SB3 Genetics


SB3a Sexual and Asexual Reproduction

  • Sexual reproduction - the fertilisation of a female sex cell by a male sex cell. 
  • Asexual reproduction - reproducing without fertilisation, which produces clones that are genetically identical to parent.
  • Advantages of sexual reproduction - combines characteristics of both parents. Variation means there is a better chance of offspring being suited to new conditions and so will be more likely to survive. 
  • Advantages and disadvantages of asexual reprodution - much faster than sexual as there is no need for a partner, but dosen't produce variation. 
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SB3b Meiosis

  • Zygote - formed when two gametes fuse during fertilisation. 
  • Genome - all DNA in an organism.
  • Chromosomes - long DNA molecule packed with proteins. 
  • Gene - sections on a chromosome that contains instructions for a specific protein.
  • Mitosis produces diploid cells but meiosis produces gametes. 
  • During meiosis, each chromosome replicates where the copies remain attached(forming a x shape). The two sets of chromosomes pair up, forming 23 pairs. the pairs then separate into two new cells, where it separates abgain, producing 4 genetically different haploid cells. 
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  • DNA molecules contain two strands which forms helix. Two strands are joined together by pairs of bases to form a double helix.
  • 4 bases - adenine, thymine, cytosine, guanine. A pairs with T, C pairs with (complementary pairs).
  • Each bases is attached to a sugar (deoxyribose) which is attached to a phosphate group. This is group is called a nucleotide.
  • Hydrogen bond - bases has a weak charge which formes an weak attraction. C only pairs with G because they form three bonds, while A and T form two.
  • The order of bases in a gene contains instructions for a protein.
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SB3d Protein Synthesis

  • Transcription - the DNA bases are used to make a strand of RNA (ribonucleic acid). An enzyme, RNA polymerase, attaches to a non-coding DNA gene and separates the two DNA strands. The enzyme moves along one DNA strand (template stand) adding complementary RNA nucleotides. It contains same bases except T is swaped for uracil (U). The nucleotides link to form a strand of messenger RNA (mRNA).
  • Translation - the mRNA travel out of the nucleus through nuclear pores (holes in membranes) and then attaches to ribosomes. A ribosome moves along the mRNA strand three bases at a time, called a codon. At each mRNA codon, a molecule of transfer RNA (tRNA) with complementary bases line up. Each tRNA molecule carries a amino acid. As the ribosome moves along, it joins the amino acids from the tRNA molecules together, forming a polypeptide. 
  • The polypeptide chain then folds to form a specific protein (such as an enzyme) with a specific shape.
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SB3e Genetic Variants and Phenotypes

  • Mutations - a change in the bases of genes that creates a genetic variant. Cause by DNA not being copied properly in cell division or evironmental factors.
  • Phenotype - observable characteristics.
  • Alleles - different versions of genes. 
  • If a mutation affects the amino acids in a polypeptide, it can stop the polypeptide from folding correctly, forming sickle cell disease (red blood cells become sickle shaped and stick together). 
  • A mutation in non-coding DNA may result in the RNA not binding well. 
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SB3f Mendel

  • In a monastery garden , Mendel observed that many characteristics were either present or absent, not a blend. He bred pea plants together and observed the characteristics. After many experiments, Mendel concluded that inherited 'factors' controlled the variation of characteristics and they exist in different version (alleles) that don't change. He found out tha some factors were more dominant and some were recessive. 
  • His work was ignored because his work didn't explain the variation in characteristics such as eye colour. They also argued that if factor's couldn't change then a species couldn;t evolve (Dawin's theory). 
  • Once chromosomes were discovered, they saw how mendel's idea could work.
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SB3g Alleles

  • There are two copies of every chromosome in a nucleus, and so ther are two copies of every gene. Each copy could be a different allele and the different combination of alleles gives us different characteristics (genetic variation).
  • If both alleles for one gene are the same, an organism is homozygous. If the alleles are different, they are heterozygous. 
  • A recessive characteristic is only seen if both alleles are recessive.
  • The alleles in an organism are its genotype and what the organism looks like is its phenotype.
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SB3h Inheritance

  • Females have two X chromosomes and males have and X and Y chromosome. 
  • Punnett square - way to demostrate inheritance, and work out probability if iffspring inheriting certain genotypes. 
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SB3i Multiple and Missing Alleles

  • ABO blood group - 4 groups of blood, A , B, AB, O. Blood group is determined by the markers outside the red blood cell: different markers include A, B or O. 
  • The gene responsible for the markers has three alleles, IA IIO  
  • IO is recessive, but A and B are dominant. Some one with A and B alleles are codominant.
  • As men have XY chromosomes, they only have one allele for some genes. If there was an allele on the X chromosome genes causes a genetic disorder, the man will develop that disorder
  • Sex-linked genetic diorder - disorders that show a different patten of inheritance in men and women. 
  • Carrier - one faulty allele and one healthy allele
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SB3j Gene Mutation

  • Mutation - a change in a gene that creates a new allele, often occuring during cell division.
  • Mutations happen when the is a mistake in copying the DNA during cell division or evironmental factors. 
  • Human Genome Project - scientist from around the world produced a map of one set of 46 chromosomes. 
  • Mapping someone's genome can indicate whether they could develop a disease or identify which medicines might be best to treat a person's illness, because the alleles we have can affect how medicines work 
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SB3k Variation

  • Genetic variation - caused by the different alleles inherited during sexual reproduction. Different alleles are produced by mutations, some which changes the phenotype. Many characteristics are affected by environmental factors - environmental variation. 
  • Acquired characteristics - characteristics that are changed by the environment during the life of an individual. 
  • Variation is grouped in 2 types: discontinuous variation - data can only take a limited set of values, and continuous variation - data can be any value in a range. 
  • Continuous data for variation forms a bell-shaped curve, known as normal distribution, because the most common value is the middle value.
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