• 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