SL

DNA

  • DNA helps with the following:
    • Forensic science
    • Paternal testing
    • DNA profiling
    • Study human migration patterns
    • Development of vaccines
    • Identification of biological remains
  • DNA stands for Deoxyribonucleic Acid and it is stored in the nucleus of the cell
  • It contains all the instructions for the function of the cell
  • DNA coils tightly to form a chromosome
    • This ensures no DNA is lost during cell division
  • Humans have 46 chromosomes in every cell
  • A gene is a segment of DNA that codes for a particular trait
    • E.g. ability to curl your tongue, hair colour, eye colour
  • Multiple genes can be found on a single chromosome (chromosome 1 has over 2000 genes on it!)
  • Nucleotide: Phosphate - Sugar - Base
  • The backbone of DNA = Phosphate and Sugar

DNA Replication

  • Helicase ==(Step 1)==
    • The ‘unzipping’ enzyme
    • Comes and unwinds the DNA
    • Topoisomerase prevents DNA from supercoiling (stops overwinding of DNA)
  • Primase ==(Step 2)==
    • Primase comes in and makes RNA primers on both strands
    • This is an important step so DNA polymerase knows where to start
  • Complementary Strands
    • Complementary strands are joined by hydrogen bonds and move in antiparallel directions
    • One strand goes 5’ to 3’ and the other goes 3’ to 5’ (based on the labelling of the sugar molecules Carbons)
  • DNA Polymerase ==(Step 3)==
    • DNA polymerase builds the new strand in the 3’ to 5’ direction (using the old template strand as a base)
    • It also has a proofreading ability, so it rarely makes mistakes
    • The leading strand is complete, but the lagging strand is built in the wrong direction and has lots of gaps in it (known as Okazaki Fragments)
  • Ligase ==(Step 4)==
    • Ligase comes along and fills in the gaps between each of the Okazaki Fragments
  • End Product
    • Two identical double-helix DNA molecules
    • This is semi-conservative because each of the two new copies contains one of the original strands of DNA

Cell Division

  • ==Cell division: the process by which one cell (parent cell) divides into two new identical daughter cells==
    • This helps them grow and produce more cells
    • It also helps repair damaged tissues
  • Before cell division occurs, cell replicates all of its DNA so each daughter cell gets a complete copy of genetic information from the parent cell
  • Asexual Reproduction
    • Many organisms, especially unicellular organisms reproduce by cell division e.g. bacteria
  • Somatic (body) cells
    • All body cells in an organism have the same kind and number of chromosomes
    • We have 22 pairs of body chromosomes and 1 pair of sex chromosomes - a total of 23 chromosomes in the human body
  • Interphase
    • Period of cell growth and development
    • DNA replication occurs during this phase
    • Cells spend most of their time in this part of the cycle, where it is able to carry out normal cell activities

Mitosis

  • Prophase
    • Chromosomes coil up
    • Nuclear envelope disappears
    • Spindle fibers form
  • Metaphase
    • Chromosomes line up in the middle of the cell
    • Spindle fibers connect to chromosomes
  • Anaphase
    • Chromosome copies divide
    • Spindle fibers pull chromosomes to opposite poles
  • Telophase
    • Chromosomes uncoil
    • Nuclear envelopes form
    • 2 new nuclei are formed
    • Spindle fibers disappear
  • Cytokinesis
    • Division of the rest of the cellar after the nucleus divides
    • Cell then returns to interphase to continue to grow and perform cell activities
  • DNA controls all cell activities including cell division
    • If the DNA is damaged/mutated, then the cell loses their ability to control cell division
    • This causes super-dividing masses called tumors
    • Benign = Not cancerous
    • Malignant = Cancerous

Mutations

  • What is a mutation?
    • When DNA replicates itself, there are sometimes errors that aren’t detected and fixed (due to mutagens or chance)
    • Changes in the letters of the code also change the meaning of the code, which can have severe effects.
    • Too much or too little proteins could be produced.
    • Proteins help control and regulate iron levels leading to too much iron in the blood which can damage organs.
  • Types of mutation
    • Substitution
    • Letters of code are changed from their original intended sequence
    • Insertion
    • Extra letters of code are inserted into the original intended sequence
    • Deletion
    • Letters of code are deleted from the original intended sequence
  • What is a mutagen?
    • A factor that triggers mutations
    • Over-exposure to X-Rays, UV rays, or chemicals in pesticides are all mutagens.
  • Are mutations inherited?
    • If the sperm and ovum carries a DNA abnormality then the child could be affected with a genetic disorder
    • Examples of genetic mutations include:
    • Red-green colour blindness
    • Haemophilia
    • Cystic Fibrosis
  • Mutations: Pros & Cons
    • Not all mutations are harmful, in fact, some species actually rely on mutations in order to survive
    • Insects that have slight variations or mutations to their genetic code may be resistant to pesticides, and these are the ones who survive while the non-resistant ones die out.
    • Sickle Cell mutation occurs when an Adenine is replaced by a Thymine in the haemoglobin code
    • If a child inherits the trait from one parent, it can be beneficial, as the child will have no/mild symptoms but is resistant to malaria
    • Two copies, one from each parent lead to misshapen red blood cells that cause pain and block vessels
    • \

Inheritance

  • Mendel
    • Our understanding of the patterns of inheritance of genetic disorders and other characteristics involving peas
    • Mendel carried out experiments on peas to determine how individual traits are inherited
  • Why did he use peas?
    • Peas grow quickly and therefore many generations can be grown in a relatively short time period
    • He used a large number of pea plants to complete his experiments
    • He found that each parent passed on half of its factors to each offspring and that certain factors were dominant over others
  • Alleles
    • Gene = proportion of DNA that determines a certain trait
    • Alleles: different versions of the same gene
    • Individuals inherit two alleles from their parents: either dominant or recessive alleles
  • Dominant or Recessive
    • Dominant allele: the allele that is expressed over the recessive allele
    • Represented by a capital letter (e.g. brown eyes are represented by ‘B’)
    • Recessive alleles: the allele that is only expressed if BOTH alleles inherited are recessive
    • Represented by a lowercase letter (e.g. blue eyes are represented by ‘b’)
    • In rare cases, both the recessive and dominant allele can be expressed and produce a blending effect (e.g. red and white carnations can produce pink carnations)
    • Represented with upper and lowercase letters (Rr)
  • Homozygous and Heterozygous
    • Let’s assume R = round pea shape (dominant trait) and r = oval pea shape (recessive trait)
    • Homozygous
    • Two copies of the same allele
    • EX: RR, rr
    • Heterozygous
    • Two different copies of an allele inherited from each parent
    • One dominant and one recessive
    • EX: Rr
  • Genotype and Phenotype
    • Genotype: Unique sequence of DNA
    • Phenotype: The expression of the genotype (i.e. how the organism looks)

Sex-Linked Traits

  • Requirements of Test Crosses
    • Large numbers of offspring are needed to produce reliable data
    • Now we have genetic screening and genome mapping, test crosses are less commonly used
    • The Human Genome Project (2003) identified all the genes in human DNA and has allowed researchers to develop genetic tests to diagnose diseases
  • Sex-Linkage
    • When a gene controlling the expression of a trait is located on a sex chromosome
    • e.g. X or Y
    • The Y chromosome is much shorter than the X and contains fewer genes
    • The X chromosome is longer and contains genes not present on the Y chromosome
    • Therefore, sex-linked conditions are usually X-linked
  • Sex-Linked Patterns
    • Sex-linked traits tend to affect males more than females because they only have one X chromosome and cannot mask the trait with a second X chromosome
    • Females have two X chromosomes and therefore can carry two different alleles on their X chromosome
    • This can be heterozygous or homozygous
    • But remember the dominant gene is always expressed
  • Rules for Sex Linkage
    • Only females can be carriers (heterozygous) for a recessive disease, males cannot be carriers because they are hemizygous and only have one X chromosome
    • Females cannot inherit an X-linked recessive condition from an unaffected father (there is only one X chromosome, so this is the dominant one)
  • Writing Conventions: Sex Linkage
    • We write the allele as a superscript to the sex chromosome (X)
    • E.g.: Haemophilia: X^H = unaffected ; X^h = affected

Pedigrees

  • A pedigree is a type of diagram used to represent the individuals in a family and track the inheritance of a particular trait in that family

  • Symbols:

    • Unshaded square = Normal male
    • Shaded square = Affected male
    • Half-shaded square = Carrier male
    • Line between a square and a circle = Parents/Married
    • Line joined ABOVE a square and a circle = Siblings
    • Unshaded circle = Normal female
    • Shaded circle = Affected female
    • Half-shaded circle = Carrier female
    • Oldest offspring on the left - offspring must be in birth order
    • A triangular line joining offspring = Identical twins
    • Line looking like ^ joining offspring = Non-identical twins

  • Autosomal Dominant

    • If both parents are affected and the offspring is unaffected, the trait must be dominant (parents must be heterozygous)
    • All affected individuals must have at least one affected parent
    • If both parents are unaffected, all offspring must be unaffected (homozygous recessive)

  • Autosomal Recessive

    • If both parents are unaffected and the offspring are affected, the trait must be recessive (both parents are heterozygous carriers)
    • If both parents show a trait, then all offspring will show the trait (homozygous recessive)

  • X-linked Dominant

    • If a male shows a trait, all daughters as well as his mother must show the trait.
    • Unaffected mothers cannot have affected sons
    • Tends to be more common in females

  • X-linked Recessive

    • If a female shows a trait, all the sons and their father must also have the trait
    • Unaffected mothers can have affected sons if she is a carrier (heterozygous)
    • X-linked recessive tend to be more common in males