Genetics Terms Starting With R

Reading Frame is one of three possible ways to partition a nucleotide sequence into consecutive, non-overlapping triplet codons, beginning at a given start position.

Frameshift mutations shift the reading frame and change every codon downstream, usually producing a nonfunctional protein. This disruption occurs because the ribosome reads mRNA strictly in triplets from a fixed starting point, so even a single inserted or deleted nucleotide scrambles all subsequent codons. Overlapping reading frames let viruses and bacteria encode multiple different proteins from the same DNA sequence, maximizing the information content of a compact genome.

The hepatitis B virus, for example, uses overlapping reading frames to produce several distinct proteins, including its surface antigen and polymerase, from a genome of only about 3,200 base pairs.

Recessive is the property of an allele whose phenotypic effect is masked by a dominant allele in the heterozygous condition, requiring two copies for expression.

Recessive alleles are expressed only in the homozygous recessive genotype, where no dominant allele is present to mask their effect. Many recessive alleles encode nonfunctional or reduced-activity proteins, and one functional copy from the dominant allele is sufficient to maintain normal phenotype. Autosomal recessive disorders such as cystic fibrosis and phenylketonuria arise from inheriting two copies of a loss-of-function allele, one from each carrier parent.

Heterozygous carriers of these alleles typically show no symptoms because a single working copy of the gene produces enough functional protein.

Recessiveness is the property of an allele that produces a detectable phenotype only when two copies are present in the homozygous state, being masked by a dominant allele in the heterozygous state.

A recessive allele typically encodes a nonfunctional or significantly reduced-function protein that cannot maintain normal phenotype on its own. One functional copy of most genes produces sufficient gene product for a normal or near-normal phenotype, so a single dominant allele completely masks the recessive allele in heterozygotes. Cystic fibrosis occurs only when an individual inherits two mutant CFTR alleles, one from each parent; heterozygous carriers produce approximately 50 percent normal CFTR protein and appear completely unaffected.

When two carrier parents mate, each child has a 25 percent chance of inheriting two recessive alleles and expressing the disease. This pattern explains why rare genetic diseases can skip generations, reappearing only when two carriers happen to reproduce together.

Recombination is the process by which genetic material is rearranged to produce new combinations of alleles, occurring naturally during meiosis through crossing over between homologous chromosomes.

Homologous recombination during meiosis I creates novel allele combinations in gametes by exchanging DNA segments between non-sister chromatids of paired homologs. This same mechanism also operates in mitotic cells, where it repairs double-strand breaks by using a homologous sequence as a template. The frequency of recombination between two loci estimates the genetic distance between them, measured in centimorgans, where 1 centimorgan corresponds to a 1 percent recombination frequency.

Human chromosome 1, the longest autosome, spans roughly 263 centimorgans despite containing about 249 million base pairs, illustrating how physical and genetic distances can differ substantially.

Restriction Enzyme is a bacterial endonuclease that recognizes and cuts double-stranded DNA at specific short palindromic sequences, producing defined fragments used in molecular cloning and analysis.

Bacteria use restriction enzymes as a defense to destroy foreign viral DNA that lacks the protective methylation marks of the host genome. Type II restriction enzymes, which cut within or near their recognition sites, are the ones used extensively in recombinant DNA technology. Different restriction enzymes recognize different four to eight base pair sequences, generating fragments with blunt ends or single-stranded overhangs called sticky ends that facilitate cloning.

EcoRI, isolated from Escherichia coli, recognizes the six-base sequence GAATTC and cuts it to leave four-nucleotide 5-prime overhangs that pair readily with compatible vector ends.

Ribosome is a large ribonucleoprotein complex found in all living cells that catalyzes protein synthesis by decoding messenger RNA and linking amino acids into a polypeptide chain.

Ribosomes consist of a large and a small subunit, each composed of ribosomal RNA and ribosomal proteins. During translation, the small subunit binds mRNA while the large subunit contains three sites, the A, P, and E sites, through which tRNA molecules carrying amino acids move as the polypeptide grows. Bacterial ribosomes sediment at 70S, composed of 50S and 30S subunits, while eukaryotic cytoplasmic ribosomes sediment at 80S, composed of 60S and 40S subunits.

This structural difference is exploited by antibiotics such as streptomycin and erythromycin, which bind bacterial ribosome subunits and block translation without affecting the eukaryotic ribosomes of the host.

RNA is a single-stranded nucleic acid molecule composed of ribonucleotides that carries genetic information from DNA to ribosomes and catalyzes biochemical reactions in cells.

The major classes of RNA include messenger RNA, transfer RNA, ribosomal RNA, and a growing list of non-coding regulatory RNAs including microRNA and long non-coding RNA, each with distinct structures and cellular roles. RNA differs from DNA in using ribose sugar instead of deoxyribose and uracil instead of thymine, and its generally single-stranded nature allows it to fold into complex three-dimensional shapes that underpin catalytic activity. The RNA world hypothesis proposes that RNA was the original molecule of life, capable of both storing genetic information and catalyzing chemical reactions before DNA and proteins evolved, a view supported by the discovery that the peptidyl transferase activity of the ribosome resides in its RNA core rather than its protein subunits.

RNA Polymerase is the enzyme that synthesizes an RNA strand complementary to a DNA template during transcription, adding ribonucleotides in the 5-prime to 3-prime direction without requiring a primer.

Bacteria have a single multi-subunit RNA polymerase that transcribes all classes of RNA, while eukaryotes have three distinct nuclear RNA polymerases with separate responsibilities: RNA Pol I transcribes the large ribosomal RNA precursor, RNA Pol II transcribes all protein-coding genes and most non-coding regulatory RNAs, and RNA Pol III transcribes transfer RNAs and 5S ribosomal RNA. Transcription begins when RNA Pol II is recruited to the promoter by a set of general transcription factors, including TFIID, TFIIB, and TFIIH, forming a pre-initiation complex that positions the enzyme at the transcription start site. This recruitment step is the primary regulatory point controlling gene expression, and the carboxy-terminal domain of RNA Pol II undergoes phosphorylation to transition the enzyme from initiation into productive elongation.