Membranes, Proteins, DNA and Gene Expression flashcards

Topic 2: Membranes, Proteins, DNA and Gene Expression

Fick's law and features of an efficient gas exchange surface

Rate of diffusion ∝ (surface area × concentration difference) ÷ diffusion distance. Alveoli: large surface area; thin walls: short distance; ventilation and blood flow: steep concentration gradient.

Fluid mosaic model

The membrane is a fluid phospholipid bilayer with a varied (mosaic) arrangement of embedded proteins, cholesterol and other components. It is a scientific model built from interpreting experimental data.

Effect of temperature and alcohol on membrane permeability (beetroot)

Both increase permeability, measured by pigment leakage from beetroot tissue. Temperature increases the kinetic energy and movement of phospholipids; alcohol disrupts the phospholipid bilayer.

Osmosis

The movement of free water molecules through a partially permeable membrane, down a water potential gradient (from higher to lower water potential). Pure water has the highest water potential.

Diffusion, facilitated diffusion and active transport compared

Diffusion: down a gradient, no ATP, no protein needed. Facilitated diffusion: down a gradient, no ATP, through channel or carrier proteins. Active transport: against a gradient, uses ATP, carrier proteins.

Endocytosis vs exocytosis

Both are bulk transport using vesicles and ATP. Endocytosis takes material into the cell; exocytosis releases it out of the cell.

Levels of protein structure

Primary: amino acid sequence. Secondary: α-helix or β-pleated sheet held by hydrogen bonds. Tertiary: 3-D shape held by hydrogen, ionic and disulfide bonds and hydrophobic interactions. Quaternary: more than one polypeptide.

Globular vs fibrous proteins

Globular: compact and soluble, with metabolic roles (for example haemoglobin, enzymes). Fibrous: long, insoluble strands with structural roles (for example collagen).

How do enzymes speed up reactions?

The substrate binds to the complementary active site, forming an enzyme–substrate complex, which lowers the activation energy of the reaction. The enzyme is unchanged at the end.

Structure of DNA

Two antiparallel polynucleotide strands of deoxyribose nucleotides, joined by phosphodiester bonds, with complementary bases (A–T, C–G) held by hydrogen bonds, coiled into a double helix.

Semi-conservative replication and the Meselson–Stahl evidence

Each new DNA molecule has one original and one new strand. Bacteria grown on ¹⁵N then moved to ¹⁴N gave only intermediate-density DNA after one generation, ruling out conservative replication, then intermediate and light bands after two, ruling out dispersive replication.

Features of the genetic code

A triplet code (three bases per amino acid), non-overlapping and degenerate (most amino acids have more than one codon). It includes start and stop codons.

Transcription vs translation

Transcription: in the nucleus, RNA polymerase makes mRNA complementary to the template strand of a gene. Translation: at a ribosome, tRNA anticodons pair with mRNA codons and amino acids are joined by peptide bonds.

Gene mutation and its effect on a protein

A change in the base sequence of DNA. It can change the amino acid sequence (primary structure), and so the tertiary structure and function of the protein. A substitution may have no effect because the code is degenerate.

Genetic terms: allele, genotype, phenotype, homozygous, heterozygous

Allele: a version of a gene. Genotype: the alleles an organism has. Phenotype: its observable characteristics. Homozygous: two identical alleles. Heterozygous: two different alleles.

How does cystic fibrosis affect the body?

A recessive allele codes for a faulty CFTR channel protein, so chloride ions are not secreted properly and mucus is thick and sticky. It blocks airways (more infections), pancreatic ducts (poor digestion) and reproductive ducts.

Codominance

Both alleles are expressed in the phenotype of a heterozygote, so neither masks the other. In ordinary dominance, the dominant allele masks the recessive allele in a heterozygote.

Why are more males than females red-green colour blind?

The recessive allele is on the X chromosome and the Y carries no matching allele. A male with one copy (XᵇY) is affected; a female needs two (XᵇXᵇ). XᴮXᵇ females are carriers with normal vision.

PGD, amniocentesis and chorionic villus sampling

PGD: embryos made by IVF, one cell tested, only unaffected embryos implanted. Amniocentesis: amniotic fluid containing fetal cells is sampled, later in pregnancy (from about 15 weeks). CVS: placental tissue is sampled, earlier (from about 10 weeks). Both prenatal tests carry a small risk of miscarriage.

Steps of DNA replication and the role of DNA polymerase

The helix unwinds and the hydrogen bonds between bases break. Each strand acts as a template; free DNA nucleotides pair with the exposed bases by complementary base pairing. DNA polymerase joins adjacent nucleotides with phosphodiester bonds.

Why does the rate of an enzyme reaction fall above the optimum temperature?

Bonds holding the tertiary structure break, so the active site changes shape and is no longer complementary to the substrate: the enzyme is denatured. Fewer enzyme–substrate complexes form.