Biology Unit 3: Genetics

Biology Unit 3: Genetics

1. 3.1 Genes

   • A gene is a heritable factor that consists of a length of DNA and influences a specific trait
   • A gene occupies a specific position on a chromosome
     • -DNA is packaged and organised into discrete structures called chromosomes
     • - The position of a gene on a particular chromosome is called the locus
   • • The various specific forms of a gene are alleles • Alleles differ from each other by one or only a few bases
     • Therefore we see different variations of a specific trait
   • New alleles are formed by mutation
     • gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait
       • Gene mutations can be:
         • Benefitial
           • change the gene sequence = create new variations of a trait
           • (missense)
         • Detrimental
           • truncate/ shorten the gene sequence = suspend the normal function of a trait
           • Nonsense
         • Neutral
           • have no effect on the functioning of the specific feature
           • (Silent)
   • Sickle cell anaemia
     • Cause
       • base substitution mutation
         • 1) Change in DNA
           • 2) Change in mRNA
             • 3) Change in Polypeptide chain in Haemoglobin
     • Consequence
       • 1) Change in Amino acid => form insoluble fibrous strand
         • Insoluble = cannot carry oxygen as effectively = feel constantly tired
       • 2) Change in shape (from RBC -> sickle cell)
         • sickle cells form clots within the capillaries = blood supply to vital organs blocked = health issues
         • sickle cells destroyed more rapidly than RBC = low red blood cell count (anaemia)
   • Genome is the whole of the genetic information of an organism
   • The entire base sequence of human genes was sequenced in the Human Genome Project
     • USES:
       • Mapping
       • Screening
       • Medicine
       • ancestory

2. 3.2 Chromosomes

   • Prokaryotes have one chromosome consisting of a circular DNA molecule
     • The region is called: NUCLEOID
   • Some prokaryotes also have plasmids but eukaryotes do not
     • Bacterial cells may exchange plasmids via their sex pili = BACTERIAL CONJUGATION
       • = Bacteria evolve new features within a generation (horizontal gene transfer)
     • plasmids: self-replicate & autonomously synthesise proteins SO good for gene manipulation
   • Eukaryotic chromosomes are linear DNA molecules associated with histone proteins
     • = more compact structure =better storage
     • 8 histone + DNA
       • Nucleosomes linked with H1 Histone
         • Solenoid condensed to form fibre
           • Fibres loop, compress and fold around protien scaffold to form chromatin
             • Chromatin supercoils during cell division to form visible when stained chromosome
   • In a eukaryote species there are different chromosomes that carry different genes
   • Homologous chromosomes - carry the same sequence of genes but not necessarily the same alleles of those genes
     • Same size and gene loci
   • Diploid = pair
     • Haploid= single
       • single gene copy/ allele
     • 2 gene copies
   • -Sex is determined by sex chromosomes -Autosomes are chromosomes that do not determine sex
   • Karyograms - chromosomes of an organism in homologous pairs of decreasing length
     • Karyograms are used to: Determine gender of an unborn child & test for chromosomal abnormalities e.g. down syndrome
   • Same species = same number of chromosomes
     • Chromosome number & Genome size does not provide a valid indication of genetic complexity

3. 3.3 Meiosis

   • One diploid nucleus divides by meiosis to produce four haploid nuclei
     • Meiosis- process by which gametes are made in reproductive organs
       • The halving of the chromosome number allows a sexual life cycle with fusion of gametes
         • If not halved - polyploidy would occur
   • Separation of pairs of homologous chromosomes in the first meiotic division halve the chromosome number
   • DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids (in the S phase of Interphase)
   • Meiosis I (diploid → haploid)
     • P-I: nuclear membrane dissolves; crossing over occurs
       • M-I: Sister chromatids align along the equator
         • A-I: Spindle fibres contract splitting sister chromatids
           • T-1: Chrom. decondense, nuclear mem. form
             • Cytokenesis
           • Non-disjunction occurs as they fail to separate correctly - E.g. Down Syndrome
         • Random/ Independent assortment occurs (gamete combination = 2^n)
       • Crossing over occurs at chiasmata (point at which homologous chromosomes are held together
         • C.O. causes each haploid daughter cell to be genetically different
     • Meiosis II
       • PMAT II
         • A-II: Sister chromatids separate into chromosomes
   • Genetic Variation occurs due to
     • Crossing over
     • Random fertilisation
       • Gametes that fertilise are random
     • Random orientation
   • Karyotyping obtain fetus cells using
     • Chronic Vili Sampling
       • remove tissue from placenta using tube inserted from cervix
       • slight chance of miscarriage (~11 weeks)
     • Amniocentesis
       • Using fluid from amniotic sack (contains fetal cells)
       • lower chance of miscarriage (~16 weeks)

4. 3.4 Inheritance

   • Mendel discovered the principles of inheritance with pea plants crossing experiments
     • Conclusions
       • Independent assortment
         • Segregation occurs independently/ randomly
       • Segregation
         • each gamete only carries one allele for each gene (Anaphase -II)
       • Dominant alleles
         • Recessive alleles masked
           • Phenotypes- observable characteristics of a specific trait that are determined by genes and environmental factors
     • Sel- fertiliser F2 gen(F3) had 3:1 dominant to recessive phenotype b/c F1 gen was heterozygous
   • 2 haploid gamete = 1 diploid zygote
     • Mom + dad = same= offspring is homozygous
       • Mom +dad = different= offspring is heterozygous
   • Genetic Disease
     • Autosomal Recessive
       • Cystic Fibrosis
         • Unusually thick and sticky mucus that clogs airways and secretory ducts of the digestive system =>respiratory failure and pancreatic cysts
       • both faulty
     • Autosomal Dominant
       • Huntingon's
         • Symptoms: muscle spasms and dementia
         • huntingtin protein misfold (many unnecessary amino acids) = cause neurodegeneration
       • one faulty copy
     • Co-dominant
       • Sickle Cell Aneamia
   • Sex linkage - gene located on sex chromosome
     • X-linked conditions
       • only females can be carriers
         • have option of dominance allele as f have XX chromosomes
           • Males always inherit X-linked trait from mother
   • Pedigree Charts
   • Gene mutation- change to the base sequence = affects structure and function of protien
     • Factors that increase the rate:
       • Radiation (UV)
       • Chemical (pollutants)
       • Bacteria and Virus
     • Nuclear bombing
       • Long-term (general)
         • increase in cancer
         • less T cells = less immunity = more infections
         • Organ-specific health effects
       • Chernobyl
         • Thyroid disease b/c release of radioactive iodide
         • 250% increase in inherited abnormalities

5. 3.5 GM and Biotech.

   • PCR - amplify small amounts of DNA
     • Heat to 95ºC to break hydrogen bonds and unzip chain
       • Annealing – cool to let DNA primers attach to the 3’ ends of the target sequence
         • Elongation – Taq DNA polymerase binds to the primer and copies the strand (72ºC)
   • Gell electrophoresis
     • Use R. Endonuclease cut DNA into fragments. Diff framents= different size
     • DNA profiling = comparing DNA
       • Paternity test
         • Father profile = Child - Mom
       • Forensics
         • DNA match completely
   • Gene Modification
     • Transgenic - new org. after gene modification
     • 1: Amplify specific gene using PCR and primers
       • OR use mRNA and reverse transcriptase and DNA sequence form lack introns (=cDNA)
       • 2: Restriction enzyme @ specific sites. Usually 'Sticky end' to ensure correct orientation
         • 3: DNA ligase joins complementary ends together
           • 4: Select bacteria with gene using antibiotic; purify and extract expressed trait
     • Crops pro/con
   • Cloning - genetically identical, from 1 o.g. parent cell
     • Natural
       • Parthenogenisis
         • embryo from unfertilised ova
         • Plants - vegetative propagation, whereby small pieces can be induced to grow independently
           • Humans- Identical twins (monozygotic) will be clones of one another (genetically identical), while non-identical twins will share 50% of the same DNA
     • Artificial
       • Embryonic cells separated
         • Put into surrogate to develop
           • Limitation: the embryo still formed by sexual reproduction
       • using SNT method for therapeutic uses (making specific tissue/ organs (genertically same = easier to transplant))
     • cloned embryos produced by Somatic cell Nuclear transfer
       • An electric current is then delivered to stimulate the egg to divide and develop into an embryo

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