DNA Replication and Protein Synthesis: Structure and Function in Human Cells

Chapter 3: DNA Replication and Protein Synthesis

Overview

This chapter explores two fundamental cellular processes: DNA replication and protein synthesis. These processes are essential for cell division and the production of specific proteins, which are critical for cellular function and organismal development.

The Cell Nucleus and DNA

Structure and Function of DNA

The nucleus is the control center of the cell, housing the genetic material in the form of DNA (deoxyribonucleic acid). DNA is organized into chromosomes and contains the instructions for all cellular activities.

• DNA Structure: DNA is a double helical molecule composed of two strands held together by weak hydrogen bonds between complementary nitrogenous bases.

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

• Nucleosome: DNA wraps around histone proteins to form nucleosomes, which further coil to create chromatin and chromosomes.

Example: The molecular structure of DNA includes a sugar-phosphate backbone and nitrogenous bases forming the rungs of the helical ladder.

DNA Replication

Process and Enzymes Involved

DNA replication occurs during the S phase of the cell cycle, ensuring that each daughter cell receives an identical copy of genetic material.

• Initiation: DNA helicase unwinds the double helix, exposing the complementary strands.

• Template Mechanism: Each strand serves as a template for the synthesis of a new complementary strand.

• RNA Primer: A short RNA primer is synthesized to provide a starting point for DNA synthesis.

• DNA Polymerase III: This enzyme adds nucleotides to the growing DNA strand, replacing the RNA primer with DNA.

• Leading and Lagging Strands: DNA polymerase works continuously on the leading strand and discontinuously on the lagging strand, forming Okazaki fragments.

• DNA Ligase: Joins the short segments of the lagging strand to create a continuous DNA molecule.

• Chromatids: After replication, two identical chromatids are joined at the centromere, marking the end of the S phase.

Equation:

$\text{DNA}_{\text{parent}} \xrightarrow{\text{Replication}} \text{DNA}_{\text{daughter}}$

Example: During cell division, each new cell receives a complete set of chromosomes due to accurate DNA replication.

Protein Synthesis

Genetic Code and Gene Expression

Protein synthesis is the process by which cells build proteins based on instructions encoded in DNA. Genes are segments of DNA that specify the sequence of amino acids in a polypeptide chain.

• Triplet Code: Each set of three nucleotide bases (codon) in DNA codes for a specific amino acid.

• Genetic Library: The collection of all possible codons and their corresponding amino acids.

Example: The codon 'AUG' codes for the amino acid methionine, which is often the start signal for protein synthesis.

From DNA to Protein

Transcription

Transcription is the process by which the genetic information in DNA is copied into messenger RNA (mRNA).

• RNA Polymerase: The enzyme that synthesizes RNA from the DNA template.

• Sense Strand: The DNA strand that serves as the template for RNA synthesis.

• Termination Signal: A sequence in DNA that signals the end of transcription.

Equation:

$\text{DNA} \xrightarrow{\text{Transcription}} \text{mRNA}$

Example: The gene for hemoglobin is transcribed into mRNA, which then directs the synthesis of the hemoglobin protein.

Translation

Translation is the process by which the sequence of bases in mRNA is converted into a sequence of amino acids, forming a protein.

• mRNA: Carries genetic information from the nucleus to ribosomes in the cytoplasm.

• tRNA: Transfers specific amino acids to the ribosome, matching its anticodon to the mRNA codon.

• rRNA: Forms the structural and functional core of ribosomes.

• Genetic Code: The set of rules by which the sequence of bases in mRNA is translated into amino acids.

• Codon-Anticodon Pairing: Each tRNA has an anticodon that pairs with the corresponding mRNA codon.

Equation:

$\text{mRNA} \xrightarrow{\text{Translation}} \text{Protein}$

Example: The mRNA sequence 'UUU' codes for the amino acid phenylalanine.

Roles of the Three Types of RNA

Type of RNA	Main Function
mRNA (Messenger RNA)	Carries genetic information from DNA to ribosomes for protein synthesis
tRNA (Transfer RNA)	Brings amino acids to the ribosome and matches them to the mRNA codon
rRNA (Ribosomal RNA)	Forms the core structure of ribosomes and catalyzes peptide bond formation

The Genetic Code

The genetic code consists of codons, which are sequences of three RNA bases that specify particular amino acids. This code is universal among almost all organisms.

Codon (mRNA)	Amino Acid
AUG	Methionine (Start)
UUU	Phenylalanine
UAA, UAG, UGA	Stop codons
Additional info: ...	Each amino acid may be coded by more than one codon

Summary

• DNA replication ensures genetic continuity during cell division.

• Protein synthesis translates genetic information into functional proteins.

• Three types of RNA play distinct roles in the process of gene expression.

• The genetic code is the basis for translating nucleotide sequences into amino acids.