topic 3 genetics

• Asexual reproduction: organisms reproduce without fertilisation, resulting in a clone (genetically identical individual)
• Sexual reproduction: organisms reproduce through fertilisation of female and male sex cells, promoting genetic variation in offspring
• DNA is found in chromosomes inside nuclei and is made of genes, which code for specific proteins
• DNA structure:

  • Consists of two strands forming a double helix
  • Strands are linked by bases (A-T, G-C) with weak hydrogen bonds
  • DNA sequence determines proteins made in the body, leading to individual differences
• Core practical - DNA extraction method:

  1. Grind/chop fruit to release cell contents

  2. Add water with detergent to break down cell membranes

  3. Heat in water bath to inactivate enzymes

  4. Cool with ice to prevent DNA degradation

  5. Filter to remove cell fragments and froth

  6. Add ice-cold ethanol to precipitate DNA
• DNA discovery:

  • Wilkins and Franklin won Nobel Prize for DNA structure discovery
  • Rosalind Franklin used X-ray crystallography
  • Edwin Chargaff showed equal amounts of A-T and C-G in DNA
  • Jerry Donohue showed bases formed hydrogen bonds
• Protein synthesis:

  • Takes place in transcription and translation stages
  • Transcription: DNA unwinds, unzips, mRNA is formed
  • Translation: mRNA attaches to ribosome, tRNA brings amino acids, polypeptide chain is formed
• Genetic code:

  • Three mRNA bases code for one amino acid
  • Multiple codons can code for the same amino acid
  • Mutations can alter phenotype and protein function
• Mutations:

  • Changes in DNA sequence can be caused by replication errors or environmental factors
  • Some mutations have no effect, others change protein shape and function
  • Mutations can be beneficial or harmful to organisms
• Mendel and inheritance:

  • Mendel's laws: gametes receive one factor randomly, some factors are more powerful
  • Inheritance involves alleles controlling variations in characteristics
• Animal cells have cell membrane, cytoplasm, and nucleus containing DNA in chromosomes

  • Human cells have 23 pairs of chromosomes (46 total) carrying genes that control characteristics
• Humans have 23 pairs of chromosomes in their nuclei, totaling 46 chromosomes
• Each chromosome carries a large number of genes
• Genes control variations in characteristics and contain information about disease likelihood
• Variation caused by genes is called inherited variation
• Alleles are different forms of the same gene
• Each person can inherit a different set of alleles from their parents
• Gametes (sex cells) have one copy of each chromosome and one allele for each gene
• In sexual reproduction, male and female gametes fuse to form an organism with 46 chromosomes and two alleles for each gene
• Dominant alleles only need one copy to show the phenotype
• Recessive alleles need two copies to show the phenotype
• Genotype refers to the alleles in an organism, while phenotype refers to what an organism looks like
• Homozygous is when both alleles in an organism are the same, heterozygous is when the alleles are different
• Punnett squares show possible genotypes produced when two organisms breed
• Inheritance of sex chromosomes determines gender
• Co-dominance occurs when both alleles in a heterozygous genotype are displayed in the phenotype
• Sex-linked disorders are caused by genes located on the X sex chromosome
• Some characteristics are both co-dominant and found on the X chromosome
• Genetic disorders like sickle cell anemia and cystic fibrosis are caused by faulty alleles
• Family pedigree charts show how genetic disorders are passed on in a family
• The Human Genome Project involved finding the sequence of the 3 billion base pairs in the human genome
• Knowing the human genome sequence has implications for improved testing, finding disease risk genes, new treatments, gene therapy, and personalized medicines
• Variation includes discontinuous and continuous variation
• Discontinuous variation is caused by genetic instructions and is plotted on a bar graph
• Continuous variation is influenced by genes and the environment, plotted on a line graph
• Transcription:

  • Takes place inside the nucleus
  • DNA first unwinds and unzips by breaking weak hydrogen bonds between bases in the double helix, separating the two DNA strands
  • One DNA strand acts as a template
  • RNA bases complementary to DNA bases link together by RNA polymerase to form mRNA
  • mRNA is complementary to the DNA template strand
  • mRNA is released from DNA and leaves the nucleus via nuclear pores
• RNA vs DNA:

  • RNA has one strand, while DNA has two
  • RNA has uracil (U) instead of thymine (T)
  • Adenine (A) in RNA pairs with uracil (U), adenine (A) in DNA pairs with thymine (T)
• Translation:

  • Takes place on ribosomes in the cytoplasm
  • mRNA attaches to ribosomes in the cytoplasm
  • Transfer RNA (tRNA) molecules in ribosomes have anticodons and amino acids
  • tRNA controls which amino acid is attached
  • tRNA contains uracil (U) instead of thymine (T)
• Process of Translation:

  • Ribosome decodes mRNA in groups of 3 (codons)
  • tRNA with complementary triplet of bases lines up with codon
  • tRNA releases amino acid to form a polypeptide chain
  • Ribosome moves to next codon until amino acid chain is complete
  • Polypeptide chains fold into specific shapes to create proteins
• Note:

  • DNA base order determines amino acid order in proteins
  • DNA base order determines mRNA sequence, which dictates tRNA alignment on mRNA