ch_06_Lecture_Presentation_DNA

Chapter Overview

Title: DNA: The Molecule of Life

Edition: Biology: The Core, Third Edition

Copyright: Pearson Education, Inc. (2020, 2017, 2015)

6.1 DNA: The Genetic Blueprint

Definition of DNA: Deoxyribonucleic Acid - a molecule composed of two strands that coil around each other to form a double helix. DNA carries genetic information vital for the growth, development, and reproduction of all living organisms.Structure: Consists of two long strands made up of nucleotides. Each strand has a sugar-phosphate backbone with nitrogenous bases attached, which provide the means for genetic coding.

Components of a Nucleotide:

• Sugar: Deoxyribose, which distinguishes DNA from RNA.

• Phosphate Group: Links neighboring nucleotides to form the DNA backbone.

• Nitrogen Base: One of four types - Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

DNA History

Early Discoveries:

• Basic rules of inheritance established in mid-1800s by Gregor Mendel, who identified how traits are passed down through generations.

• Identification of chromosomes as carriers of hereditary information marked a significant advancement in genetics.

DNA's Role Identified:

• In the mid-20th century, DNA was recognized as the hereditary material, largely due to the work of scientists such as Avery, MacLeod, and McCarty.

• Experiments by Hershey and Chase solidified the understanding that DNA, not protein, is the genetic material.

Structure Elucidation:

• In 1953, James Watson and Francis Crick proposed the double helix model of DNA based on X-ray diffraction data produced by Rosalind Franklin, whose contributions were critical yet initially underappreciated.

6.1 Nucleotide Structure

Comparison:Nucleotides can be visualized as links on a charm bracelet, where alternating sugars and phosphate groups form the backbone while the nitrogenous bases operate as charms that give unique properties to the DNA strand.

6.1 Double Helix Structure

Formation of DNA:

• The strands of DNA twist around each other like a corkscrew, creating a helical shape that is essential for its stability and function.

• Hydrogen bonds between complementary nitrogenous base pairs (A-T and G-C) secure the two strands together, allowing for both stability and the flexibility required for replication and transcription.

6.1 Base Pairing Rules

Key Base Pairings:

• Adenine (A) pairs with Thymine (T) through two hydrogen bonds.

• Guanine (G) pairs with Cytosine (C) through three hydrogen bonds, making this pairing stronger.

• The complementarity creates a base sequence critical for replication and transcription processes.

6.2 DNA Replication

Process:

• DNA replicates prior to cell division, ensuring that each new cell receives a complete set of genetic material.

• The replication process begins at specific locations known as origins of replication, where the double helix unwinds, and each strand serves as a template for the synthesis of a new complementary strand, guided by the base pairing rules.

6.2 Genome Size Variation

DNA Content Across Species:

• An example of the diversity in genomic content can be seen with onions, which possess a greater amount of DNA than humans despite being simpler organisms.

• Gene count varies significantly among species, and this variability raises questions about non-coding regions of DNA and their potential roles.

6.11 Coding and Non-Coding DNA

Coding DNA:

• Provides the necessary instructions to assemble proteins, which perform a variety of functions within an organism.

• Coding regions are characterized by sequences of nucleotides that are transcribed into mRNA and later translated into proteins.

Non-Coding DNA:

• Consists of regions that do not code for proteins but play regulatory roles, such as enhancer and silencer sequences that influence gene expression.

• Non-coding DNA accounts for a substantial portion of the genome, leading to ongoing research regarding their functions and significance in evolutionary biology.

6.3 Role of RNA

RNA Overview:

• Ribonucleic Acid (RNA) serves as the intermediate molecule between DNA and proteins, facilitating the flow of genetic information in a process termed gene expression.

• RNA's structure differs from DNA in several critical ways:

  • It is single-stranded, allowing for a greater variety of shapes and functions.

  • It contains ribose sugar rather than deoxyribose.

  • The base uracil (U) replaces thymine (T), further distinguishing RNA.

Protein Synthesis: Transcription and Translation

Transcription:

• It involves copying a specific gene’s nucleotide sequence from DNA into messenger RNA (mRNA). This occurs within the cell nucleus in eukaryotic cells.

Translation:

• mRNA moves to the ribosome where it directs the assembly of amino acids into proteins, the final product of gene expression, heavily influencing the phenotype traits of the organism.

6.4 Gene Expression Regulation

Gene Regulation:

• Critical for cellular function, allowing cells to respond to internal and external stimuli by turning genes on or off based on necessity, exemplified by lactase expression in certain populations as a response to dairy consumption.

6.6 Genetic Code and Mutation Impact

Genetic Code:

• The genetic code is composed of triplet codons (three-nucleotide sequences), each corresponding to a specific amino acid. For example, the codon ACU corresponds to the amino acid Threonine.

Mutations:

• Changes in the DNA sequence can lead to mutations, which may affect growth, development, and cell function. Certain mutations within regulatory genes can lead to conditions such as cancer due to hypercellular growth.

6.13 Genetic Manipulation Techniques

Selective Breeding:

• An age-old technique that involves choosing specific plants or animals for reproduction to enhance desirable traits, such as the development of modern corn from ancestral wild grasses.

Genetic Engineering:

• Involves direct manipulation of an organism's genes, including techniques like CRISPR-Cas9, facilitating advancements such as the potential growth of organs for transplant. This represents a significant ethical consideration in modern biotechnology.

6.15 Genetically Modified Organisms (GMOs)

Definition:Organisms whose genetic material has been altered through genetic engineering to exhibit traits not naturally theirs.Usage:GMOs are prevalent in agriculture, with crops like corn and soybeans engineered for pest resistance and increased yield.

Transgenic Organisms:

• These organisms contain genes from other species, exemplified by goats engineered to produce human enzymes, showcasing the potential for cross-species gene applications.

Biotechnology and Stem Cells

Pluripotent Cells:

• Stem cells that have the capability to differentiate into any cell type, offering immense potential for regenerative medicine. Sources include embryonic stem cells and induced pluripotent stem cells derived from adult tissues.

Potential Applications:

• Addressing conditions such as spinal cord injuries, diabetes, and neurodegenerative diseases, revolutionizing future therapeutic strategies.

6.13 DNA Cloning Techniques

PCR (Polymerase Chain Reaction):

• A technique that enables the amplification of DNA fragments, facilitating various applications from genetic testing to forensic analysis.

Restriction Enzymes:

• Enzymes that cut DNA at specific sequences, enabling scientists to analyze and manipulate DNA fragments effectively, vital for cloning and genetic engineering applications.

6.14 Intellectual Property in Genetics

Case Study:

• BRCA gene patents associated with breast cancer risk, reflecting complex legal and ethical challenges around gene ownership. The U.S. Supreme Court ruling on gene patents has opened new avenues for genetics research and testing.

6.17 DNA Profiling

STRs (Short Tandem Repeats):

• DNA profiling analyzes repeated sequences in DNA (STRs) for forensic applications and personal identification.

Electrophoresis:

• This technique separates DNA samples based on size, enabling comparison and identification through visualizing band patterns, critical in criminal investigations and paternity cases.

Genomics and Proteomics

Genomics:

• The comprehensive study of all genes within an organism's genome, with the human genome containing approximately 21,000 genes, provides insights into genetic diseases and therapeutic targets.

Proteomics:

• The study of the entire complement of proteins expressed by a genome, crucial for understanding biological functions and cellular processes, providing a more profound insight into how genes control phenotypic variations.