The characteristic structure of DNA, consisting of two antiparallel polynucleotide strands coiled around a common axis, stabilized by hydrogen bonds between complementary bases.
This structure is fundamental to DNA's function, allowing for precise replication and stable storage of genetic information.
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Nucleotide
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The basic monomeric unit of nucleic acids (DNA and RNA), composed of a nitrogenous base, a five-carbon sugar (deoxyribose or ribose), and one or more phosphate groups.
Understanding the components of a nucleotide is crucial for grasping how DNA and RNA polymers are built and function.
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Semiconservative Replication
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The mechanism of DNA replication where each new DNA molecule consists of one original (parental) strand and one newly synthesized (daughter) strand.
This mechanism ensures that genetic information is accurately passed down, as each new helix retains half of the original template.
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DNA Polymerase
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An enzyme that synthesizes new DNA strands by adding nucleotides complementary to a template strand, also possessing proofreading activity to correct errors.
DNA polymerase is the primary enzyme responsible for building new DNA, ensuring high fidelity in genetic inheritance.
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Origin of Replication
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A specific sequence on a DNA molecule where DNA replication begins, characterized by the local unwinding of the double helix to form a replication bubble.
Multiple origins of replication allow for efficient and timely duplication of large eukaryotic genomes.
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Lagging Strand
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The DNA strand synthesized discontinuously in short segments called Okazaki fragments, moving away from the replication fork due to the 5' to 3' synthesis direction constraint of DNA polymerase.
Its discontinuous synthesis highlights the directional nature of DNA polymerase and the need for DNA ligase to join the fragments.
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DNA Ligase
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An enzyme that catalyzes the formation of a phosphodiester bond to join DNA fragments, such as Okazaki fragments on the lagging strand, or to repair breaks in the DNA backbone.
Think of DNA ligase as the 'molecular glue' that seals gaps in the DNA strand, essential for complete and stable DNA molecules.
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Helicase
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An enzyme that unwinds and separates the two strands of the DNA double helix during replication, using ATP hydrolysis to break hydrogen bonds between base pairs.
Helicase creates the replication fork, making the single-stranded DNA templates accessible for DNA polymerase.
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Central Dogma
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The fundamental principle of molecular biology stating that genetic information flows in a specific direction: from DNA to RNA through transcription, and from RNA to protein through translation.
This concept defines the core pathway by which genetic information is expressed to create functional molecules in a cell.
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Transcription
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The process of synthesizing an RNA molecule from a DNA template, where the genetic information of a gene is copied into an RNA sequence by RNA polymerase.
Transcription is the first step in gene expression, creating an RNA copy that can then be translated into protein or perform other cellular functions.
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RNA Polymerase
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An enzyme that synthesizes RNA by reading a DNA template strand and assembling a complementary RNA strand, without requiring a primer.
Unlike DNA polymerase, RNA polymerase can initiate synthesis de novo, which is critical for starting transcription at specific gene promoters.
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Promoter
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A specific DNA sequence located upstream of a gene that acts as a binding site for RNA polymerase, initiating the process of transcription.
The promoter sequence dictates where transcription begins and often influences the rate of gene expression.
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Intron
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A non-coding segment of an RNA transcript that is removed by splicing before the mature mRNA leaves the nucleus for translation.
Introns allow for alternative splicing, generating multiple protein isoforms from a single gene, increasing protein diversity.
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Exon
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A coding segment of an RNA transcript that remains after splicing and is ultimately expressed as part of the protein sequence.
Exons are the 'expressed' regions, containing the genetic information that codes for amino acids in the final protein.
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Splicing
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The post-transcriptional process of removing introns from a pre-mRNA molecule and joining together the remaining exons to form a mature messenger RNA.
Splicing is essential for generating functional mRNA in eukaryotes and contributes to the complexity of gene expression.
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Codon
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A sequence of three nucleotides in an mRNA molecule that specifies a particular amino acid or a stop signal during protein synthesis.
Codons are the 'words' of the genetic code, dictating the sequence of amino acids in a polypeptide chain.
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Anticodon
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A sequence of three nucleotides on a transfer RNA (tRNA) molecule that is complementary to a specific codon on an mRNA molecule, facilitating the delivery of the correct amino acid.
The anticodon-codon pairing ensures that the correct amino acid is incorporated into the growing polypeptide chain during translation.
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Translation
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The process by which genetic information encoded in messenger RNA (mRNA) is used by ribosomes to synthesize a polypeptide chain (protein).
Translation converts the nucleic acid language of mRNA into the amino acid language of proteins, enabling gene expression to yield functional products.
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Operon
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A functional unit of DNA in prokaryotes containing a cluster of genes under the control of a single promoter and operator, often involved in a common metabolic pathway.
Operons allow for coordinated regulation of multiple genes whose products are needed simultaneously, such as in metabolic pathways.
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Repressor
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A regulatory protein that binds to an operator region within an operon, blocking RNA polymerase from transcribing the downstream genes.
Repressors act as 'brakes' on gene expression, preventing unnecessary protein synthesis when gene products are not required.
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Enhancer
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A DNA sequence that can be located far from a gene, binding specific transcription factors to increase the rate of transcription of that gene.
Enhancers provide a mechanism for fine-tuning gene expression in eukaryotes, often responding to specific cellular signals or developmental stages.
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Histone Modification
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Covalent alterations to histone proteins (e.g., acetylation, methylation) that affect chromatin structure and gene expression without changing the underlying DNA sequence.
These modifications influence how tightly DNA is packed, thereby regulating the accessibility of genes for transcription.
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Epigenetics
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The study of heritable changes in gene expression that occur without a change in the primary DNA sequence, often involving DNA methylation or histone modification.
Epigenetic changes explain how environmental factors can influence gene expression and phenotype, sometimes across generations, without altering the genetic code itself.
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Point Mutation
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A type of gene mutation involving a change in a single nucleotide base pair in the DNA sequence, which can lead to a silent, missense, or nonsense mutation.
Point mutations are the smallest scale of genetic alteration but can have significant effects on protein function, as seen in sickle cell anemia.
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Frameshift Mutation
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A type of gene mutation caused by an insertion or deletion of nucleotides that are not a multiple of three, altering the reading frame of the mRNA and typically leading to a non-functional protein.
This mutation is often more severe than a point mutation because it changes every codon downstream, usually resulting in a premature stop codon.
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Autosomal Recessive Inheritance
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A pattern of inheritance where a trait or disorder only manifests if an individual inherits two copies of the recessive allele, one from each parent, while heterozygotes are carriers.
Individuals must be homozygous for the recessive allele to express the trait, meaning affected offspring can arise from unaffected parents who are carriers.
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X-linked Recessive Inheritance
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A pattern of inheritance where a trait or disorder is caused by a recessive allele on the X chromosome, affecting males more frequently and severely than females because males only have one X chromosome.
Males are hemizygous for X-linked genes, so a single recessive allele on their X chromosome will result in the trait, unlike females who have a second X chromosome.
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Incomplete Dominance
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A type of inheritance where the heterozygous phenotype is intermediate between the two homozygous phenotypes, rather than one allele completely masking the other.
For example, a red flower crossed with a white flower produces pink offspring, demonstrating a blending of traits.
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Pleiotropy
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The phenomenon where a single gene affects multiple distinct phenotypic traits that appear unrelated.
An example is the gene for cystic fibrosis, which can affect the lungs, pancreas, and sweat glands due to a single protein defect.