Ohio Biology EOC B.H (Molecular Genetics): DNA structure and replication, chromosomes and alleles, protein synthesis, gene expression, and mutations

What the molecular genetics topics demand

The Heredity strand (B.H) on Ohio's Biology EOC has a molecular side and an inheritance side. This guide covers the molecular side: the structure of DNA, how it is copied, how it codes for proteins, how its expression is regulated, and how it changes. (The inheritance side, with meiosis, Punnett squares, inheritance patterns, and biotechnology, is in the heredity and inheritance guide.) The content statements here are B.H.1 (genes are segments of DNA on chromosomes), B.H.3 (traits result from inherited genes interacting with the environment), B.H.4 (the molecular structure of DNA determines its information), and B.H.5 (the structure of DNA determines the structure of proteins). The recurring crosscutting concept is structure and function.

This guide ties together the matching topic pages, each with its own practice questions: DNA structure and replication, chromosomes, genes, and alleles, protein synthesis, gene expression and regulation, and mutations and genetic variation.

DNA structure and replication

DNA is a double helix of two strands, each a chain of nucleotides (sugar, phosphate, and a base). The base-pairing rule is A with T and G with C, which makes the two strands complementary. Because each base has one partner, DNA can be copied accurately: in replication the helix unzips and each old strand acts as a template for a new one, giving two identical molecules each with one old and one new strand (semi-conservative). This base-pairing rule is the single most useful fact in the strand.

Chromosomes, genes, and alleles

DNA is coiled and packaged into chromosomes, and a gene is a segment of DNA on a chromosome that codes for a product (B.H.1). An allele is a version of a gene. Body cells are diploid, with chromosomes in homologous pairs (one from each parent) that carry the same genes but possibly different alleles. The genotype is the set of alleles (RR, Rr, rr); the phenotype is the observable trait. Homozygous means two identical alleles; heterozygous means two different ones. This vocabulary is the language of every inheritance problem.

Protein synthesis

The structure of DNA determines the structure of proteins (B.H.5) through two steps. Transcription (in the nucleus) copies a gene's DNA into mRNA, using uracil in place of thymine. Translation (at a ribosome) reads the mRNA three bases at a time: each codon specifies one amino acid, transfer RNA brings the amino acids, and they are joined into a chain that folds into a protein. The order of codons sets the order of amino acids, which sets the protein's structure and function. The flow is DNA to mRNA to protein.

Gene expression and the environment

Every body cell has the same DNA, but each cell type expresses a different subset of genes, so it makes different proteins and specializes (B.H.3). This regulation is the mechanism behind differentiation. The environment also interacts with genes: factors such as nutrition, temperature, and soil chemistry can change the phenotype, so the same genotype can give different phenotypes in different conditions. The EOC point is that traits come from inherited genes and the environment together.

Mutations

A mutation is a change in the DNA base sequence (B.H.4). Point mutations include substitution (swap a base), insertion (add a base), and deletion (remove a base); insertions and deletions can shift the reading frame and change many amino acids. Because DNA codes for protein, a mutation can change a protein and a trait. Mutations are caused by mutagens (UV and other radiation, some chemicals) or by replication errors, and they can be harmful, neutral, or beneficial. They are the original source of new alleles, which links molecular genetics to evolution.

Check your knowledge

A mix of recall and reasoning questions covering the molecular genetics topics. Attempt them under timed conditions, then check against the solutions.

1. State the base-pairing rule in DNA. (1 mark)
2. Explain why DNA replication is described as semi-conservative. (2 marks)
3. State the difference between a gene and an allele. (2 marks)
4. A DNA template strand reads 3'-TAC-GGA-5'. Write the mRNA sequence. (2 marks)
5. State where transcription and translation each occur. (2 marks)
6. Explain why a nerve cell and a skin cell with the same DNA look different. (2 marks)
7. Name the three types of point mutation. (3 marks)
8. State the three possible effects a mutation can have on an organism. (3 marks)