Module 2: Cell Structure and Function (Part 1)

Topics Overview

This module covers the fundamental processes of DNA synthesis and the cell cycle, which are essential for understanding cell structure and function in human anatomy and physiology.

• DNA Synthesis

• Cell Cycle

DNA Synthesis

Chromosome Structure and Genome Organization

Each chromosome consists of a single DNA molecule and several histone proteins that help package the DNA. The complete set of DNA in an organism's cell is called its genome.

• Chromosome: A structure composed of DNA and proteins (mainly histones) found in the nucleus of most cells (except red blood cells and sex cells).

• Genome: The total genetic material (DNA) present in a cell.

• Gene: A segment of DNA that contains instructions for the synthesis of a specific protein or RNA molecule.

• In humans, less than 2% of the genome codes for proteins; the rest has regulatory or unknown functions.

Genes and Genetic Information

Genes are distinct units of information within DNA. Each gene provides the instructions for making a specific protein or RNA molecule.

• Gene: A sequence of DNA that encodes a functional product (protein or RNA).

• Genes are the basic units of heredity and are responsible for inherited traits.

Overview of Protein Synthesis

Protein synthesis is the process by which cells produce proteins, essential for cell structure and function. This process occurs in three main steps:

1. Transcription: The process of copying genetic information from DNA to messenger RNA (mRNA).

2. Translation: The process by which ribosomes read the mRNA sequence and assemble the corresponding amino acids into a polypeptide chain.

3. Protein Folding: The polypeptide chain folds into its functional three-dimensional structure, often with the help of the Golgi complex.

Summary Diagram: DNA → mRNA (transcription) → Polypeptide (translation) → Protein (folding)

DNA Structure

DNA (deoxyribonucleic acid) is a double-stranded nucleic acid composed of long chains of nucleotides. Each nucleotide consists of three components:

• Phosphate group

• Deoxyribose sugar

• Nitrogenous base

The four types of nitrogenous bases in DNA are:

• Adenine (A)

• Thymine (T)

• Cytosine (C)

• Guanine (G)

DNA strands are held together by hydrogen bonds between complementary nitrogenous bases:

• Adenine (A) pairs with Thymine (T)

• Cytosine (C) pairs with Guanine (G)

This is known as the rules of complementarity.

Comparison of DNA and RNA

Both DNA and RNA are nucleic acids, but they differ in structure and function.

Types of RNA in Protein Synthesis

Protein synthesis requires three main types of RNA:

• Messenger RNA (mRNA): Encodes the amino acid sequence of a protein; acts as a "disposable copy" of DNA.

• Ribosomal RNA (rRNA): Forms the primary structure of ribosomes, which catalyze translation.

• Transfer RNA (tRNA): Delivers amino acids to the ribosome to construct the polypeptide chain.

The Genetic Code

The sequence of bases in mRNA is read in groups of three, called codons. Each codon specifies a particular amino acid.

• Codon: A three-base sequence in mRNA that codes for a specific amino acid.

• Anticodon: A three-base sequence in tRNA that is complementary to an mRNA codon.

• The genetic code is universal (shared by almost all organisms).

Transcription

Transcription is the process by which genetic information is copied from DNA to mRNA. This produces a single-stranded mRNA molecule that serves as a template for protein synthesis.

• Occurs in the nucleus.

• Uses base pairing rules (A pairs with U in RNA, C with G).

• Produces a "disposable copy" of the gene.

Translation

Translation is the process by which ribosomes use the sequence of codons in mRNA to assemble amino acids into a polypeptide chain.

• Occurs in the cytoplasm at the ribosome.

• tRNA molecules bring specific amino acids to the ribosome, matching their anticodon to the mRNA codon.

• The polypeptide chain is synthesized and then folded into a functional protein.

Protein Modification

After translation, the polypeptide chain undergoes folding and modification, often in the Golgi complex, to become a functional protein.

• Folding is essential for protein function.

• Additional modifications (e.g., addition of carbohydrate groups) may occur.

The Cell Cycle

Overview of the Cell Cycle

The cell cycle includes all events that occur during a cell's lifetime, from its formation to its division into daughter cells.

• Some cells never divide (e.g., most neurons).

• Most cells go through a period of growth and division to produce new cells.

• The cell spends most of its life in interphase (growth and DNA replication).

Phases of the Cell Cycle

Key Events in Mitosis

• Prophase: Chromosomes condense and become visible; nuclear envelope breaks down.

• Metaphase: Chromosomes line up at the center of the cell.

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

• Telophase: Nuclear envelopes reform around the two sets of chromosomes.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Note: The cell cycle ensures that each daughter cell receives an identical set of genetic material.

Summary Table: DNA vs. RNA

Key Equations and Concepts

• Base Pairing Rules:

• Central Dogma of Molecular Biology:

Example: If a DNA sequence is ATCG, the complementary mRNA sequence (after transcription) would be UAGC.

Additional info: The cell cycle and protein synthesis are tightly regulated processes, and errors can lead to diseases such as cancer or genetic disorders.