Molecular Biology Terms Starting With M
Molecular Biology Glossary: M
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MicroRNA
/ MY-kroh ar-en-ay / · Greek: mikros (small) + RNA abbreviation
MicroRNA is a small non-coding RNA molecule of approximately 22 nucleotides that post-transcriptionally represses gene expression by binding to complementary sequences in target mRNAs and recruiting the RNA-induced silencing complex.
MicroRNAs are transcribed as longer primary miRNA transcripts, processed in the nucleus by the enzyme Drosha into pre-miRNA hairpins of about 60 to 70 nucleotides, then exported to the cytoplasm and cleaved by Dicer to produce the mature miRNA duplex. One strand is loaded into the RISC complex, where it guides transcript cleavage or translational repression of mRNAs sharing complementary sequences. A single miRNA can regulate hundreds of different target mRNAs, coordinating entire gene networks.
Humans encode more than 2,600 mature miRNAs, and dysregulation of miRNA expression is documented in cancers, cardiovascular disease, and neurological disorders.
Victor Ambros and Gary Ruvkun discovered the first microRNA, lin-4, in the roundworm Caenorhabditis elegans in 1993, but the finding was considered a nematode curiosity for nearly a decade until miRNAs were found to be conserved across animals. Their work was recognized with the Nobel Prize in Physiology or Medicine in 2024.
Small RNAs are too short to have regulatory significance. MicroRNAs of just 22 nucleotides can silence hundreds of target genes simultaneously by guiding RISC to complementary mRNA sequences.
In zebrafish (Danio rerio), the microRNA miR-430 is transcribed in large quantities shortly after fertilization and clears hundreds of maternally deposited mRNAs from the embryo within the first few hours of development. This rapid clearance affects more than 300 maternal transcripts and is required for the embryo to transition control of gene expression from maternal to zygotic products.
Mobile Genetic Element
/ MOH-bul jeh-NET-ik EL-eh-ment / · English: mobile + genetic + element
Mobile Genetic Element is a DNA sequence that can move to a different location within a genome through enzymatic mechanisms, altering gene expression or genome structure when it inserts at a new site.
DNA transposons use a cut-and-paste mechanism in which the transposase enzyme excises the element and inserts it at a new site. Retrotransposons replicate by a copy-and-paste mechanism through an RNA intermediate that is reverse transcribed and inserted at a new location, increasing copy number with each round. Mobile elements make up nearly half of the human genome and have driven genome evolution through insertional mutagenesis, chromosomal rearrangements, and co-option of transposon-derived sequences as regulatory elements.
Barbara McClintock first described mobile elements in maize (Zea mays) in the 1940s and 1950s, work that earned her the Nobel Prize in Physiology or Medicine in 1983.
The Alu element, a retrotransposon about 300 nucleotides long, has accumulated more than 1 million copies in the human genome and accounts for roughly 11 percent of total genomic DNA. New Alu insertions occasionally disrupt gene function and have been linked to inherited diseases including hemophilia and certain cancers.
All DNA stays in one fixed chromosomal position throughout an organism's lifetime. Mobile genetic elements actively relocate within genomes, and new transposition events occur in human germ cells at a measurable rate.
The piggyBac transposon, originally isolated from the cabbage looper moth (Trichoplusia ni), has been adapted as a tool for genetic engineering in mammalian cells. Researchers use it to insert DNA sequences of interest at defined genomic locations, with individual constructs up to 100 kilobases successfully delivered in experimental systems.
Molecular Cloning
/ moh-LEK-yoo-ler KLOH-ning / · Greek: molekula + klon (twig)
Molecular Cloning is the process of inserting a specific DNA fragment into a replicating vector and propagating it in a host organism to produce large quantities of the identical sequence for study or protein production.
The basic workflow involves cutting both the target DNA and a vector, typically a plasmid, with restriction enzymes, joining them by ligation, and transforming the recombinant molecule into a bacterial host such as Escherichia coli. Each bacterial colony that grows from a transformed cell carries many identical copies of the insert, which can be isolated and sequenced to confirm identity. Expression cloning extends this approach by driving transcription and translation of the cloned sequence to produce recombinant protein.
Modern assembly methods, including Gibson assembly and Golden Gate cloning, allow construction of multi-fragment constructs without relying on restriction site compatibility, greatly expanding the complexity of sequences that can be assembled.
The first recombinant DNA molecule was constructed in 1972 by Paul Berg's laboratory at Stanford University using DNA from two different viruses. This experiment, along with the development of plasmid cloning by Herbert Boyer and Stanley Cohen in 1973, launched the recombinant DNA era.
Molecular cloning copies entire organisms. The technique copies a specific DNA sequence, not a whole organism, and the host cell replicates only the vector carrying the inserted fragment.
Human insulin cDNA was cloned into a bacterial expression plasmid in 1978 by researchers at Genentech, and the resulting engineered Escherichia coli strain produced functional human insulin protein. By 1982, recombinant human insulin became the first genetically engineered drug approved by the U.S. Food and Drug Administration.
mRNA
/ em-ar-en-AY / · Abbreviation: messenger ribonucleic acid
mRNA is a single-stranded RNA molecule transcribed from DNA that carries protein-coding information to ribosomes for translation into a polypeptide.
In eukaryotes, mRNA is produced from a pre-mRNA that receives a 7-methylguanosine cap, has introns removed by splicing, and gains a poly-A tail at the 3-prime end before export from the nucleus. Ribosomes read the mature mRNA in triplet codons from a start codon to a stop codon, with transfer RNAs delivering the corresponding amino acids. mRNA abundance is controlled by transcription rate, processing efficiency, export, localization, translation, and decay.
Half-lives vary widely, from less than 30 minutes for unstable regulatory transcripts to more than 10 hours for stable mRNAs encoding abundant structural or secreted proteins.
The mRNA encoding human factor VIII spans about 9 kilobases of coding sequence. Cloning and expressing this mRNA sequence enabled recombinant factor VIII production for hemophilia A treatment without relying on pooled human plasma.
mRNA permanently alters the DNA of a cell. mRNA is usually a transient working copy that is translated and degraded without changing the genome.
Building Blocks of Nucleic Acids →In pancreatic beta cells, insulin mRNA is translated into preproinsulin, a 110-amino-acid precursor. The insulin mRNA has a half-life of roughly 30 hours in beta cells, helping sustain insulin production between meals.