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