Follow DNA through chromosome copying and cell division, see how meiosis reshuffles alleles, then use inheritance probabilities to connect a gene variant with a trait.
Cell division explains how DNA is copied and inherited. Gene expression adds another set of steps that can connect a DNA region with a cellular effect. This is a model of one possible route for a protein-coding gene, not a deterministic pipeline for every gene or trait.
Promoters and other regulatory DNA help control when, where, and how much a gene is transcribed. A protein-coding gene can include exons and introns; its translated open reading frame is only part of the mature message. An exon can also contain an untranslated region.
RNA polymerase copies a gene into a primary RNA transcript. In many eukaryotic protein-coding transcripts, RNA processing adds a 5′ cap and poly-A tail and removes introns by splicing. Alternative processing can produce different RNA isoforms; many genes also produce functional noncoding RNAs.
After processing, a mature messenger RNA can be exported to the cytoplasm. Its stability, location, and abundance are regulated, so a DNA sequence does not guarantee that a transcript is always present or used at the same level.
For a protein-coding mRNA, a ribosome reads codons in the open reading frame and assembles an amino-acid chain. Translation is one possible outcome of gene expression: not every RNA is translated, and a new chain may need folding, chemical modification, or transport before it functions.
Beta-globin is one example: HBB encodes beta-globin, which joins other globin chains to form hemoglobin. A variant may change protein sequence or amount, but its cellular effect depends on the resulting molecule and the conditions in which it operates.
In sickle cell disease, deoxygenated hemoglobin S can polymerize and alter red-cell shape and behavior. Genotype, hemoglobin composition, oxygenation, development, environment, and other factors influence outcomes; one molecular change does not determine every feature of a person’s phenotype.
A sequence change in a promoter or enhancer can affect transcription-factor binding and gene expression in particular cell types or states. The direction and size of an effect depend on the exact sequence, regulatory partners, and cell context; the word variant alone does not predict the result.
A gene is a region of DNA. An One DNA-sequence version at a particular locus. An allele can affect a trait, but a sequence difference is not automatically harmful. is a sequence version at a particular A specific position or region in a genome, such as the location of a gene or genetic variant.. A DNA difference can matter, have a small effect, or have no known effect; the word variant alone does not mean harmful.
Before Nuclear division that separates replicated chromosomes into daughter nuclei, usually preserving the chromosome-set number. or A cell-division program with two nuclear divisions after one DNA-replication phase; it reduces chromosome-set number and usually creates varied haploid products., a cell copies its DNA during S phase. In the 2n = 4 model below, four DNA molecules packaged with proteins. Each replicated chromosome is counted as one until its sister chromatids separate. become four duplicated DNA molecules packaged with proteins. Each replicated chromosome is counted as one until its sister chromatids separate. containing eight DNA molecules and their associated proteins within chromosomes. Sister chromatids are the replicated copies joined before they separate.. The A DNA molecule packaged with proteins. After replication, one chromosome contains two sister chromatids until they separate. count is still four because each duplicated A DNA molecule packaged with proteins. After replication, one chromosome contains two sister chromatids until they separate. is counted as one until its The two DNA-containing copies of a replicated chromosome. They remain joined until they separate during cell division. separate.
Nuclear division that separates replicated chromosomes into daughter nuclei, usually preserving the chromosome-set number. uses one nuclear division to distribute The two DNA-containing copies of a replicated chromosome. They remain joined until they separate during cell division. and usually makes two daughter nuclei with the same A DNA molecule packaged with proteins. After replication, one chromosome contains two sister chromatids until they separate.-set number as the starting nucleus. A cell-division program with two nuclear divisions after one DNA-replication phase; it reduces chromosome-set number and usually creates varied haploid products. uses two nuclear divisions after one DNA-replication phase: A pair with the same genes in corresponding locations, one chromosome inherited from each parent; the alleles need not be identical. separate in A cell-division program with two nuclear divisions after one DNA-replication phase; it reduces chromosome-set number and usually creates varied haploid products. I, then The two DNA-containing copies of a replicated chromosome. They remain joined until they separate during cell division. separate in A cell-division program with two nuclear divisions after one DNA-replication phase; it reduces chromosome-set number and usually creates varied haploid products. II.
Track chromosomes and chromatids in a symbolic cell with two homologous pairs (2n = 4).
The drawings show counts, not exact chromosome positions or allele combinations. A duplicated chromosome is counted as one chromosome until its sister chromatids separate.
Meiosis. Before S phase, step 1 of 4.
The model starts with two homologous chromosome pairs: four chromosomes and four chromatids. A human starting cell has 46 chromosomes; this reduced example uses 2n = 4 so each separation can be counted.
In prophase I, A pair with the same genes in corresponding locations, one chromosome inherited from each parent; the alleles need not be identical. pair. Exchange of corresponding DNA segments between nonsister chromatids of homologous chromosomes during meiosis I. can exchange corresponding DNA segments between nonsister DNA molecules and their associated proteins within chromosomes. Sister chromatids are the replicated copies joined before they separate.. At metaphase I, each homolog pair can orient in either direction, producing different combinations of A DNA molecule packaged with proteins. After replication, one chromosome contains two sister chromatids until they separate. copies in Reproductive cells with one chromosome set in many sexual life cycles; their formation differs across organisms.. The tendency of different homologous chromosome pairs to orient independently in meiosis I. Nearby linked genes do not always follow this simple pattern. is a useful model for unlinked loci; nearby loci on the same A DNA molecule packaged with proteins. After replication, one chromosome contains two sister chromatids until they separate. tend to be inherited together, although Exchange of corresponding DNA segments between nonsister chromatids of homologous chromosomes during meiosis I. can separate them.
For a specified A specific position or region in a genome, such as the location of a gene or genetic variant., a A reproductive cell with one chromosome set in many sexual life cycles; its formation differs among organisms and sexes. normally receives one One DNA-sequence version at a particular locus. An allele can affect a trait, but a sequence difference is not automatically harmful. copy. A Punnett square lists the combinations predicted from the parental genotypes. Its fractions describe a probability for each conception under the stated model; they do not promise an exact ratio among a small number of siblings.
The HbS variant changes A protein subunit that joins alpha-globin and heme to make hemoglobin., a subunit of The protein in red blood cells that binds and carries oxygen.. In a common two-One DNA-sequence version at a particular locus. An allele can affect a trait, but a sequence difference is not automatically harmful. teaching cross, two A common hemoglobin genotype with one HbA and one HbS beta-globin allele; it is usually called sickle cell trait, which differs from sickle cell disease. parents can each pass an HbA or HbS One DNA-sequence version at a particular locus. An allele can affect a trait, but a sequence difference is not automatically harmful.: each pregnancy has a 1-in-4 chance of A hemoglobin genotype with two HbA alleles in the simplified HbA/HbS inheritance example., a 1-in-2 chance of A common hemoglobin genotype with one HbA and one HbS beta-globin allele; it is usually called sickle cell trait, which differs from sickle cell disease. (Usually the HbAS genotype: one HbA and one HbS allele. It is different from sickle cell disease.), and a 1-in-4 chance of A hemoglobin genotype with two HbS alleles; HbSS is one common form of sickle cell disease.. A group of inherited hemoglobin disorders. HbSS is one form; HbSC and HbS/beta-thalassemia are others. also includes genotypes such as HbSC and HbS/beta-thalassemia, and clinical outcomes vary.
Each parent can pass either HbA or HbS. This grid shows the four equally likely allele combinations in the simplified cross.
| Parent 1 ↓ · Parent 2 → | HbA | HbS |
|---|---|---|
| HbA | HbAAHbA / HbA | HbASSickle cell trait |
| HbS | HbASSickle cell trait | HbSSSickle cell disease |
25%HbAA
50%HbAS · sickle cell trait
25%HbSS · sickle cell disease
These are probabilities for each conception under this HbA/HbS model, not a guaranteed ratio among siblings. Sickle cell disease also includes HbSC and HbS/beta-thalassemia. This teaching example is not genetic testing or counseling.
NIH · Sickle Cell Disease: Causes and Risk FactorsFailure of homologous chromosomes or sister chromatids to separate as expected, which can produce cells with an abnormal chromosome number. occurs when DNA molecules packaged with proteins. Each replicated chromosome is counted as one until its sister chromatids separate. fail to separate as expected. The timing matters: homologs can fail to separate in A cell-division program with two nuclear divisions after one DNA-replication phase; it reduces chromosome-set number and usually creates varied haploid products. I, while The two DNA-containing copies of a replicated chromosome. They remain joined until they separate during cell division. can fail to separate in A cell-division program with two nuclear divisions after one DNA-replication phase; it reduces chromosome-set number and usually creates varied haploid products. II or Nuclear division that separates replicated chromosomes into daughter nuclei, usually preserving the chromosome-set number.. Many traits involve multiple genes, regulation, development, and environment, so a one-gene chart is a teaching model rather than a universal explanation.