deoxyribose
a five-carbon sugar molecule with a hydrogen atom rather than a hydroxyl group in the 2′ position; the sugar component of DNA nucleotides
The building blocks of DNA are nucleotides, which are made up of three parts: a deoxyribose (5-carbon sugar), a phosphate group, and a nitrogenous base (Figure 9.3).
double helix
the molecular shape of DNA in which two strands of nucleotides wind around each other in a spiral shape
Watson and Crick proposed that the DNA is made up of two strands that are twisted around each other to form a right-handed helix, called a double helix.
nitrogenous base
a nitrogen-containing molecule that acts as a base; often referring to one of the purine or pyrimidine components of nucleic acids
The building blocks of DNA are nucleotides, which are made up of three parts: a deoxyribose (5-carbon sugar), a phosphate group, and a nitrogenous base (Figure 9.3).
phosphate group
a molecular group consisting of a central phosphorus atom bound to four oxygen atoms
The building blocks of DNA are nucleotides, which are made up of three parts: a deoxyribose (5-carbon sugar), a phosphate group, and a nitrogenous base (Figure 9.3).
DNA ligase
the enzyme that catalyzes the joining of DNA fragments together
As synthesis proceeds, an enzyme removes the RNA primer, which is then replaced with DNA nucleotides, and the gaps between fragments are sealed by an enzyme called DNA ligase.
DNA polymerase
an enzyme that synthesizes a new strand of DNA complementary to a template strand
During elongation, an enzyme called DNA polymerase adds DNA nucleotides to the 3′ end of the template.
helicase
an enzyme that helps to open up the DNA helix during DNA replication by breaking the hydrogen bonds
Certain proteins bind to the origin of replication while an enzyme called helicase unwinds and opens up the DNA helix.
lagging strand
during replication of the 3′ to 5′ strand, the strand that is replicated in short fragments and away from the replication fork
The strand with the Okazaki fragments is known as the lagging strand.
leading strand
the strand that is synthesized continuously in the 5′ to 3′ direction that is synthesized in the direction of the replication fork
This continuously synthesized strand is known as the leading strand.
mismatch repair
a form of DNA repair in which non-complementary nucleotides are recognized, excised, and replaced with correct nucleotides
Most mistakes are corrected during replication, although when this does not happen, the mismatch repair mechanism is employed.
mutation
a permanent variation in the nucleotide sequence of a genome
You isolate a cell strain in which the joining together of Okazaki fragments is impaired and suspect that a mutation has occurred in an enzyme found at the replication fork.
nucleotide excision repair
a form of DNA repair in which the DNA molecule is unwound and separated in the region of the nucleotide damage, the damaged nucleotides are removed and replaced with new nucleotides using the complementary strand, and the DNA strand is resealed and allowed to rejoin its complement
Nucleotide excision repair is particularly important in correcting thymine dimers, which are primarily caused by ultraviolet light.
replication fork
the Y-shaped structure formed during the initiation of replication
As the DNA opens up, Y-shaped structures called replication forks are formed (Figure 9.10).
semiconservative replication
the method used to replicate DNA in which the double-stranded molecule is separated and each strand acts as a template for a new strand to be synthesized, so the resulting DNA molecules are composed of one new strand of nucleotides and one old strand of nucleotides
This is known as semiconservative replication.
telomerase
an enzyme that contains a catalytic part and an inbuilt RNA template; it functions to maintain telomeres at chromosome ends
The discovery of the enzyme telomerase (Figure 9.11) helped in the understanding of how chromosome ends are maintained.
telomere
the DNA at the end of linear chromosomes
The darkly staining regions usually contain genes that are not active, and are found in the regions of the centromere and telomeres.
Okazaki fragments
the DNA fragments that are synthesized in short stretches on the lagging strand
Because DNA polymerase can only synthesize DNA in a 5′ to 3′ direction, the other new strand is put together in short pieces called Okazaki fragments.
primer
a short stretch of RNA nucleotides that is required to initiate replication and allow DNA polymerase to bind and begin replication
Because DNA polymerase can only add new nucleotides at the end of a backbone, a primer sequence, which provides this starting point, is added with complementary RNA nucleotides.
exon
a sequence present in protein-coding mRNA after completion of pre-mRNA splicing
Eukaryotic genes are composed of protein-coding sequences called exons (ex-on signifies that they are expressed) and intervening sequences called introns (int-ron denotes their intervening role).
intron
non–protein-coding intervening sequences that are spliced from mRNA during processing
Eukaryotic genes are composed of protein-coding sequences called exons (ex-on signifies that they are expressed) and intervening sequences called introns (int-ron denotes their intervening role).
mRNA
messenger RNA; a form of RNA that carries the nucleotide sequence code for a protein sequence that is translated into a polypeptide sequence
These include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)—molecules that are involved in the production of proteins from the DNA code.
nontemplate strand
the strand of DNA that is not used to transcribe mRNA; this strand is identical to the mRNA except that T nucleotides in the DNA are replaced by U nucleotides in the mRNA
The mRNA product is complementary to the template strand and is almost identical to the other DNA strand, called the nontemplate strand, with the exception that RNA contains a uracil (U) in place of the thymine (T) found in DNA.
promoter
a sequence on DNA to which RNA polymerase and associated factors bind and initiate transcription
The DNA sequence onto which the proteins and enzymes involved in transcription bind to initiate the process is called a promoter.
RNA polymerase
an enzyme that synthesizes an RNA strand from a DNA template strand
Termination liberates the mRNA and occurs by mechanisms that stall the RNA polymerase and cause it to fall off the DNA template.
splicing
the process of removing introns and reconnecting exons in a pre-mRNA
The process of removing introns and reconnecting exons is called splicing (Figure 9.18).
template strand
the strand of DNA that specifies the complementary mRNA molecule
Transcription always proceeds from one of the two DNA strands, which is called the template strand.
transcription bubble
the region of locally unwound DNA that allows for transcription of mRNA
The region of unwinding is called a transcription bubble.
codon
three consecutive nucleotides in mRNA that specify the addition of a specific amino acid or the release of a polypeptide chain during translation
Each amino acid is defined by a three-nucleotide sequence called the triplet codon.
genetic code
the amino acids that correspond to three-nucleotide codons of mRNA
Various RNA molecules function in the process of forming proteins from the genetic code in DNA.
rRNA
ribosomal RNA; molecules of RNA that combine to form part of the ribosome
These include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)—molecules that are involved in the production of proteins from the DNA code.
stop codon
one of the three mRNA codons that specifies termination of translation
Termination of translation occurs when a stop codon (UAA, UAG, or UGA) is encountered.
start codon
the AUG (or, rarely GUG) on an mRNA from which translation begins; always specifies methionine
In addition to specifying the amino acid methionine, it also serves as the start codon to initiate translation.
tRNA
transfer RNA; an RNA molecule that contains a specific three-nucleotide anticodon sequence to pair with the mRNA codon and also binds to a specific amino acid
These include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)—molecules that are involved in the production of proteins from the DNA code.
alternative RNA splicing
a post-transcriptional gene regulation mechanism in eukaryotes in which multiple protein products are produced by a single gene through alternative splicing combinations of the RNA transcript
In the 1970s, genes were first observed that exhibited alternative RNA splicing.
epigenetic
describing non-genetic regulatory factors, such as changes in modifications to histone proteins and DNA that control accessibility to genes in chromosomes
Eukaryotic cells change the accessibility (epigenetic), transcription, or translation of a gene.
gene expression
processes that control whether a gene is expressed
The process of turning on a gene to produce RNA and protein is called gene expression.
post-transcriptional
control of gene expression after the RNA molecule has been created but before it is translated into protein
Gene expression in prokaryotes is regulated only at the transcriptional level, whereas in eukaryotic cells, gene expression is regulated at the epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels.
post-translational
control of gene expression after a protein has been created
Gene expression in prokaryotes is regulated only at the transcriptional level, whereas in eukaryotic cells, gene expression is regulated at the epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels.
These terms were extracted from Concepts of Biology – 1st Canadian Edition by Charles Molnar and Jane Gair, eCampusOntario (originally published by BCcampus) which is licensed under a Creative Commons Attribution 4.0 International License. Chapter content adapted from Concepts of Biology by OpenStax (© Rice University), licensed under CC BY 4.0.