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Genetics
Assumed Knowledge
• The central dogma• Basic genetic terminology• DNA replication, transcription, translation• Chromosome structure• Mitosis and Meiosis• Mendel’s laws of inheritance/ Hardy-Weinberg
The central dogma
Some Basic Terminology
DNA – stands for deoxyribonucleic acid. DNA is a nucleic acid that contains the genetic material of a cell.
DNA is composed of two strands of nucleic acids arranged in a double helix.
RNA – stands for ribonucleic acid. Involved in the conversion of genetic material in DNA to proteins.
A single stranded sequence of nucleic acids.
Nucleic Acids – these are linear strands of nucleotides
Nucleotides – these have three parts• A phosphate group• A sugar group• A base
DNARNA
Sugar DeoxyriboseRibose
Base Adenine (A)Adenine (A)
Thymine (T)Uracil (U)
Guanine (G)Guanine (G)
Cytosine (C)Cytosine (C)
Codon – these are sequences of three adjacent nucleotides
Each codon encodes for a specific amino acide.g. AAA will encode lysine, AGU will encode serine
Anticodon – these are sequences of three adjacent nucleotides that are complimentary to a specific codon
Each codon encodes for a specific amino acide.g. UUU will be complimentary to AAA and encode lysine, UCA will be complimentary to AGU and encode serine
Genome – the entire genetic content of an organism.
Basically, all the DNA.
Genes – these are the regions of DNA that code for proteins.
Not every bit of DNA gets converted to protein – DNA contains a lot of junk (~1.5% of the genome encodes for proteins)
1 Gene = 1 Protein
Chromatin – a complex of DNA and protein.
Chromosomes – These are large condensed segments of DNA formed from the condensation of chromatin.
Each chromosome contains many genes.
Humans have 46 chromosomes – 23 pairs
Karyotype – the number and appearance of chromosomes in the nucleus of a cell.
Allele – these are alternative forms of the same gene
e.g. ABO blood groupsThere are 3 alleles that are recognised - A, B, OAs every individual has two copies of there are six possible genotypes (combinations of the alleles)AA, AO, BB, BO, AB, OO
Genotype – this is the genetic make up of an individual
e.g. is their ABO blood group AA, AB, BB???
Phenotype – this is an organism’s observable traits
e.g. is their eye colour blue, green, brown
Alleles may be dominant or recessive
Dominant alleles require only one allele of the two copies to be present for the phenotype to be observed
Recessive alleles require both copies of the allele to be present for the phenotype to be observed
DNA Replication
DNA is compose of two complimentary strands
Sugar group and phosphate group form a backbone with the bases projecting into the center of the helix to form bonds with bases on the opposite strand
Backbone reads as sugar, phosphate,sugar, phosphate
DNA strands have a 5’ and a 3’ end. This gives the strand directionality (a head end and a tail end!!!):• 5’ > 3’ or• 3’ > 5’
The two complimentary strands of DNA are arranged in opposite directions
In RNA uracil is the complimentary base to adenine, not thymine
DNA Replication• Unwinding of the two strands of DNA
(helicase)• Each parent strand acts as a template for a
new strand of DNA• Formed by adding complimentary nucleotides
(DNA polymerase)
Yields two identical strands of DNA, each with one old strand and one new strand
Semi-conservative
http://www.youtube.com/watch?v=zdDkiRw1PdU&feature=related
Transcription & Translation
TranscriptionDNA > RNA
TranslationRNA > Protein
Transcription• Takes place in the nucleus• Unwinding of the DNA strands• Using one strand of DNA as a template, there
is formation of a complimentary strand of messenger RNA (mRNA)
• RNA contains uracil instead of thymine• All steps catalyzed by RNA polymerase
Splicing – this is the removal of the junk DNA• Remember 1 gene = 1 protein• Genes contain useful regions (exons) and junk
regions (introns)• Introns are removed from the mRNA by
splicing• Ensures only the exons carry on to translation
and protein formation
Transcription• Performed by the ribosome in the cytoplasm
and also involves tRNA (transfer RNA)• Every three nucleotides on mRNA will encode
an amino acid (codons)• tRNA contains a specific anticodon and also
has attached an amino acid corresponding to that anticodon
• tRNA anticodons bind to their complimentary sequences on mRNA
• The ribosome removes the amino acid attached to the tRNA and forms a chain of amino acids – a protein
Chromosome Structure & Number
• DNA wraps around specialised proteins called histones – beads on a string
• These condense to form chromatin (DNA + protein)
• Further condensation to form a compact chromosome
• During the non-divided phase of the cell cycle DNA exists as uncondensed chromatin that is accessible for transcription
• During the dividing phase of the cell, chromatin condenses to form a four arm chromosome consisting of two identical sister chromatids joined in the center at the centromere
• Remember, DNA replication forms an identical copy of DNA – a chromatid is one of these copies
Humans have 46 chromosomes – 23 pairs• 22 autosomal pairs• 1 sex pair
Females = XX Males = XY
Monosomy – one copy of the chromosome is missingTrisomy – an extra copy of the chromosomes is present
Mitosis and Meiosis
Mitosis – cell division required for production of an identical set of chromosomes, forms two genetically identical diploid daughter nuclei Meiosis – cell division required for sexual reproduction, forms four genetically different haploid daughter nucleiHaploid (n) – only one copy of the chromosome is presentDiploid (2n) – two copies of the chromosome are present, they exist in pairs
Cell Cycle• Interphase – cell growth, protein production, DNA
replication• Nucleus division – via mitosis or meiosis
• Cytokinesis – division of the cytoplasm to form two separate cells
Mitosis• Prophase – chromatin condensation to form chromosomes
composed of identical sister chromatids. Formation of spindle fibers
• Metaphase – alignment of the chromosomes along the equatorial plane. Attachment of spindle fibers to the centromere of the chromosome
• Anaphase – shortening of spindle fibers pulls sister chromatids apart to opposite poles of the cell
• Telophase – formation of a new nuclear membrane around each chromosome group
Pneumonic – Pour Me Another Tea
Meiosis – composed of meiosis 1 and meiosis 2
Meiosis 1• Prophase 1 - chromatin condensation to form
chromosomes composed of identical sister chromatids. Formation of spindle fibers
• Recombination – chromosome crossover resulting in non-identical sister chromatids. Provides genetic variation in gametes
• Metaphase 1 • Anaphase 1• Telophase 1
Meiosis 2• Prophase 2• Metaphase 2• Anaphase 2• Telophase 2• Cytokinesis
There is no DNA replication during meiosis 2, this means each daughter cell will be haploid and only contain 23 chromosomes
Mendel’s Law of Inheritance
Law of dominance – offspring that inherit a dominant gene will have only the dominant phenotype
Law of segregation – during formation of the gamete, genes responsible for a trait are separated and only one is passed to the offspring. During fertilization, maternal genes are combined with paternal genes producing the genotype of that individual
In eye colour where the dominant gene is for brown eyes (B) and blue eyes are recessive (b), if both parents have the genotype Bb both will have brown eyes
Patterns of inheritance are
Offspring may have brown eyes – BB, Bbor blue eyes – bb
B bB BB Bbb Bb bb
Hardy-Weinberg In a population with two alleles A (wild type) & a (defective) • The frequency of the wild type allele is p• The frequency of the defective allele is q• As there are only two alleles p + q = 1
Chance that offspring are
- AA is (p x p) = p2
- Aa is (p x q) + (p x q) = 2pq
- aa is (q x q) = q2
A (p) a (q)A (p) AA (p2) Aa (pq)a (q) Aa (pq) Aa (q2)
Factors required for the Hardy-Weinberg equilibrium • Large population• No migration in or out of the population• Random mating• Mutation rate remains constant• No selection of alleles
