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Chromatin

A practical hub for learning what chromatin is, what it does, where it is found, and how packaging connects to gene expression.

DNA

Learn DNA structure, base pairing, replication, and how DNA is packaged into chromatin without treating DNA and chromatin as synonyms.

Genetics

A hub for classical genetics language—alleles, dominance, Punnett squares, and how genotype relates to phenotype—connected to molecular topics.

Chromosomes

Understand chromosomes as segregatable DNA packages built from chromatin, including centromeres, telomeres, and sister chromatids.

Cell Biology

Connect cells, nuclei, the cell cycle, and chromosome segregation—the cellular stage on which chromatin biology plays out.

Epigenetics

Explore DNA methylation, histone modification, and chromatin states that influence gene activity without changing the underlying DNA sequence.

Gene Expression

Follow the path from genes to RNA and protein, including regulation points that chromatin accessibility can influence.

Histones

Learn what histones are, how H2A/H2B/H3/H4 form the nucleosome core, and why histone tails are important for regulation.

Nucleosomes

Master nucleosome structure—DNA wrapped around a histone octamer—and how nucleosome positioning relates to accessibility.

Mitosis

Walk prophase through cytokinesis and connect mitotic chromosome condensation to chromatin packaging concepts.

Meiosis

Learn meiosis I and II, homologous chromosomes, and why meiosis differs from mitosis in ploidy and variation.

intro

Chromatin

Chromatin is the DNA–protein complex that packages eukaryotic genomes and helps regulate gene access.

intermediate

Chromatin structure

Chromatin structure describes how DNA and histones are organized from nucleosomes to higher-order domains.

intermediate

Chromatin function

Chromatin functions in packaging the genome, regulating access for transcription, and supporting replication and repair.

intermediate

Chromatin organization

Chromatin organization describes spatial and regional patterns of genome packaging in the nucleus.

intermediate

Chromatin compaction

Chromatin compaction is the folding of DNA–protein fibers into denser forms, culminating in mitotic chromosomes.

intermediate

Chromatin accessibility

Chromatin accessibility describes how easily proteins can contact DNA within packaged chromatin.

advanced

Chromatin remodeling

Chromatin remodeling uses specialized complexes to slide, eject, or restructure nucleosomes and change DNA access.

intermediate

Chromatin and the cell cycle

Chromatin organization changes across the cell cycle, from accessible interphase forms to condensed mitotic chromosomes.

intro

DNA

DNA stores genetic information as a sequence of nucleotide bases in a double helix.

intro

DNA structure

DNA structure features a sugar-phosphate backbone and complementary base pairs forming a double helix.

intermediate

DNA replication

DNA replication copies the genome by using each strand as a template for a new complementary strand.

intro

DNA packaging

DNA packaging describes how long DNA molecules are folded with proteins to fit and function in cells.

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DNA base pairing

DNA base pairing pairs A with T and G with C, enabling complementary strands and accurate copying.

intro

Double helix

The double helix is the twisted-ladder structure of two complementary DNA strands.

intermediate

DNA mutation

A DNA mutation is a change in DNA sequence that can alter RNA and protein products or regulatory sites.

intermediate

DNA repair

DNA repair pathways detect and correct DNA damage or replication errors to protect genome integrity.

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Gene

A gene is a DNA segment that contains information for a functional product, typically an RNA or protein.

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Gene expression

Gene expression uses information in a gene to make a functional RNA or protein product.

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Gene regulation

Gene regulation controls when, where, and how much gene products are made.

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Promoter

A promoter is a regulatory DNA region where transcription machinery assembles to start transcription.

intermediate

Enhancer

An enhancer is a regulatory DNA element that increases transcription of a target gene, often from a distance.

intermediate

Transcription factor

Transcription factors are proteins that bind specific DNA sequences to activate or repress transcription.

intro

Coding DNA

Coding DNA is sequence that specifies the amino-acid sequence of a protein via codons in mRNA.

intro

Noncoding DNA

Noncoding DNA does not specify protein amino-acid sequence but includes many functional regulatory and structural elements.

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Genome

A genome is the complete genetic material of an organism or cell type under study.

intermediate

Genomics

Genomics is the study of genomes using sequencing, comparison, and large-scale functional analysis.

intro

Histones

Histones are the core packaging proteins of chromatin that form the nucleosome octamer.

intermediate

Histone H1

Histone H1 is a linker histone that binds linker DNA and helps stabilize higher-order chromatin folding.

intermediate

Histone H2A

Histone H2A is a core histone that pairs with H2B in the nucleosome octamer.

intermediate

Histone H2B

Histone H2B is a core histone that pairs with H2A in the nucleosome octamer.

intermediate

Histone H3

Histone H3 is a core histone central to the nucleosome and rich in regulatory tail modifications.

intermediate

Histone H4

Histone H4 is a highly conserved core histone that forms a tetramer with H3 in the nucleosome.

intermediate

Histone modification

Histone modifications are chemical marks on histone proteins that help regulate chromatin behavior.

intermediate

Histone acetylation

Histone acetylation adds acetyl groups to lysines and often correlates with more accessible, active chromatin.

intermediate

Histone methylation

Histone methylation adds methyl groups to residues such as lysine; outcomes depend on site and context.

intro

Nucleosome

The nucleosome is the basic repeating unit of chromatin: DNA wrapped around a histone octamer.

intermediate

Nucleosome structure

Nucleosome structure details the histone octamer, wrapped DNA, and protruding histone tails.

advanced

Nucleosome positioning

Nucleosome positioning refers to where nucleosomes sit along DNA and how that placement affects access to sequences.

intro

Linker DNA

Linker DNA is the DNA segment connecting adjacent nucleosomes in a chromatin array.

intermediate

Euchromatin

Euchromatin is generally less compact, more accessible chromatin often enriched for active genes.

intermediate

Heterochromatin

Heterochromatin is generally more compact, less accessible chromatin associated with reduced transcription.

advanced

Constitutive heterochromatin

Constitutive heterochromatin remains compact across most cell types and is often rich in repeats.

advanced

Facultative heterochromatin

Facultative heterochromatin is compacted in some cell contexts and can be used for developmental gene silencing.

intro

Chromosome

A chromosome is an organized package of DNA and proteins that carries genetic information.

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Chromatid

A chromatid is one copy of a duplicated chromosome; sister chromatids are joined before anaphase separation.

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Sister chromatids

Sister chromatids are the two identical copies of a duplicated chromosome produced by DNA replication.

intermediate

Centromere

The centromere is the chromosome region where kinetochores assemble for spindle attachment during segregation.

intermediate

Telomere

Telomeres are protective DNA–protein structures at the ends of linear eukaryotic chromosomes.

intro

Homologous chromosomes

Homologous chromosomes are chromosome pairs with the same genes at corresponding loci, one from each parent.

intermediate

Chromosome condensation

Chromosome condensation is the compaction of chromatin into discrete mitotic or meiotic chromosomes.

intro

Karyotype

A karyotype is a display of an organism’s condensed chromosomes arranged for analysis of number and structure.

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RNA

RNA (ribonucleic acid) is a nucleic acid used in gene expression, including mRNA, tRNA, rRNA, and more.

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mRNA

Messenger RNA (mRNA) carries genetic coding information from DNA to ribosomes for translation.

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tRNA

Transfer RNA (tRNA) delivers amino acids to the ribosome by matching anticodons to mRNA codons.

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rRNA

Ribosomal RNA (rRNA) is a structural and catalytic core component of ribosomes.

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Transcription

Transcription synthesizes RNA from a DNA template using RNA polymerase.

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Translation

Translation is ribosomal synthesis of a polypeptide directed by mRNA codons.

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RNA processing

RNA processing converts eukaryotic primary transcripts into mature RNAs through capping, splicing, and more.

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Codon

A codon is a three-nucleotide mRNA sequence that specifies an amino acid or a translation stop signal.

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Anticodon

An anticodon is the three-base tRNA sequence that base-pairs with an mRNA codon during translation.

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Cell

The cell is the basic unit of life; eukaryotic cells package their genome in a nucleus as chromatin.

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Cell nucleus

The nucleus is the eukaryotic organelle that houses chromosomal DNA and is the site of transcription.

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Cell cycle

The cell cycle is the ordered series of stages a cell passes through to grow and divide.

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Interphase

Interphase is the G1, S, and G2 portion of the cell cycle when the cell grows and replicates DNA.

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G1 phase

G1 is the first gap phase after mitosis/cytokinesis when the cell grows and performs normal functions before DNA replication.

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S phase

S phase is the synthesis phase when genomic DNA is replicated and sister chromatids are produced.

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G2 phase

G2 is the second gap phase after DNA replication when the cell prepares for mitosis.

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Mitosis

Mitosis is nuclear division that segregates sister chromatids into two daughter nuclei.

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Prophase

Prophase begins mitosis with chromosome condensation and spindle organization.

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Metaphase

Metaphase aligns chromosomes at the metaphase plate with spindle attachments.

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Anaphase

Anaphase separates sister chromatids toward opposite poles.

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Telophase

Telophase reforms nuclei around separated chromosome sets.

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Cytokinesis

Cytokinesis divides the cytoplasm to produce two daughter cells.

intermediate

Meiosis

Meiosis is a two-division process that produces haploid cells and increases genetic variation.

intermediate

Epigenetics

Epigenetics studies heritable gene-activity states that do not change DNA sequence.

intermediate

Epigenome

The epigenome is the genome-wide set of epigenetic features such as DNA methylation and histone marks in a cell state.

intermediate

DNA methylation

DNA methylation is a chemical modification of DNA bases—commonly cytosine in CpG contexts—in many organisms.

advanced

CpG island

CpG islands are GC-rich genomic regions with elevated CpG density, often found near mammalian promoters.

advanced

Chromatin-remodeling complex

Chromatin-remodeling complexes are multi-protein machines that use energy to reposition or alter nucleosomes.

intermediate

Gene silencing

Gene silencing is the stable reduction or shutdown of gene expression by regulatory mechanisms.

advanced

X-chromosome inactivation

X-chromosome inactivation silences most genes on one X chromosome in female mammals for dosage compensation.

advanced

Genomic imprinting

Genomic imprinting causes certain genes to be expressed in a parent-of-origin-specific manner.

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Allele

An allele is an alternative version of a gene found at the same locus on homologous chromosomes.

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Genotype

A genotype is an organism’s genetic makeup at one or more loci—the alleles it carries.

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Phenotype

A phenotype is an organism’s observable traits resulting from genotype and environment.

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Dominant allele

A dominant allele is expressed in the phenotype even when only one copy is present in a heterozygote.

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Recessive allele

A recessive allele determines phenotype only when two copies are present (in simple diploid Mendelian cases).

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Homozygous

Homozygous means having two identical alleles at a locus (AA or aa in simple notation).

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Heterozygous

Heterozygous means having two different alleles at a locus (Aa in simple notation).

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Mendelian genetics

Mendelian genetics describes inheritance patterns based on segregation and independent assortment of alleles.

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Punnett square

A Punnett square is a diagram that predicts offspring genotype probabilities from parental gametes.

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Mutation

A mutation is a heritable change in DNA sequence that can create new alleles and affect phenotypes.

intermediate

Single-nucleotide variant

A single-nucleotide variant (SNV) is a DNA difference at one base position among individuals.

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Inheritance

Inheritance is the transmission of genetic information from parents to offspring.