AP Biology Unit 6: Gene Expression & Regulation

AP Biology Unit 6: Gene Expression & Regulation

• This video is a study resource for the AP Biology exam and a Unit 6 test.
• Key concepts of gene expression and regulation are covered.
• Topics include DNA/RNA structure and function, DNA replication, transcription, translation, the genetic code, prokaryotic (operons) and eukaryotic gene expression, mutations, horizontal gene transfer, and biotechnology.
• Mr. W (Glenn Wolkenfeld), a retired AP biology teacher, created the video.
• Additional study material, a checklist, is available at APbios.com.

DNA Structure

• DNA is a double-stranded helix with nucleotide monomers.
• Each nucleotide comprises deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, guanine, cytosine).
• DNA strands run antiparallel (5' to 3' and 3' to 5').
• Complementary base pairing (A-T, G-C) via hydrogen bonds.
• This pairing is crucial for DNA replication.

DNA as the Molecule of Heredity

• DNA structure enables storage, replication, and transmission of genetic information.
• Information is stored as a base sequence, dictating protein amino acid order.
• Replication is accomplished by complementary base pairing, creating new strands.
• DNA stability protects the base sequence for accurate transmission.
• Mutability allows for genetic variation.

RNA Structure and Function

• RNA is a single-stranded nucleic acid with ribonucleotides.
• Ribonucleotides contain ribose sugar, a phosphate group, and nitrogenous bases (adenine, uracil, guanine, cytosine).
• RNA types include mRNA (carries genetic information), tRNA (brings amino acids), rRNA (part of ribosomes), and small RNAs (gene regulation).

DNA and RNA in Prokaryotes and Eukaryotes

• Prokaryotes have a single, looped circular chromosome.
• Eukaryotes have multiple linear chromosomes.
• Plasmids are small, extrachromosomal DNA loops in bacteria, involved in horizontal gene transfer.

DNA Replication

• DNA replication is a semiconservative process; each new DNA is one original and one new strand.
• Key enzymes include helicase (unwinds DNA), primase (synthesizes RNA primers), DNA polymerase (adds nucleotides), and ligase (joins fragments).
• The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously in Okazaki fragments.

Transcription

• Transcription synthesizes RNA from a DNA template.
• RNA polymerase is the key enzyme.
• The template strand is transcribed; the coding strand has the same sequence as mRNA (except for uracil instead of thymine).
• Promoter regions initiate transcription, while terminator regions end it.

Translation

• Translation synthesizes proteins from mRNA.
• Key components include mRNA, ribosomes, and tRNA.
• The genetic code consists of codons (3-nucleotide triplets) that specify amino acids.
• Translation has initiation (assembly), elongation (adding amino acids), and termination (stop codon) steps.

The Central Dogma of Molecular Genetics

• DNA → RNA → Protein describes the flow of genetic information.
• This process facilitates genetic information transmission and protein synthesis.

Gene Regulation: Operons

• E. coli, a colon bacterium, has ~4000 genes, ~4 million base pairs.
• Operons are clusters of genes, transcribed as a single RNA molecule (primarily in prokaryotes).
• Operons have regulatory elements for gene expression.
• Operons have structural genes, operators (repressor binding), promoters (RNA polymerase site), and regulatory genes.

The Tryptophan Operon (Trp Operon)

• The Trp operon synthesizes the amino acid tryptophan.
• Repressible, meaning tryptophan presence stops its expression.
• Trp absence allows gene transcription; Trp presence stops it via the repressor protein. Tryptophan acts as a corepressor, changes the repressor, which in turn binds to the operator.

The Lac Operon

• The Lac operon codes for lactose-digesting enzymes.
• Inducible, meaning lactose presence starts transcription.
• Lactose absence prevents transcription via the repressor protein. Lactose acts as an inducer, changing the repressor protein.

Regulation of Gene Expression: Eukaryotes

• Eukaryotic gene regulation is more complicated.
• Most eukaryotic DNA is noncoding, with introns in genes.
• Epigenetics plays a role (methylation silences, acetylation activates).
• Epigenetics explains cell differentiation, with different genes expressed in different tissues.

Transcription Regulation in Eukaryotes

• Eukaryotic gene expression uses promoter regions, enhancers, activator proteins, mediator proteins, and general transcription factors.
• Various genes can be affected by a single hormone.

Introns and Exons in Eukaryotic Cells

• Introns are noncoding sequences in genes, excised from pre-mRNA.
• Exons are coding sequences, joined to form mature mRNA.
• Alternative splicing creates multiple protein variants from a single pre-mRNA.

Small RNAs

• Small RNAs regulate gene expression.
• microRNAs (miRNAs) affect post-transcriptional regulation.
• They can degrade mRNA or inhibit translation depending on the complementary match.

Mutations

• Mutation is a random DNA sequence change.
• Point mutations change a single nucleotide.
  • Silent mutations don't alter amino acid sequence.
  • Nonsense mutations introduce stop codons.
  • Missense mutations change amino acids (conservative or non-conservative).
  • Frameshift mutations alter the reading frame due to insertion or deletion.
• Sickle cell disease results from a missense mutation in the hemoglobin gene.
• Mutation effects vary from positive (enhanced fitness) to negative (reduced fitness) to neutral.

Germline vs. Somatic Mutations

• Germline mutations occur in gamete-producing cells, passed to offspring.
• Somatic mutations occur in non-reproductive cells, not passed to offspring.

Horizontal Gene Transfer

• Vertical gene transfer is inheritance from parent to offspring.
• Horizontal gene transfer moves genes between non-related organisms.
• Mechanisms in bacteria include conjugation (direct contact), transformation (uptake from environment), and transduction (via viruses).

Viral Recombination

• Viral recombination shuffles genetic material between viral strains.
• This occurs upon dual infection.

Biotechnology

• Recombinant DNA combines genetic material from different sources.
• Restriction enzymes cut DNA, creating sticky ends.
• Joining is facilitated by DNA ligase.
• Recombinant plasmids facilitate protein production (insert human gene, bacteria express protein).
• DNA fingerprinting (restriction fragment analysis) identifies and characterizes DNA, uses in forensics and genetics.
• PCR (polymerase chain reaction) amplifies a specific DNA sequence, crucial for diagnosis, sequencing.
• DNA sequencing determines the nucleotide order of DNA, critical for research and medicine.

Prepare for the AP Biology exam with this comprehensive Unit 6 quiz on gene expression and regulation. Topics include DNA structure, RNA functions, genetic code, and how gene expression is regulated in prokaryotes and eukaryotes. Utilize the study checklist available at APbios.com for effective learning.