Processes

Describe the process of natural selection

• Variation

• Overproduction

• Competition

• Selection

• Inheritance

• Evolution

• Adaptation

• Individuals in populations show inherited variation

• More offspring produced than can survive

• Competition occurs due to limited resources

• Individuals with advantageous traits survive/reproduce more successfully

• Advantageous alleles passed to offspring

• Frequency of advantageous alleles increases over generations

• Population becomes adapted

Describe the process of antibiotic resistance

• Mutation

• Selection pressure

• Survival

• Reproduction

• Evolution

Random mutation creates resistant bacteria

antibiotic kills susceptible bacteria

resistant bacteria survive

resistant bacteria reproduce

resistance allele frequency increases

Describe the process of allopatric speciation

• Isolation

• Reduced gene flow

• Divergence

• Reproductive isolation

• Speciation

Geographic barrier separates populations

gene flow interrupted

mutations/selection/drift cause divergence

reproductive isolation evolves

populations become separate species

Describe the process of sympatric speciation

• Same environment

• Isolation

• Reduced gene flow

• Divergence

• Speciation

Populations remain in same area

reproductive barriers develop

gene flow reduced

genetic divergence increases

new species forms

Describe reinforcement in hybrid zones

• Hybrid disadvantage

• Selection

• Reduced hybridisation

• Outcome

Hybrids have reduced fitness

selection favours prezygotic isolation

hybridisation decreases

speciation strengthened

Describe fusion in hybrid zones

• Gene flow

• Barrier weakening

• Outcome

Gene flow continues

reproductive barriers weaken

populations merge

Describe stability in hybrid zones

• Hybrid formation

• Outcome

Hybrids continue being produced

hybrid zone maintained

Describe the process of morphogen gradients

• Release

• Gradient formation

• Detection

• Gene activation

• Differentiation

Morphogen released from source

concentration gradient forms

cells detect different concentrations

different genes activated

different cell fates develop

Describe the stages of mitosis

• Prophase

• Metaphase

• Anaphase

• Telophase

• Cytokinesis

Chromosomes condense (prophase)

chromosomes align at equator (metaphase)

sister chromatids separate (anaphase)

nuclei reform (telophase)

cytokinesis produces 2 identical daughter cells

Describe meiosis

• Synapsis

• Crossing over

• Alignment

• Separation I

• Haploid formation

• Alignment II

• Separation II

• Outcome

Homologous chromosomes pair

crossing over occurs

homologous pairs align at equator

homologous chromosomes separate

2 haploid cells produced

chromosomes align individually

sister chromatids separate

4 genetically different haploid cells formed

Describe the process of crossing over

• Pairing

• Exchange

• Recombination

• Outcome

• Variation

Homologous chromosomes pair

non-sister chromatids cross

DNA exchanged

recombinant chromosomes formed

genetic variation increases

Describe the process of non-disjunction

• Separation failure

• Abnormal gametes

• Outcome

Chromosomes/sister chromatids fail to separate

gametes receive abnormal chromosome number

fertilisation produces aneuploidy

Describe the process of X-inactivation

• Random inactivation

• Barr body formation

• Maintenance

• Compensation

One X chromosome randomly inactivated in female embryo

inactivated chromosome condenses into Barr body

inactivation maintained in daughter cells

dosage compensation achieved

Describe the process of DNA replication

• Unwinding

• Priming

• Elongation

• Leading strand synthesis

• Lagging strand synthesis

• Primer replacement

• Joining

Helicase unwinds DNA

primase adds RNA primers

DNA polymerase synthesises new DNA 5′→3′

leading strand continuous

lagging strand forms Okazaki fragments

primers removed/replaced

ligase joins fragments

Describe semi-conservative DNA replication

• Strand separation

• Template action

• Complementary synthesis

• Outcome

DNA strands separate

each strand acts as template

complementary strand synthesised

daughter DNA contains 1 old + 1 new strand

Describe the process of transcription

• Initiation

• Unwinding

• Elongation

• Termination

• Release

RNA polymerase binds promoter

DNA unwinds

RNA synthesised 5′→3′ using template strand

RNA polymerase reaches terminator

RNA released

Describe how the lac operon is switched OFF

• Absence of lactose

• Repression

• Blocking

• Outcome

No lactose present

repressor binds operator

RNA polymerase blocked

no transcription

Describe how the lac operon is switched ON

• Lactose conversion

• Repressor inactivation

• Operator release

• Transcription

Lactose converted to allolactose

allolactose binds repressor

repressor released from operator

RNA polymerase transcribes operon

Describe mRNA processing

• Capping

• Polyadenylation

• Splicing

• Exon joining

• Export

5′ cap added

poly-A tail added

introns removed

exons joined

mature mRNA exported

Describe translation initiation

Ribosome binding

Start codon recognition

Assembly

Small ribosomal subunit binds mRNA

initiator tRNA binds start codon

large subunit joins

Describe translation elongation

• tRNA entry

• Pairing

• Peptide formation

• Translocation

tRNA enters A site → codon–anticodon pairing occurs → peptide bond forms → ribosome translocates

Describe translation termination

• Stop codon recognition

• Release factor binding

• Release

Stop codon reached → release factor binds → polypeptide released

Describe the process of PCR

• Denaturation

• Annealing

• Extension

• Cycling

• Amplification

DNA heated → strands separate (denaturation) → primers bind target DNA (annealing) → Taq polymerase extends strands (extension) → cycle repeats → DNA amplified exponentially

Describe the process of RT-PCR

• RNA isolation

• Reverse transcription

• Amplification

• Analysis

RNA isolated → reverse transcriptase synthesises cDNA → PCR amplifies cDNA → gene expression measured

Describe gel electrophoresis

• Loading

• Migration

• Charge movement

• Separation

• Visualisation

DNA loaded into gel → electric current applied → DNA moves toward positive electrode → smaller fragments move faster → bands visualised using fluorescent dye

Describe DNA fingerprinting

• Amplification

• Separation

• Analysis

• Identification

STR regions amplified by PCR → fragments separated by electrophoresis → band pattern analysed → DNA profile compared between individuals

Describe molecular cloning

• DNA cutting

• Insert preparation

• Ligation

• Transformation

• Replication

• Outcome

Plasmid cut with restriction enzyme → target DNA cut with same enzyme → DNA ligase joins fragments → recombinant plasmid introduced into bacteria → bacteria reproduce → clones produced

Describe Southern blotting

• Digestion

• Separation

• Transfer

• Hybridisation

• Detection

DNA cut into fragments → electrophoresis separates fragments → DNA transferred to membrane → labelled probe hybridises → probe detected

Describe how microarrays work

• Probe preparation

• Sample addition

• Hybridisation

• Detection

• Analysis

DNA probes fixed on chip → labelled sample added → hybridisation occurs → fluorescence measured → expression levels analysed

Describe shotgun sequencing

• Fragmentation

• Sequencing

• Overlap identification

• Assembly

Genome fragmented randomly → fragments sequenced → overlapping regions identified → sequences assembled computationally

Describe DNA transposition

• Excision

• Insertion

Transposon excised → inserted elsewhere in genome

Describe retrotransposition

Fragmentation

Sequencing

Overlap identification

Assembly

Transcribed into RNA → reverse transcribed into DNA → inserted into genome