General Biology Exam Review: DNA, Cell Cycle, Replication, and Molecular Genetics

DNA Structure and Replication

Basic Structure of DNA

DNA (deoxyribonucleic acid) is the hereditary material in almost all living organisms. Its structure consists of two antiparallel strands forming a double helix, with each strand composed of nucleotides.

• Nucleotide: Consists of a phosphate group, a deoxyribose sugar, and a nitrogenous base (A, T, C, G).

• Base Pairing: Adenine (A) pairs with Thymine (T); Cytosine (C) pairs with Guanine (G).

• Antiparallel Strands: One strand runs 5' to 3', the other 3' to 5'.

• Phosphodiester Bonds: Link nucleotides together in a strand.

Example: The diagram showing base pairing and directionality of DNA strands.

DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division. It is semi-conservative, meaning each new DNA molecule contains one old and one new strand.

• Origin of Replication: Specific sequence where replication begins.

• Replication Fork: Y-shaped region where DNA is unwound and new strands are synthesized.

• Enzymes Involved:

  • Helicase: Unwinds the DNA double helix.

  • Primase: Synthesizes RNA primers.

  • DNA Polymerase: Adds nucleotides to the growing DNA strand.

  • Ligase: Joins Okazaki fragments on the lagging strand.

• Leading vs. Lagging Strand:

  • Leading Strand: Synthesized continuously in the 5' to 3' direction.

  • Lagging Strand: Synthesized discontinuously as Okazaki fragments.

Equation:

$\text{DNA Polymerase}: \text{dNTPs} + \text{template strand} \rightarrow \text{new DNA strand}$

Example: Diagrams showing replication forks and Okazaki fragments.

Cell Cycle and Mitosis

Phases of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide.

• Interphase: Includes G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis).

• Mitosis: Division of the nucleus, followed by cytokinesis (division of the cytoplasm).

Stages of Mitosis:

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelope reforms, chromosomes decondense.

Example: Diagrams showing chromosome movement during mitosis.

Gene Expression and Mutations

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA → RNA → Protein.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into a polypeptide (protein).

Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Types of Mutations

Mutations are changes in the DNA sequence that can affect gene function.

• Point Mutation: Change in a single nucleotide.

• Insertion/Deletion: Addition or loss of nucleotides, may cause frameshift.

• Silent Mutation: Does not change the amino acid sequence.

• Missense Mutation: Changes one amino acid.

• Nonsense Mutation: Creates a premature stop codon.

Example: Questions about the effect of mutations on protein synthesis.

Genetic Engineering and Plasmids

Plasmids and Transformation

Plasmids are small, circular DNA molecules found in bacteria, often used in genetic engineering.

• Transformation: Uptake of foreign DNA by a cell.

• Selectable Markers: Genes (e.g., antibiotic resistance) used to identify transformed cells.

• Restriction Enzymes: Cut DNA at specific sequences, allowing insertion of genes into plasmids.

Example: Diagrams and questions about plasmid use in E. coli and gene cloning.

Tables and Diagrams

Comparison of DNA Replication Features

The following table summarizes key differences between leading and lagging strand synthesis:

Feature	Leading Strand	Lagging Strand
Synthesis Direction	Continuous (5' to 3')	Discontinuous (Okazaki fragments)
Primer Requirement	One primer	Multiple primers
DNA Ligase	Not required	Required to join fragments

Stages of Mitosis

Stage	Main Event
Prophase	Chromosomes condense, spindle forms
Metaphase	Chromosomes align at equator
Anaphase	Sister chromatids separate
Telophase	Nuclear envelope reforms

Additional info:

• Some questions referenced the use of restriction enzymes and plasmids in genetic engineering, which is covered in Ch. 20.

• Diagrams and questions about chromosome movement and cell division relate to Ch. 12 (The Cell Cycle) and Ch. 13 (Meiosis).

• Questions about DNA replication, Okazaki fragments, and enzyme function are relevant to Ch. 15 (DNA and the Gene: Synthesis and Repair).