Topic 4 - Genetic information, variation and relationships

• Describe the structure of a DNA nucleotide

  Phosphate molecule, deoxyribose sugar, nitrogenous base - adenine, thymine, guanine, cytosine
• Describe the structure of an RNA nucleotide

  Phosphate molecule, ribose sugar, nitrogenous base - adenine, uracil, guanine, thymine
• How is DNA arranged in prokaryotic cells? (and chloroplasts/mitochondrion)

  Short circular strands with no histone association
• How is DNA arranged in the eukaryotic nucleus?

  Longer linear strands, with histone association
• What is a chromosome?

  Two polynucleotide strands, twisted into a double helix around associated histones (proteins)
• What is the structure of a DNA molecule?

  A DNA molecule is structured as a double helix, composed of two strands of nucleotides connected by hydrogen bonds between complementary base pairs.
• What is a gene?


  A gene is a segment of DNA that codes for amino acids. Located at the locus of a chromosome, a chromosome contains many genes.
• What is a genome?


  The complete set of genetic information in a cell or organism
• What is a proteome?


  the full range of proteins that a cell can produce
• What are the 3 characteristics of the genetic code?

  Universal, degenerate, non-overlapping
• What does universal mean?

  Every organism uses the same genetic code, meaning all genetic information is transferable between species
• What does non-overlapping mean?

  Each codon only codes for its particular amino acid, so each base in a sequence is read once
• What does degenerate mean?


  Multiple triplet codons can code for the same amino acid during protein synthesis.
• Describe the process of DNA replication

  DNA helicase unwinds the DNA helix, free floating nucleotides align opposite to their complementary pair and form hydrogen bonds, DNA polymerase forms phosphodiester bonds between the adjacent nucleotides.
• What does it mean that DNA replication is semi-conservative?

  Two DNA helixes are made, one strand is conserved from the original DNA molecule, one strand is made from new nucleotides, for each helix.
• What was the Meselson-Stahl experiment?


  An experiment on E. Coli to study the replication of DNA
• Describe the Meselson-Stahl experiment

  They used two different isotopes of nitrogen, N-15 and N-14, to label the DNA molecules. They grew E.coli in a medium containing N-15 and then transferred it to a medium containing N-14. They separated the DNA molecules by a centrifuge and observed the pattern of the isotopes. They found that DNA replicated semi-conservatively, meaning that each strand served as a template for a new strand.
• What is the structure of mRNA?

  Single stranded helix molecule, longer chain than tRNA, single polynucleotide chain, contains ribose, and the bases A, U, G, C.
• What is the structure of tRNA?

  Clover-leaf shaped, amino acid attachment site and anticodon binding site, short chain, single polynucleotide chain, contains ribose sugar and bases A, U, G, C.
• What is protein synthesis?

  A DNA template is transcribed into an mRNA in the nucleus, the mRNA base sequence is then translated into an amino acid sequence in association with tRNA, in the ribosomes.
• Describe the process of transcription


  DNA molecule unwinds, catalysed by DNA helicase, only one chain acts as a template strand. Free mRNA nucleotides align opposite to exposed DNA bases, RNA polymerase binds together the RNA nucleotides to create pre-mRNA polymer, non-coding regions are spliced.
• What is splicing?


  Splicing is the process of removing introns (non-coding regions) and joining exons (coding regions) together in pre-mRNA to produce mature mRNA.
• Describe the process of translation

  mRNA leaves the nucleus, attaches to a ribosome at the start codon, tRNA with complimentary anti-codon aligns opposite to the mRNA, ribosome moves along the mRNA chain, two amino acids on the tRNA molecules are joined by a peptide bond (requires ATP), continues until ribosome reaches a stop codon, then detaches, protein enters golgi for processing and packaging.
• What is a mutation?

  A spontaneous change in the base sequence of chromosomes.
• Outline 2 ways mutation can occur in DNA replication

  Base insertion/ deletion or base substitution
• What is base insertion/ deletion?

  One base is inserted or deleted, causing the triplet composition to change along the entire chain, known as a frameshift, changing all triplets can change most/all amino acids/ genes coded for.
• What is base substitution?

  One base is substituted for another base, causing a change in the genetic code for that triplet codon.
• Why do some base substitutions result in a protein change?

  It can change the original triplet code into another which codes for a different amino acid. This results in a different primary protein structure which can change the tertiary structure and function of the protein.
• Why don't all mutations result in a protein change?

  The genetic code is degenerate, some triplets code for the same amino acid.
• What are mutagenic agents?

  Chemicals or radiation that increase the rate of gene mutation
• What are some examples of mutagenic agents?

  UV, gamma rays, some viruses/ microorganisms (HPV), unhealthy diet (alcohol), mutagenic chemicals (benzene, asbestos, tobacco tar).
• What is random fertilisation in meiosis?

  Any random haploid gamete can be fertilised, increasing genetic variation
• What is non-disjunction?


  When homologous pairs or sister chromatids fail to separate
• When can non-disjunction occur?

  Anaphase I: homologous pairs

  Anaphase II: sister chromatids
• How many cells does non-disjunction affect?

  Four daughter cells if occurs in anaphase I, two daughter cells if occurs in anaphase II
• What is crossing over in meiosis?

  When homologous pairs of chromosomes cross over, and it results in the equivalent portions of the chromatid having been exchanged
• What is independent segregation?


  Where genes are inherited separately to one another.
• Is independent segregation and crossing over exclusive to meiosis?

  Yes
• How does independent segregation lead to variation?

  Alleles are reshuffled and new combinations of chromosomes/ alleles are created
• How does crossing over lead to variation?

  Results in a new combination of alleles