Year 10 Science - Extension Biology Revision: Genetics, Evolution, and DNA

Essential Biological Terminology and Definitions

• Allele: An alternative form of a gene that exists at a specific position on a chromosome.

• Chromosomes: Thread-like structures made of DNA and proteins that carry genetic information in the form of genes.

• Co-dominance: A form of inheritance wherein the alleles of a gene pair in a heterozygote are fully expressed, resulting in offspring with a phenotype that is neither dominant nor recessive (e.g., spots).

• Diploid: A cell or organism that contains two complete sets of chromosomes ($2n$), one from each parent.

• DNA (Deoxyribonucleic Acid): The molecule that carries the genetic instructions for life, composed of two strands forming a double helix.

• Dominant: An allele that expresses its phenotype even when paired with a different allele (denoted by capital letters, e.g., $T$).

• Genotype: The genetic makeup of an organism, specifically the alleles it carries (e.g., $TT$, $Tt$, or $tt$).

• Haploid: A cell or organism that contains a single set of unpaired chromosomes ($n$), typically found in gametes.

• Heterozygous: Having two different alleles for a particular gene (e.g., $Tt$).

• Homozygous: Having two identical alleles for a particular gene (e.g., $TT$ or $tt$).

• Incomplete Dominance: A genetic situation in which one allele does not completely dominate another, resulting in a new, blended phenotype (e.g., wavy hair from curly and straight parents).

• Meiosis: A type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, producing gametes.

• Mitosis: A type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, used for growth and repair.

• Monohybrid Cross: A genetic cross between two individuals involving one pair of contrasting traits.

• Mutation: A change in the DNA sequence that can lead to variations in traits.

• Nucleotide: The basic building block of DNA and RNA, consisting of a sugar, a phosphate group, and a nitrogenous base.

• Phenotype: The observable physical or biochemical characteristics of an organism, determined by its genotype and environment.

• Recessive: An allele that is only expressed when the genotype is homozygous (denoted by lowercase letters, e.g., $t$).

• Sex-linked Inheritance: Inheritance of traits located on the sex chromosomes (usually the $X$ chromosome).

• Transcription: The first step of protein synthesis where a segment of DNA is copied into mRNA.

• Translation: The second step of protein synthesis where the genetic code in mRNA is read by a ribosome to assemble amino acids into a protein.

• Variation: Differences between individuals of the same species.

• Natural Selection: The process by which organisms better adapted to their environment tend to survive and produce more offspring.

• Gamete: A mature haploid male or female germ cell which is able to unite with another of the opposite sex in sexual reproduction to form a zygote.

The Structure and Function of DNA

• Double Helix: The name given to the double-spiraled shape of the DNA molecule.

• Molecular Components:

  • Bases: Adenine ($A$), Thymine ($T$), Cytosine ($C$), and Guanine ($G$). These are held together by hydrogen bonds.

  • Backbone: Made of alternating sugar (deoxyribose) and phosphate molecules.

  • Nucleotide Structure: A single unit containing one phosphate, one sugar, and one nitrogenous base.

• Complementary Base Pairing: The principle that in DNA, Adenine ($A$) always bonds with Thymine ($T$), and Cytosine ($C$) always bonds with Guanine ($G$). This ensures the genetic code is copied accurately.

• Sequence Practice: The complementary DNA strand for the sequence $AATCGTTGCCAGGTCCTA$ is $TTAGCAACGGTCCAGGAT$.

DNA Replication and the Cell Cycle

• DNA Replication Steps:

  1. The DNA double helix unwinds and "unzips" as hydrogen bonds between bases break.

  2. Free nucleotides in the nucleus pair up with their complementary partners on the exposed strands.

  3. Two identical DNA molecules are formed, each consisting of one original strand and one newly synthesized strand.

• Semi-conservative Process: Replication is called semi-conservative because each new DNA molecule saves (conserves) one of the original parent strands.

• Purpose of Replication: DNA must replicate prior to cell division so that each daughter cell will have a complete and identical copy of the genetic material (Option B from the worksheet).

• Chromosome Structures: Chromosomes have an X-shape because they are made of two identical sister chromatids (Option D) joined at a centromere.

• The Cell Cycle Order: The correct sequence for eukaryotes is $G1 \rightarrow S \rightarrow G2 \rightarrow M$.

• Detailed Phases:

  • Interphase: Includes the $G1$, $S$, and $G2$ phases where the cell prepares for division.

  • Growth Phase 1 (G1): The cell grows rapidly and performs routine metabolic processes. Most routine metabolism occurs here.

  • Synthesis Phase (S): The phase where all nuclear DNA is copied (DNA replication).

  • Growth Phase 2 (G2): The cell makes final preparations to divide.

  • Mitosis (M Phase): The process in which the nucleus of the cell divides.

  • Cytokinesis: The division of the cytoplasm, resulting in two distinct daughter cells.

Protein Synthesis and Mutations

• Transcription:

  • The first step of protein synthesis.

  • Occurs in the nucleus.

  • DNA is transcribed into messenger RNA (mRNA).

• Translation:

  • The second step of protein synthesis.

  • Occurs at the ribosomes in the cytoplasm.

  • mRNA is translated into an amino acid sequence (protein).

• The Genetic Code:

  • Nitrogen bases are read three bases at a time (called a codon).

  • Start Codon: $AUG$ (Methionine).

  • Stop Codons: $UAA$, $UAG$, and $UGA$.

• Mutations:

  • Point Mutations (Substitution): One base is swapped for another. This may change one amino acid or have no effect (silent mutation).

  • Frameshift Mutations (Insertion or Deletion): A base is added or removed, causing the entire reading frame to shift. This usually changes every amino acid following the mutation, potentially creating a premature stop signal or destroying protein function.

Mendelian and Non-Mendelian Genetics

• Zork Genetics Application:

  • Traits: Height (Tall $T$, Short $t$), Eye Number (One $E$, Three $e$), Lip Color (Purple $L$, Green $l$), Fang Number (One $F$, Two $f$).

  • Genotypes like $TT$ or $EE$ are homozygous dominant (Purebred).

  • Genotypes like $Ee$ or $Ll$ are heterozygous (Hybrid).

  • George and Marge Scenario: George is heterozygous ($Tt$). Marge's genotype depends on her parents; if her father is $Tt$ and mother is $tt$, Marge is $Tt$ (since the problem states she is tall).

• Incomplete Dominance (Blending):

  • Example: Hair type in humans. Curly ($CC$) and Straight ($SS$) produce Wavy ($CS$).

  • Cross between two wavy parents ($CS \times CS$) results in: $1/4$ Curly ($CC$), $1/2$ Wavy ($CS$), and $1/4$ Straight ($SS$).

• Co-dominance (Both show surface traits):

  • Example: Cat fur color. Tan ($TT$) and Black ($BB$) produce Tabby (spotted $TB$).

  • Example: Sickle Cell Anemia. $AA$ (Normal), $SS$ (Sickle Anemia), $AS$ (Carrier with some sickle cells).

• Blood Types:

  • Type $A$ ($I^A$) and Type $B$ ($I^B$) are co-dominant to each other but dominant over Type $O$ ($i$).

  • A Type $O$ mother ($ii$) and Type $AB$ father ($I^A I^B$) can only have Type $A$ ($I^A i$) or Type $B$ ($I^B i$) children.

Sex-Linked Inheritance

• Symbols: $X^C$ (Normal), $X^c$ (Color-blind), $Y$ (Male chromosome).

• Carriers: Females with genotype $X^C X^c$ have normal vision but carry the recessive trait.

• Scenario Work:

  • If a normal female ($X^C X^C$) marries a color-blind male ($X^c Y$), all female offspring will be carriers ($X^C X^c$) and all male offspring will be normal ($X^C Y$). Chance of color-blind offspring is $0\%$.

  • If a carrier female ($X^C X^c$) marries a normal male ($X^C Y$), there is a $50\%$ chance that male offspring will be color-blind ($X^c Y$).

Pedigree Analysis and GMOs

• Pedigree Symbols:

  • Square: Male.

  • Circle: Female.

  • Darkened/Shaded: Individual expresses the trait (e.g., Hemophilia or Huntington's).

  • Horizontal Line: Marriage.

  • Vertical Line: Offspring.

  • Roman Numerals (I, II, III): Represent generations.

• Inheritance Patterns:

  • If a child has an autosomal dominant trait, at least one parent must have the trait.

  • If two parents have an autosomal recessive trait, all of their children must have the trait.

  • Autosomal recessive traits can skip generations, while dominant traits typically do not.

• Genetically Modified Organisms (GMO):

  • Definition: An organism whose genetic material has been altered using genetic engineering techniques.

  • Positives: Enhanced nutrient content (e.g., Golden Rice), increased crop yields, and resistance to pests/herbicides.

  • Negatives: Ecological concerns (cross-pollination with wild species), potential for unknown allergic reactions, and corporate control over food supplies.