Biology 1113 Main Ideas

Incomplete dominance

Neither allele is dominant; heterozygote phenotype is an intermediate mix of both alleles

Codominance

Both alleles are fully expressed in a heterozygote

ABO blood type inheritance

O is recessive; A and B are codominant to each other

Sickle cell heterozygote advantage

Heterozygotes produce both normal and sickle hemoglobin and are resistant to malaria

Polygenic inheritance

A trait influenced by multiple genes whose combined effects determine phenotype

Why most traits are polygenic

Multiple genes interact with environment to influence phenotype

Linked genes

Genes close together on the same chromosome that tend to be inherited together

Effect of crossing over on inheritance

Crossing over can separate linked genes and alter expected genotype ratios

Pleiotropy

A single gene affects multiple aspects of phenotype

Epistasis

One gene masks or alters the expression of another gene

Mitochondrial inheritance

Genes inherited maternally; mutations often affect energy-demanding tissues

Carrier

A heterozygous individual who carries a recessive allele without showing the phenotype

Why lethal recessive alleles persist

They can be passed through heterozygous carriers

Why lethal dominant alleles are rare

They reduce reproductive success unless effects occur after reproduction

Pedigree

A diagram that traces inheritance of traits across generations

Recessive inheritance pattern in pedigrees

Trait often skips generations

Dominant inheritance pattern in pedigrees

Trait appears in every generation

Sex-linked inheritance pattern

Males and females show different inheritance probabilities

Why dominance can be subjective

Expression depends on molecular vs organism-level phenotype

Semiconservative DNA replication

Each daughter DNA molecule contains one original strand and one new strand

DNA polymerase directionality

DNA polymerase adds nucleotides only in the 5’ to 3’ direction

Origin of replication

DNA sequence where replication begins

Replication fork

Y-shaped region where DNA strands are separated and replicated

Leading strand

DNA strand synthesized continuously toward the replication fork

Lagging strand

DNA strand synthesized discontinuously away from the replication fork

Okazaki fragments

Short DNA fragments synthesized on the lagging strand

Role of helicase

Unwinds the DNA double helix

Role of topoisomerase

Relieves torsional strain ahead of the replication fork

Role of primase

Synthesizes RNA primers needed for DNA polymerase

Role of DNA polymerase I

Replaces RNA primers with DNA

Role of DNA ligase

Joins Okazaki fragments together

Why telomeres shorten

DNA polymerase cannot fully replicate the 3’ end of linear chromosomes

Telomerase

Enzyme that extends telomeres using an RNA template

Why telomerase is active in cancer

Prevents telomere shortening and allows unlimited division

Central dogma

DNA is transcribed into RNA, which is translated into protein

Why central dogma has exceptions

Some RNAs are functional and some genes produce multiple proteins

Gene expression

The process by which information in DNA produces functional RNA or protein

Transcription

Copying DNA information into RNA

Translation

Using mRNA to build a protein

Codon

Three-nucleotide mRNA sequence that specifies an amino acid or stop signal

Reading frame

The grouping of codons determined by the start codon

Genetic code redundancy

Multiple codons can code for the same amino acid

Role of tRNA

Matches codons with the correct amino acids

Mutation

A permanent change in DNA sequence

Germline mutation

Mutation that can be passed to offspring

Somatic mutation

Mutation affecting body cells only

Sources of mutation

Replication errors, radiation, free radicals

DNA polymerase proofreading

Removal of incorrectly paired nucleotides during replication

Mismatch repair

Post-replication correction of base-pair mismatches

Mutator genes

Genes involved in DNA repair whose loss increases mutation rate

Why mutations are not always harmful

Effects depend on protein function and environment

Point mutation

Mutation affecting one or a few nucleotides

Silent mutation

Mutation that does not change amino acid sequence

Missense mutation

Mutation that changes one amino acid

Nonsense mutation

Mutation that introduces a premature stop codon

Frameshift mutation

Insertion or deletion that alters the reading frame

Why frameshifts are severe

They alter all downstream amino acids

Aneuploidy

Abnormal number of chromosomes

Chromosomal inversion

Segment of chromosome reversed in orientation

RNA polymerase

Enzyme that synthesizes RNA

Key difference between RNA and DNA polymerase

RNA polymerase does not require a primer

Stages of transcription

Initiation, elongation, termination

Sigma factor

Bacterial protein that helps RNA polymerase bind promoters

Transcription factors

Eukaryotic proteins that regulate transcription

Polyribosomes

Multiple ribosomes translating the same mRNA

RNA processing

Modification of pre-mRNA before translation

Introns

Noncoding RNA sequences removed during processing

Exons

Coding RNA sequences spliced together

Alternative splicing

Producing different proteins from the same gene

5’ cap function

Protects mRNA and aids ribosome binding

Poly(A) tail function

Stabilizes mRNA and aids translation

Aminoacyl tRNA

tRNA with an attached amino acid

Aminoacyl-tRNA synthetase

Enzyme that attaches correct amino acid to tRNA

Wobble pairing

Flexibility in the third codon position

Translation initiation

Small ribosomal subunit binds mRNA and initiator tRNA

Translation elongation

Amino acids are added to the growing polypeptide

Translation termination

Release factor binds stop codon and ends translation

Operon

Group of genes regulated together in prokaryotes

Repressor

Protein that inhibits transcription

Activator

Protein that increases transcription

Lac operon logic

Lactose presence removes repression and allows transcription

Trp operon logic

Tryptophan presence represses its own synthesis

Regulon

Multiple genes regulated by shared control mechanisms

Epigenetics

Heritable changes in gene expression without DNA sequence changes

Chromatin

DNA wrapped around histone proteins

Histone acetylation

Opens chromatin and promotes transcription

Histone methylation

Often condenses chromatin and represses transcription

Why epigenetics is reversible

Chemical tags can be added or removed

Why epigenetics matters in development

Determines cell differentiation

Master regulators

Transcription factors that control cell fate

Apoptosis

Programmed cell death during development

Theory

Broad, well-supported explanation supported by extensive evidence