choose not to have the screening tests so they do not have to make decisions
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