DNA Replication and Structure Quiz

DNA Replication

• DNA carries genetic instructions for all living organisms' growth, development, functioning, and reproduction.
• DNA is shaped like a twisted ladder (double helix).
• Sugar-phosphate backbones form the sides of the ladder.
• Nitrogenous base pairs (A-T, C-G) form the rungs.
• Adenine (A) pairs with Thymine (T).
• Cytosine (C) pairs with Guanine (G).
• Hydrogen bonds hold the base pairs together.
• Nearly 99.9% of human DNA is identical across species, despite variations making individuals unique.
• DNA is found in the nucleus and mitochondria. Mitochondria DNA is inherited maternally.
• A single human cell contains about 2 meters of DNA
• DNA replication is semi-conservative, meaning each new DNA molecule consists of one original strand and one newly synthesized strand.

DNA Structure

• DNA is made of a sugar (deoxyribose), a phosphate group and a nitrogenous base.
• The sugar and phosphate form the backbone of the DNA double helix; the bases face inwards.
• Hydrogen bonds connect the base pairs of the DNA molecule.
• The four nitrogenous bases are: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C).

Enzymes involved in DNA Replication

• Helicase unwinds and separates the double-stranded DNA.
• Topoisomerase relieves supercoiling tension in the DNA ahead of the replication fork.
• Single-Strand Binding Proteins (SSBs) prevent separated strands from reannealing.
• Primase synthesizes a short RNA primer to provide a starting point for DNA polymerase.
• DNA Polymerase III adds new nucleotides to the growing strand, extending from the RNA primer (5’ to 3’ direction).
• DNA Polymerase I removes RNA primers and replaces them with DNA nucleotides forming phosphodiester bonds.
• Ligase seals gaps between fragments, such as Okazaki fragments.

Central Dogma

• The central dogma describes the flow of genetic information from DNA to RNA to protein.
• Replication is the process for copying DNA.
• Transcription is the process of making RNA from DNA.
• Translation is the process of making protein from RNA.

Differences between DNA and RNA

• DNA is deoxyribonucleic acid; RNA is ribonucleic acid
• DNA is very stable (long life); RNA is less stable (short life)
• DNA is found in the nucleus, and mitochondria, plastids; RNA is found in nucleus and cytosols
• DNA has a double helix structure; RNA has a single strand structure
• DNA has deoxyribose sugar; RNA has ribose sugar
• DNA has A-T, C-G; RNA replaces T with U (A-U, C-G)
• DNA is usually very resistant to UV radiation; RNA is more sensitive to UV radiation

Protein Synthesis-Transcription

• Transcription takes place in the cytoplasm of prokaryotes and the nucleus of eukaryotes.
• DNA is used as a template to create an mRNA molecule.
• The mRNA molecule is complementary to a DNA strand.

Protein Synthesis-Translation

• The RNA sequence is translated into a sequence of amino acids to form a protein.
• The ribosome reads three bases (a codon) at a time from the RNA and translates them to an amino acid.
• There are twenty amino acids. Eleven are non-essential, synthesizable by the body; nine are essential, non-synthesizable by the body.
• Examples of non-essential amino acids include alanine, asparagine, etc. Examples of essential are tryptophan, methionine, etc.

Types of Gene Mutations

• Point Mutation (Substitution): A single nucleotide is replaced with another. Silent mutations do not change the amino acid sequence due to redundancy in the genetic code(example: AGA to AGG). Missense mutations cause a change in the amino acid sequence (example: GAG to GUG). Nonsense mutations result in a premature stop codon (example: UAC to UAG).
• Frameshift Mutation: Insertion or deletion of nucleotides shifts the reading frame of codons, altering the protein sequence. An example of an insertion mutation: AUG-AAA-GGC to AUG-GAA-AGG-C.
• Expanding Repeats Mutation: A repeated sequence of nucleotides expands, causing genetic disorders (e.g. Huntington's disease).