Chapter 3 Bio Exam

Gene

A segment of DNA that codes for a protein or RNA product. Genes comprise both structural and regulatory elements.

Chromosome

A complex of DNA-binding proteins and DNA. Chromosomes are circular in most bacteria and archaea and linear in eukaryotes.

Haploid number

The number of different types of chromosomes present in a cell. Haploid number is indicated with an n. Humans have 23 different types of chromosomes, so in that species n = 23.

Ploidy

The number of each type of chromosome present in a cell. Ploidy is written as a number before the n that presents haploid number. Humans, for example, are 2n, meaning that they have two of each type of chromosomes.

Haploid

Having one of each type of chromosome.

Diploid

Having two of each type of chromosome.

Homologous Chromosomes (or simply homologs)

Different versions of the same chromosome type

Replicated chromosomes

A chromosome comprised of two identical DNA molecules, joined together.

Sister Chromatids

The identical copies in a replicated chromosome.

Alleles

Different versions of a gene that have slight variations in the DNA sequence found in other alleles of the same gene. 

Transcription

The process that creates an RNA molecule from information stored in a gene.

Translation

The process that uses the information in messenger RNA (mRNA) to synthesize a protein.

DNA

A molecule that stores information in code form. 

RNA

Molecules that are similar to DNA in terms of chemical structure. Different types of RNAs have different functions in organisms, but they do not serve in long-term information storage like DNA.

Proteins

Molecules that are widely variable in size, shape, and other aspects of structure, but which consist of the same underlying building blocks. Many or most of the structures and molecular machines inside organisms are made of proteins.  

Mutation

Any change in DNA. Mutations create new alleles. 

Genotype

A listing of the alleles present in an individual. 

Phenotype

What an individual looks or acts like. 

Fitness

The ability to produce viable offspring. 

DNA Polymerase

An enzyme that catalyzes the formation of phosphodiester linkages between deoxyribonucleotides, forming the primary structure of DNA.

DNA Ligase

An enzyme that catalyzes the formation of phosphodiester linkages in gaps between two existing nucleotides in double-stranded DNA.

Origin of replication

A location (a point) where enzymes open the DNA double helix so replication can begin.

Replication Fork

The y-shaped structure visible in micrographs of DNA during replication. This is the site where the replisome works, synthesizing the leading and lagging strands.

Primase

An enzyme that adds a short RNA primer to single-stranded DNA, so DNA polymerase can start adding complementary bases.

Replisome

The multi-molecular machine that works at the replication fork, copying DNA.

Helicase

An enzyme in the replisome that opens the DNA double helix, creating single-stranded template DNA for leading and lagging strand synthesis.

Topoisomerase

An enzyme that is important in DNA replication. It breaks and rejoins DNA to relieve the tension created when helicase continuously opens the double helix.

Telomerase

An enzyme that catalyzes the addition of deoxyribonucleotides to lagging strands at either end of a linear chromosome, thus helping to solve the end-replication problem.

Stem Cells

A “professional divider” cell. After each round of mitosis and cell division, one daughter cell becomes a stem cell and the other becomes a specialized cell type that no longer divides. Different types of stem cells give rise to daughter cells which then mature into specific cell types.

DNA proofreading

The ability of DNA polymerase to respond when it adds the wrong base by mistake by 1) pausing synthesis of the new strand, 2) removing the mismatched base with an exonuclease activity, and 3) adding the correct base.

DNA mismatch repair

Proteins and enzymes work on newly synthesized DNA molecules to recognize and repair mismatched bases.

DNA excision repair

Proteins and enzymes work on DNA molecules long after replication is complete to recognize and repair damaged areas as they arise.

Mitosis

A process in eukaryotes that is responsible for distributing a copy of each chromosome to each daughter cell. It occurs after chromosomes have been replicated and prior to the physical division of the parent cell into two daughter cells.

M phase

The portion of the cell cycle when mitosis and cell division occur.

Interphase

The portion of the cell cycle when growth and DNA replication occur.

Prophase

The initial phase of mitosis, when replicated chromosomes condense and the nuclear envelope breaks down.

Metaphase

The middle phase of mitosis, when replicated chromosomes move to the middle of the cell.

Anaphase and telophase

The late phases of mitosis, when sister chromatids separate and one of each type of chromosome ends up at either end of the parent cell, then nuclear envelopes re-form around each set of chromosomes.

Germ Cells

Cells that directly give rise to sperm or eggs, or are sperm or eggs themselves.  Germ cells undergo meiosis to produce haploid daughter cells.

Somatic Cells

Cells that form the body, and that only undergo mitosis.

Meiosis

A process in eukaryotes that distributes one of each homologous chromosome pair to daughter cells, resulting in a halving of chromosome number.  

Gamete

Cells that fuse to form a zygote. In many species, these are sperm and egg.

Zygote

A fertilized egg

Maternal Chromosome

Any chromosome inherited from the mother.

Paternal chromosome

Any chromosome inherited from the father.

Crossing over and recombination

Physical exchange of DNA strands between non-sister chromatids during Meiosis I. Leads to recombinant chromosomes containing alleles from both maternal and paternal chromosomes.

Sexual reproduction

Production of offspring that is based on meiosis and fusion of gametes

Aesexual reproduction

Production of offspring that is based on mitosisInd

Independent assortment

The observation that maternal and paternal homologs line up at random before separating at the end of Meiosis I, producing a wide array of possible combinations in daughter cells.

F₁ generation

The initial generation from an experimental cross—meaning, the offspring of the original parents.

F₂ generation

The second generation from an experimental cross—the offspring of F₁s.

Recessive allele

An allele with a phenotype that does not appear in heterozygous individuals.

Dominant allele

An allele with a phenotype that appears in heterozygous individuals.

Homozygous

Having two copies of the same allele for a gene.

Heterozygous

Having two different alleles for a gene.

Principle of segregation

The observation that alleles of the same gene separate from each other prior to gamete formation. Caused by the separation of homologous chromosomes at the end (anaphase and telophase) of Meiosis I.

Principle of independent assortment

The observation that alleles of different genes go into gametes independently of each other, making every combination of alleles from different genes equally likely. Caused by different chromosomes lining up independently of each other in the middle (metaphase) of Meiosis I, and holds if the genes in question are found on different chromosomes.

Wild Type

The typical or most common version of an allele or phenotype, as opposed to novel, unusual, or low-fitness forms.

Co-dominant alleles

Heterozygotes have the phenotype associated with both alleles present.

Loss of function alleles

Alleles that lead to a protein or RNA product that cannot function normally or is completely absent.  

Sex Chromosome

A chromosome associated with the sex of the individual.

Autosome

Any chromosome that is not a sex chromosome.

Sex-linked genes

Genes found on a sex chromosome.

Linked genes

Genes found on the same chromosome.

Polygenic Trait

A trait that shows continuous variation—usually normally distributed—due to the involvement of products from many different genes. Also called a quantitative trait.

Genetic locus

The physical location of a gene on a chromosome. The plural form is loci and is pronounced LOW-sigh.

Genome

All of the DNA, and thus all genes, in an individual of a particular species.

Pedigree

A diagram summarizing the incidence of a genetic disease or other trait in a family. Pedigrees are used to identify the pattern of inheritance of specific traits and predict how likely offspring from a particular set of parents are to have a specific phenotype.

X-linked recessive phenotypes

shows a trait that primarily affects males, often skipping generations through unaffected carrier females

Autosomal dominant phenotypes

Autosomal dominant phenotypes usually show up in every generation in a pedigree. Every offspring with the autosomal dominant phenotype must have had at least one parent who also had the trait, except in rare cases where a change in DNA has created a new dominant allele. There is no sex bias with this inheritance pattern, as the gene is autosomal—both males and females are equally likely to have the trait. 

Autosomal recessive phenotypes

Males and females are equally likely to have an autosomal recessive trait, and individuals with the phenotype must be homozygous. One big clue that an inheritance pattern is autosomal recessive is if an offspring with the phenotype has parents who are not affected. In this case, both parents were heterozygous, or "carriers" of the trait: they did not show the recessive phenotype themselves, but could pass the recessive allele to their offspring.