Genes and the genetic code

• what is an allele?

  a different version of a gene.
• what is a chromatid?

  one of the two strands of a chromosome that are joined together by a single centromere prior to cell division.
• what is a chromatin

  the material that makes up chromosomes. It consists of DNA and the protein histone.
• what is a chromosome

  a thread-like structure made of protein and DNA by which hereditary information is physically passed from one generation to the next.
• what are exons 


  regions of a gene that code for a polypeptide
• whats a gene? 


  section of DNA that codes for a polypeptide and so contributes to the characteristics of an organism.
• what is a gene mutation?

  a change to one or more nucleotide bases in DNA resulting in a change in genotype which may be inherited.
• what are introns?


  regions of a gene that do not code for a polypeptide. Spliced out during mRNA processing.
• what is a locus

  location of a gene on a chromosome.
• what is a genetic code?

  term we use for the way that the four bases of DNA - the A, C, G, and T - are strung together in a way that the ribosome can read them and turn them into a protein.
• what is a triplet?

  3 nucleotides in a row and code for a single amino acid
• how many codons code for an amino acid?

  3 codons
• How many amino acids regularly occur in proteins?

  20
• Does each amino acid have its own code of bases on the DNA?


  yes
• Can an amino acid be coded for by more than one base sequence?


  yes
• what is a degenerate code?

  most amino acids are coded for by more than one triplet.
• Are triplets overlapping in the genetic code?


  no
• What does it mean that the genetic code is universal between organisms?

  The genetic code is universal between organisms, providing evidence of evolution.
• what are prokaryotic cells?

  Prokaryotic cells are simple, single-celled organisms that lack a nucleus and other membrane-bound organelles.
• What are eukaryotic cells?

  have a nucleus and other membrane-bound organelles, like mitochondria and the endoplasmic reticulum
• When can chromosomes be seen?

  When a cell is dividing/condensing
• What do chromosomes look like when they become visible?

  they appear as two threads AKA chromatids

  joined at a single centre point
• What is the DNA in chromosomes held by?

  histones
• describe DNA's structure

  • highly coiled
  • folded
  • double helix
  • combined with histones to form a DNA-histone complex.
• how is DNA packed into a chromosome?

  1. histones hold the dna in place.
  2. It is then coiled.
  3. The coil is then looped and then further coiled into a single chromosome(all the loops and coils allow a lot to be packed in to a chromosome)
• what are homologous chromosomes?

  two pieces of DNA within a diploid organism which carry the same genes, one from each parental source.
• What is true about homologous pairs in terms of genes and alleles?


  A homologous pair carries the same genes, but not necessarily the same alleles.
• what happens during meiosis to homologous pairs

  number of chromosomes half, being haploid.

  When haploid cells combine, during sexual reproduction, homologous chromosomes that are diploid are restored
• contrast the DNA in a prokaryotic cell then in a eukaryotic cell
• contrast the nucleus in a PC then in a EC

  PC = not in nucleus, short, circled, no chromosomes

  EC = in nucleus, long, linear, chromosomes