Glossary for Understanding Eukaryotic Genes

"3 prime"; Refers to carbon 3 of the nucleic acid sugar component (either ribose in RNA or deoxyribose in DNA) to which additional nucleotides may be added by polymerase, often used to refer to that end of a single-stranded DNA or RNA molecule where the 3' carbon retains its hydroxyl group (-OH) and no further nucleotides are bonded.

"5 prime"; Refers to carbon 5 of the nucleic acid sugar component (either ribose in RNA or deoxyribose in DNA), to which the triphosphate is attached in a nucleotide triphosphate, often used to refer to that end of a single-stranded DNA or RNA molecule where the 5' carbon’s phosphate group(s) is/are unattached to a preceding nucleotide.

The inclusion or exclusion of certain exons in the splicing reactions that determine the sequences included in the final mRNA product. This mechanism is utilized to generate a series of closely related protein isoforms, which differ by the inclusion or exclusion of the particular protein regions encoded by those exons. Alternative splicing is directed by RNA-binding proteins that may block, or stimulate, utilization of a particular splice site.

The basic building block of proteins, a small molecule with a -C-C- core, an amine group (-NH₂) at one end and a carboxylic acid group (-COOH) at the other end. The general structure can be represented as NH₂-CHR-COOH, where R can be any of 20 different functional groups of acidic, basic, or nonpolar character.

Gene annotation is the process of notating the location, structure, and identity of genes in a genome. As initial attempts may be based on incomplete information, gene annotations are constantly changing as further data becomes available. Gene annotation databases are updated regularly, and different databases may refer to the same gene/protein by different names, reflecting new knowledge and improved understanding of protein function.

Although formally incorrect (the nitrogenous base that defines A, C, G, T and U is only part of the whole nucleotide), this is often used as a synonym for "nucleotide" in referring to the A, C, G, T, and U components of DNA and RNA.

The hydrogen bonding of one of the bases (A, C, G, T, U) with another, as dictated by optimal hydrogen bond formation in DNA (A-T and C-G) or in RNA (A-U and C-G). Two polynucleotide strands, or regions thereof, in which all the nucleotides form such base pairs are said to be complementary. In achieving complementarity, each strand of DNA can serve as a template for synthesis of its partner strand — the secret of DNA replication’s extremely high accuracy and thereby of inheritance.

In agreement with existing principles and standards generated from data and evidence. For example, the canonical splice donor sequence is GT. In rare instances, however, the sequence GC is used instead; since GC is not the "standard", it would be referred to as non-canonical.

"complementary DNA"; a double-stranded DNA molecule prepared in vitro ("outside of the body"; i.e., in a test tube) by employing an RNA molecule as a template to synthesize DNA using reverse transcriptase. The RNA component of the resulting RNA-DNA hybrid is enzymatically degraded, and the complementary strand then synthesized by DNA polymerase. The resulting double-stranded DNA can be used for cloning and analysis.

"coding sequence"; that part of the DNA sequence of a gene that is translated into protein.

In a gene, any exon that contains some part of the CDS; in contrast, an exon that has no part translated into protein is called a "non-coding exon."

In a gene, the DNA strand that has the sequence found in the RNA molecule. Also called the sense, positive, or non-template strand.

The sequence of three nucleotides in DNA or RNA that specifies a particular amino acid.

Numerical position within a biological sequence; for example, the first base in a DNA sequence would have the coordinate "1".

Refers to the genomic region that comes after the feature being examined.

In eukaryotes, a contiguous segment of DNA that corresponds to a portion of the mature (processed) RNA product of that gene. Exons in eukaryotic genomes are often, but not always, separated by introns. Although exons are transcribed with the introns, the latter are spliced out during RNA processing and degraded.

Any region of defined structure/sequence in a genomic fragment of DNA. Inherent features would include genes, pseudogenes, and repetitive elements. A feature may also be predicted by computational algorithms, such as those aimed at identifying protein-coding genes.

Non-coding section of a eukaryotic nucleic acid sequence found between exons. Introns are removed ("spliced out") from the primary transcript/pre-mRNA after transcription and before the molecule is exported to the cytoplasm for translation.

Potentially different versions of a protein encoded by a single gene. Isoforms result from alternative splicing of a particular pre-mRNA, and/or the use of a different transcription start site.

Mature messenger RNA that has been completely processed and is ready for translation; it has a 7-methylguanosine cap at its 5' end, a poly(A) tail at its 3' end, and has all its introns spliced out.

Also called the anti-sense, template, or non-coding strand. This strand of the DNA sequence of a single gene is the complement of the 5' to 3' DNA strand known as the positive, sense, non-template, or coding strand. The term loses meaning for longer DNA sequences with genes on both strands.

The basic building block of DNA (A, C, G, T) and RNA (A, C, G, U). Nucleotides consist of a nitrogenous base, a 5-carbon sugar (either ribose in RNA or deoxyribose in DNA), and phosphate group(s).

"Open reading frame"; a long stretch of codons in the same reading frame uninterrupted by termination codons; an ORF may reflect the presence of a gene.

The phase describes the number of bases between the end of the exon (defined by the splice site) and the full codon nearest that splice site. The number of bases between the adjacent full codon and an exon/splice site can be 0, 1 or 2. The phase of an upstream exon will determine which frame is translated in the downstream exon by indicating how many bases after the splice acceptor site are needed to create a full codon of 3 bases.

About 250 adenine nucleotides that are post-transcriptionally added by poly(A) polymerase to the 3' end of eukaryotic transcripts, following cleavage of the newly synthesized RNA ~20 nucleotides downstream of an AAUAAA polyadenylation signal sequence.

The initial transcript from a protein-coding gene that contains both introns and exons. Pre-mRNA requires the addition of a 5' cap and 3' poly (A) tail and the removal of introns to produce the final mRNA molecule containing joined exons.

A segment of DNA to which RNA polymerase binds to initiate transcription of the downstream gene(s).

Something that may be predicted or inferred but that requires more evidence to confirm or refute.

A raw DNA sequence.

A frame is a single series of adjacent nucleotide triplets in DNA or RNA: one frame would have bases at positions 1, 4, 7, etc. as the first base of sequential codons. There are three possible reading frames in an mRNA strand and six in a double stranded DNA molecule due to the two strands from which transcription is possible. Different computer programs number these frames differently, so care should be taken when comparing designated frames from different programs. One common way is to refer to the three possible left-to-right reading frames as +1, +2, and +3 and the three possible right-to-left reading frames as -1, -2, and -3.

The process by which introns are removed and exons are joined to produce a mature, functional RNA (mRNA) from a primary transcript. Some RNAs are self-splicing, but most require a specific ribonucleoprotein complex to catalyze the reaction.

The splicing site at the 3' end of an intron, at the boundary between an intron and the exon immediately downstream. The canonical splice acceptor site dinucleotide sequence is AG.

The splicing site at the 5' end of an intron, at the boundary between an intron and the exon immediately upstream. The canonical splice donor site dinucleotide sequence is GT; in rare cases, the non-canonical sequence GC is used instead.

Either a splice acceptor site or a splice donor site.

The first codon of a CDS. In eukaryotes this is almost always ATG, which codes for methionine (one of the 20 amino acids).

A codon that specifies the termination of protein synthesis; sometimes called a "nonsense codon" since it does not specify an amino acid.

The process of copying one strand of a DNA double helix by RNA polymerase, creating a complementary strand of RNA called the transcript.

The process by which codons in an mRNA are "read" by the ribosome and tRNAs to direct protein synthesis.

The location in DNA, generally upstream of a gene’s coding sequence, where RNA polymerase begins transcription.

"Untranslated region"; a segment of DNA (or RNA) that is transcribed and present in the mature mRNA but is not translated into protein. UTRs may be found at either or both of the 5' and 3' ends of a gene or transcript.

Refers to the genomic region prior to the feature being examined.