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purines
Bases with a double-ring structure.
pyrimidines
Bases with a single-ring structure.
mutation
Changes in the nucleotide sequence in DNA.
allele
One of the alternative forms of a gene that governs a characteristic, such as hair color.
asexual reproduction
One parent produces a genetically identical offspring by mitosis.
autosomes
Chromosomes that are not directly involved in determining the sex of an individual.
crossing over
Nonsister chromatids exchanging DNA segments.
diploid cell
Has two sets of chromosomes.
gametes
A haploid cell such as an egg or sperm that unite during sexual reproduction to produce a diploid zygote.
homologous chromosomes
Pair of chromosomes that are the same size, same appearance and same genes.
independent assortment
The random distribution of the pairs of genes on different chromosomes to the gametes.
karyotype
Photograph of chromosomes grouped in order and in pairs.
locus
The specific site of a particular gene on its chromosome.
sexual reproduction
When two parents give a unique combination of genes to offspring.
zygote
Fertilized egg. Carries one set of chromosomes from each parent.
true-breeding
Organisms that, when reproducing, create offspring of all the same variety.
P generation
The name for the true-breeding parents.
F1 Generation
The hybrid offspring of true-breeding parents.
The Law of Segregation
Two alleles separate during gamete formation and end up in different gametes because they are on homologous chromosomes.
recessive allele
An allele that is masked when a dominant allele is present.
homozygous
An organism having a pair of identical alleles for a character, either dominant or recessive.
phenotype
An organism's traits.
genotype
An organism's genetic makeup.
testcross
The result of breeding a recessive homozygote with an organism of dominant phenotype but unknown genotype.
codominance
When the phenotypes of both alleles are exhibited in the heterozygote.
incomplete dominance
Creates a blended phenotype; one allele is not completely dominant over the other.
pedigree
A diagram that shows the occurrence of a genetic trait in several generations of a family.
sex-linked genes
Genes located on the sex chromosomes.
linked genes
Genes located on the same chromosome that tend to be inherited together in genetic crosses.
nondisjunction
Error in meiosis in which homologous chromosomes fail to separate.
semiconservative model
Type of DNA replication in which the replicated double helix consists of one old strand, derived from the old molecule, and one newly made strand.
DNA polymerase
An enzyme that catalyzes the formation of the DNA molecule.
lagging strand
A discontinuously synthesized DNA strand that elongates by means of Okazaki fragments, each synthesized in a 5' to 3' direction away from the replication fork.
leading strand
The new continuous complementary DNA strand synthesized along the template strand in the mandatory 5' to 3' direction.
Okazaki fragments
Small fragments of DNA produced on the lagging strand during DNA replication, joined later by DNA ligase to form a complete strand.
helicase
An enzyme that untwists the double helix at the replication forks, separating the two parental strands and making them available as template strands.
transcription
Synthesis of an RNA molecule from a DNA template.
messenger RNA (mRNA)
Carries genetic message from the DNA to the protein-synthesizing machinery of the cell.
ribosomes
Complex particles that facilitate the orderly linking of amino acids into polypeptide chains.
template strand
The DNA strand that provides the template for ordering the sequence of nucleotides in an mRNA transcript.
promoter
A specific nucleotide sequence in DNA that binds RNA polymerase and indicates where to start transcribing mRNA.
RNA polymerase
Enzyme that links together the growing chain of ribonucleotides during transcription.
terminator
In prokaryotes, a special sequence of nucleotides in DNA that marks the end of a gene.
5' cap
The 5' end of a pre-mRNA molecule modified by the addition of a cap of guanine nucleotide.
exons
Coding segments of eukaryotic DNA.
codons
mRNA base triplets.
RNA splicing
Process by which the introns are removed from RNA transcripts and the remaining exons are joined together.
introns
Noncoding segments of nucleic acid that lie between coding sequences.
transfer RNA (tRNA)
Interpreter of a series of codons along a mRNA molecule.
ribosomal P site
Site that holds tRNA carrying the growing polypeptide chain.
ribosomal A site
Site that holds the tRNA carrying the next amino acid to be added to the chain.
Ribosomal E site
Site where discharged tRNAs leave the ribosome.
mutations
Random errors in gene replication that lead to a change in the sequence of nucleotides. The source of all genetic diversity.
point mutations
Chemical changes in just one base pair of a gene.
missense mutations
Most common type of mutation, a base pair mutation in which the new codon makes sense in that it still codes for an amino acid.
nonsense mutations
A mutation that changes an amino acid codon to one of three stop codons, resulting in a shorter and usually nonfunctional protein.
frameshift mutation
Mutation occurring when the number of nucleotides inserted or deleted is not a multiple of three, resulting in improper grouping of nucleotides into codons.
polymerase chain reaction (PCR)
A technique for amplifying DNA in vitro by incubating with special primers, DNA polymerase molecules, and nucleotides.
gel electrophoresis
The separation of nucleic acids or proteins, on the basis of their size and electrical charge, by measuring their rate of movement through an electrical field in a gel.
stem cell
Unspecialized cell that can both reproduce itself indefinitely and differentiate into specialized cells of one or more types.