SBI4U Unit 3: Molecular Genetics Definitions

Ribonucleic Acid

• A single-stranded nucleic acid involved in protein synthesis and gene regulation. Contains ribose sugar and the nitrogenous base uracil.

• Also known as RNA

Deoxyribonucleic Acid

• A double-stranded helical molecule that stores genetic information in cells. Contains deoxyribose sugar and uses thymine instead of uracil.

• also known as DNA

Chromosome

A threadlike structure composed of DNA and proteins that carries genetic information. Found in the nucleus of eukaryotic cells.

Nucleotide

The building block of nucleic acids, consisting of a phosphate group, a sugar (ribose or deoxyribose), and a nitrogenous base.

Nitrogenous

A nitrogen-containing molecule that forms part of a nucleotide. Includes adenine, thymine, cytosine, guanine, and uracil.

Adenine

• A purine nitrogenous base that pairs with thymine in DNA and with uracil in RNA.

• represented by the letter A

Guanine

• A purine nitrogenous base that pairs with cytosine in both DNA and RNA.

• Represented by the letter G

Cytosine

• A pyrimidine nitrogenous base that pairs with guanine.

• Represented by the letter C

Thymine

• A pyrimidine nitrogenous base found only in DNA that pairs with adenine.

• Represented by the letter T

Uracil

• A pyrimidine nitrogenous base found only in RNA that pairs with adenine.

• Represented by the letter U.

Pruine

• A type of nitrogenous base with a double-ring structure. Includes adenine and guanine.

Pyrimidine

A nitrogenous base with a single-ring structure. Includes cytosine, thymine, and uracil.

Complementary Base Pairing

The specific pairing of nitrogenous bases (A with T/U, C with G) due to hydrogen bonding.

Antiparallel

Refers to the opposite orientation of the two strands in a DNA molecule (5' to 3' and 3' to 5').

Semi-Conservative Theory

The accepted model of DNA replication where each new DNA molecule consists of one original strand and one new strand.

Conservative Theory

A disproven model suggesting the original DNA molecule remains intact and a completely new copy is made.

DNA Polymerase

An enzyme that adds complementary nucleotides to a growing DNA strand during replication.

Replication forks

The Y-shaped regions where DNA is unwound to allow for replication.

Helicases

Enzymes that unwind the DNA double helix by breaking hydrogen bonds between base pairs.

Primer

A short RNA sequence that provides a starting point for DNA synthesis.

Okazaki Fragments

Short DNA fragments synthesized on the lagging strand during DNA replication.

Leading Strand

The DNA strand synthesized continuously in the 5' to 3' direction toward the replication fork.

DNA ligase

An enzyme that joins Okazaki fragments by forming phosphodiester bonds.

Lagging strand

The DNA strand synthesized in short fragments away from the replication fork.

Primase

An enzyme that synthesizes RNA primers during DNA replication.

Gene

A segment of DNA that codes for a specific protein or functional RNA.

Genome

The complete set of an organism’s genetic material.

Exon

A coding region of a gene that remains in the final mRNA.

Intron

A non-coding region of a gene that is removed during RNA processing.

Codon

A sequence of three nucleotides on mRNA that codes for a specific amino acid.

Transcription

The process by which the genetic information encoded in DNA is copied into messenger RNA (mRNA). This occurs in the nucleus of eukaryotic cells.

Translation

The process in which the sequence of nucleotides in mRNA is decoded to synthesize a specific polypeptide (protein), occurring in the ribosome.

Initiation

The first phase of transcription or translation where key molecules assemble.

Elongation

The phase in transcription or translation where RNA or a polypeptide chain is extended.

mRNA

• stands for Messenger RNA

• A type of RNA that carries genetic instructions from DNA to the ribosome.

Termination

The final phase of transcription or translation where the process ends and the product is released.

Processing

Modifications made to pre-mRNA, including 5’ capping, splicing, and polyadenylation.

Promoter Sequence

A DNA region where RNA polymerase binds to start transcription.

RNA polymerase

The enzyme responsible for synthesizing RNA from a DNA template.

5’cap

A modified guanine nucleotide added to the 5’ end of mRNA to protect it and assist with ribosome binding.

poly-A-tail

A string of adenine nucleotides added to the 3’ end of mRNA to increase stability.

tRNA

• transfer RNA

• A type of RNA that brings amino acids to the ribosome during translation.

Anticodon

A sequence of three bases on tRNA complementary to an mRNA codon.

Ribosome

A cellular structure made of rRNA and protein that assembles amino acids into proteins.

Mutations

A change in the nucleotide sequence of DNA.

Point Mutation

A mutation affecting a single nucleotide pair in the DNA sequence.

Silent mutation

A mutation that changes a nucleotide but does not alter the amino acid.

Mis-sense Mutation

A mutation that results in a different amino acid being added to the protein.

Nonsense Mutation

A mutation that converts a codon into a stop codon, ending translation early. This leads to a truncated protein that is usually nonfunctional.

Frameshift Mutation

A mutation caused by the insertion or deletion of nucleotides that alters the reading frame.

Mutagen

An agent (chemical, physical, or biological) that causes mutations in DNA.