Biology: DNA Structure, Function and Replication

• Module 5 of a biology course typically covers genetics, heredity, and DNA.

DNA Structure and Function

• DNA (deoxyribonucleic acid) carries genetic information.
• It is a double helix composed of nucleotides.
• Nucleotides consist of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
• The four nitrogenous bases are adenine (A), guanine (G), cytosine (C), and thymine (T).
• A pairs with T, and C pairs with G, through hydrogen bonds (Chargaff's rules).
• DNA's primary function is to store and transmit genetic information.

DNA Replication

• DNA replication is the process of copying DNA.
• It is semiconservative, meaning each new DNA molecule consists of one original strand and one newly synthesized strand.
• Replication begins at specific sites called origins of replication.
• DNA polymerase is the main enzyme involved, adding nucleotides to the 3' end of the growing strand.
• Leading strand is synthesized continuously
• Lagging strand is synthesized in fragments (Okazaki fragments).
• DNA ligase joins the Okazaki fragments.

Transcription

• Transcription is the process of synthesizing RNA from a DNA template.
• RNA polymerase binds to the promoter region of a gene.
• RNA polymerase moves along the DNA, unwinding the double helix and synthesizing a complementary RNA molecule.
• In eukaryotes, the initial RNA transcript (pre-mRNA) undergoes processing.
• Splicing removes introns (non-coding regions).
• A 5' cap and a 3' poly-A tail are added.

Translation

• Translation is the process of synthesizing a polypeptide from an mRNA template.
• Ribosomes are the site of translation.
• mRNA codons (three-nucleotide sequences) specify particular amino acids.
• tRNA molecules bring the correct amino acids to the ribosome, matching their anticodons to the mRNA codons.
• Translation occurs in three stages: initiation, elongation, and termination.

Genetic Code

• The genetic code is the set of rules by which information encoded in genetic material (DNA or RNA) is translated into proteins (amino acid sequences) by living cells.
• It is a triplet code, where three nucleotides (a codon) specify an amino acid.
• The code is degenerate, meaning that more than one codon can specify the same amino acid.
• There are start (AUG) and stop codons (UAA, UAG, UGA).

Gene Expression

• Gene expression is the process by which the information encoded in a gene is used to synthesize a functional gene product (protein or RNA).
• It involves transcription and translation.
• Gene expression can be regulated at various levels.
• Transcription factors can enhance or repress transcription.
• RNA processing, translation, and post-translational modifications can also be regulated.

Mutations

• Mutations are changes in the DNA sequence.
• They can be spontaneous or induced by mutagens.
• Point mutations involve changes in a single nucleotide.
• Substitutions, insertions, or deletions.
• Frameshift mutations result from insertions or deletions that alter the reading frame.
• Mutations can be harmful, beneficial, or neutral.

Mendelian Genetics

• Gregor Mendel is considered the father of genetics.
• He studied inheritance in pea plants.
• Mendel's laws of inheritance include:
• Law of segregation: each individual has two alleles for each trait, and these alleles separate during gamete formation.
• Law of independent assortment: alleles of different genes assort independently of one another during gamete formation.
• Genotype is the genetic makeup of an organism.
• Phenotype is the observable characteristics of an organism.
• Homozygous means having two identical alleles for a gene.
• Heterozygous means having two different alleles for a gene.
• Dominant alleles mask the expression of recessive alleles.

Inheritance Patterns

• Autosomal inheritance involves genes located on autosomes (non-sex chromosomes).
• Sex-linked inheritance involves genes located on sex chromosomes (X and Y chromosomes).
• X-linked recessive traits are more common in males.
• Incomplete dominance is when the heterozygote phenotype is intermediate between the two homozygote phenotypes.
• Codominance is when both alleles are expressed in the heterozygote.
• Polygenic inheritance involves multiple genes affecting a single trait.
• Pleiotropy is when a single gene affects multiple traits.

Chromosomal Inheritance

• Chromosome number and structure affect inheritance patterns
• Alterations in chromosome number can be inherited, e.g. Down Syndrome
• Nondisjunction is the failure of chromosomes to separate properly during meiosis.
• Aneuploidy results from nondisjunction
• One or more chromosomes are extra or missing.
• Polyploidy involves having more than two sets of chromosomes.
• Chromosomal mutations include deletions, duplications, inversions, and translocations.

Genetic Variation

• Genetic variation is essential for evolution.
• It arises through mutation, gene flow, and sexual reproduction.
• Gene flow is the movement of genes between populations.
• Sexual reproduction introduces new combinations of alleles through crossing over, independent assortment, and random fertilization.

Biotechnology

• Biotechnology involves the use of biological systems to develop products.
• DNA technology includes techniques such as:
• DNA cloning: producing multiple copies of a DNA segment.
• Polymerase chain reaction (PCR): amplifying DNA sequences.
• Gel electrophoresis: separating DNA fragments by size.
• DNA sequencing: determining the nucleotide sequence of DNA.
• Genetic engineering involves modifying an organism's genes.
• Applications of biotechnology include:
• Medicine (e.g., gene therapy, drug development).
• Agriculture (e.g., genetically modified crops).
• Forensics (e.g., DNA fingerprinting).

This module covers the structure, function, and replication of DNA. DNA is a double helix composed of nucleotides, which include a deoxyribose sugar, a phosphate group, and nitrogenous bases (A, T, C, G). DNA replication is semiconservative, with DNA polymerase adding nucleotides to the 3' end.