DNA, Genes and Chromosomes

• The structure of DNA is different in prokaryotes and eukaryotes.
• Prokaryotes possess a single, short and circular chromosomal DNA molecule that is sometimes referred to as a nucleoid.
• Prokaryotes usually have one or more plasmids, which are very small, circular DNA molecules that usually only contain a few genes.
• What are plasmids?


  Small, circular DNA molecules that usually only contain a few genes and can replicated independently
• Plasmids are more accessible for proteins required for gene expression and therefore contain genes that are required often and/or quickly, for example: antibiotic resistance genes.
• What is an advantage of plasmids in prokaryotes?

  They are accessible as they contain genes that are required often and/or quickly
• The DNA within prokaryotic cells is not associated with any proteins.
• The DNA in prokaryotic cells coil around themselves in a process called supercoiling, which condenses the circular chromosomes so that they can fit inside the cell.
• Why does prokaryotic DNA supercoil?

  So that it can fit inside the cell
• What is the process called when prokaryotic DNA coils around itself?

  Supercoiling
• Prokaryotic cells have no nucleus, so the DNA is located in the cytoplasm of the cell.
• Where is DNA located in prokaryotic cells?


  Cytoplasm
• DNA and RNA synthesis in prokaryotes occurs together alongside protein synthesis in the same compartment, the advantage of this is that RNA and protein synthesis occurs much quicker, replicating nucleotides at a 100x faster rate than eukaryotes.
• What is an advantage of prokaryotes having DNA, RNA and protein synthesis occur in the same compartment?


  Makes the processes much quicker
• The DNA in eukaryotes is located in the nucleus
• Where is DNA located in eukaryotes?

  Nucleus
• Diploid eukaryotes have homologous pairs of chromosomes.
• What does homologous mean in reference to chromosome pairs?


  The chromosome from the mother and father have the same genes at the same loci, but the alleles may be different at those loci
• The DNA in eukaryotes is tightly wrapped around proteins called histones, coiling around and condensing to form a chromosome.
• What is the protein called that DNA wraps around in eukaryotes?

  Histones
• What does histones and DNA form?

  Chromosomes
• What are chromosomes formed by?


  DNA coiled around histones
• What biological molecule are histones?

  Proteins
• The chromosomes in eukaryotes are long and linear
• In eukaryotes, DNA and RNA synthesis occur in a separate compartment from protein synthesis. This compartmentalisation enables the building of more complex proteins and RNA products.
• The mitochondria and chloroplasts of eukaryotic cells contain DNA which is short, circular and not associated with proteins, similar to the DNA of prokaryotes.
• What is the DNA like in mitochondria and chloroplasts?

  Short, circular and not associated with proteins
• The mitochondria is the site of aerobic respiration within eukaryotic cells
• What is the function of the mitochondria?


  It is the site of aerobic respiration in eukaryotic cells
• Mitochondria are just visible with a light microscope
• What are the major structures within mitochrondria?


  Double membrane (inner and outer), cristae and matrix
• The mitochondria is surrounded by a double membrane, with the inner membrane folded to form cristae, the matrix formed by the cristae contains enzymes needed for aerobic respiration, producing ATP.
• What is the function of the matrix in mitochondria?


  Contains the enzymes needed for aerobic respiration, producing ATP
• Small circular pieces of DNA, known as mitochondrial DNA and ribosomes are found in the matrix of the mitochondria, which are needed for replication.
• Chloroplasts are found in the green parts of the plant, with the green colour being as a result of the photosynthetic pigment chlorophyll.
• What causes the green colour in plants?


  The photosynthetic pigment chlorophyll in chloroplasts
• Chloroplasts are larger than mitochondria
• Chloroplasts are surrounded by a double membrane
• What are some of the main structures of chloroplasts?

  Contain membrane bound components called thylakoids (which contain chlorophyll), that stack to form structures called grana, that are joined together by lamellae, which are thin and flat thylakoid membranes.
• Chloroplasts are the site of photosynthesis, with the light dependent stage taking place in the thylakoids and the light independent stage (known as the Calvin cycle) taking place in the stroma.