Genes and Cellular Function

DNA Structure and Function

Deoxyribonucleic acid (DNA) is the molecule that carries genetic instructions for the development, functioning, growth, and reproduction of all known living organisms. DNA is a polymer made up of nucleotides, each consisting of a phosphate group, a deoxyribose sugar, and a nitrogenous base. The bases pair in a complementary fashion (adenine with thymine, cytosine with guanine), forming a double helix structure.

• Structure: Double helix composed of nucleotides

• Function: Encodes instructions for protein synthesis

• Complementary base pairing: Ensures accurate replication and transcription

Chromatin and Chromosomes

DNA in the cell exists in two main forms: chromatin and chromosomes. Chromatin is a complex of DNA and proteins (mainly histones) that helps package DNA into a compact, organized structure. During cell division, chromatin condenses to form chromosomes, which are visible under a microscope.

• Chromatin: DNA and proteins, loosely packed

• Chromosomes: Tightly wound chromatin, visible during cell division

RNA Structure and Function

Ribonucleic acid (RNA) is a polymer of ribonucleotides. It plays a crucial role in interpreting the genetic code in DNA and synthesizing proteins. There are three main types of RNA involved in protein synthesis:

• mRNA (messenger RNA): Carries genetic information from DNA to ribosomes

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation

• rRNA (ribosomal RNA): Forms the core of the ribosome's structure and catalyzes protein synthesis

What is a Gene?

A gene is a unit of heredity, defined as a segment of DNA that contains the information necessary to produce a functional product, usually a protein. Genes are located on chromosomes and are responsible for the transmission of traits from one generation to the next.

• Gene: Information-containing segment of DNA

• Role: Directs synthesis of one or more proteins

The Human Genome and Genetic Variation

The human genome consists of 46 chromosomes arranged in two sets of 23. While 99.9% of the DNA sequence is identical among all humans, the remaining 0.1% accounts for individual variation. This variation is often due to single nucleotide polymorphisms (SNPs).

• Chromosome number: 46 (23 pairs)

• Genetic similarity: 99.9% identical among humans

• Variation: SNPs contribute to differences in traits

The Genetic Code

The genetic code is the set of rules by which information encoded in DNA is translated into proteins. All proteins are made from 20 amino acids, and the sequence of these amino acids is determined by the sequence of nucleotides in genes. Three DNA nucleotides (a triplet) code for one amino acid; when transcribed into RNA, this triplet is called a codon.

• Codon: Sequence of three nucleotides in mRNA that specifies an amino acid

• Universal code: Used by all living organisms

Transcription

Transcription is the process by which genetic instructions in DNA are copied into messenger RNA (mRNA). This occurs in the nucleus and is facilitated by the enzyme RNA polymerase. The mRNA then migrates to the cytoplasm for translation.

• Enzyme: RNA polymerase

• Location: Nucleus

• Product: mRNA

Translation

Translation is the process by which the genetic code carried by mRNA is used to synthesize proteins. Ribosomes bind to mRNA and use tRNAs to assemble amino acids in the correct sequence, forming a polypeptide chain.

• Ribosome: Site of protein synthesis

• tRNA: Brings amino acids to ribosome

• Speed: One ribosome can add 2-6 amino acids per second

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA is transcribed into RNA, and RNA is translated into protein. This process is fundamental to all cellular functions.

• DNA → RNA → Protein

Gene Regulation

Gene regulation refers to the mechanisms that control when and how genes are expressed. Genes can be turned on or off in response to environmental signals, developmental cues, or cellular needs. Some genes are permanently silenced in certain cell types.

• Activator: Turns genes on

• Repressor: Turns genes off

Genes and Traits

Traits are determined by the specific sequence of amino acids in proteins, which are encoded by genes. Variations in gene sequences lead to differences in traits such as eye color, hair color, height, lactose tolerance, and alcohol metabolism.

• Trait examples: Eye color, hair color, height, lactose tolerance, alcohol metabolism

Genes and Mutations

Mutations are changes in the DNA sequence that can affect gene function and lead to altered traits or diseases. Two examples are sickle cell anemia and cystic fibrosis:

• Sickle cell anemia: Single amino acid substitution in hemoglobin causes red blood cells to become sickle-shaped

• Cystic fibrosis: Deletion of three nucleotides in the CFTR gene leads to thick, sticky mucus in organs

Causes of Mutations

Mutations can occur spontaneously due to errors in DNA replication or be induced by environmental factors such as chemicals, radiation, or UV light.

• Spontaneous mutations: Replication errors

• Induced mutations: Chemicals, radiation, UV light

Epigenetics

Epigenetics involves changes in gene expression that do not alter the underlying DNA sequence. Environmental factors and behaviors can modulate gene expression, and these changes can be inherited. DNA is wrapped around histones, and chemical modifications such as methylation and acetylation affect whether genes are accessible for transcription.

• Methylation: DNA is tightly coiled and inaccessible, gene is inactive

• Acetylation: DNA unwinds and is accessible, gene is active

• Environmental factors: Diet, stress, exposure to toxins, exercise

Epigenetics: Dutch Hunger Winter Example

The Dutch Hunger Winter (1944-1945) is a classic example of epigenetic effects. Severe malnutrition experienced by pregnant women led to increased health risks in their children, such as heart disease, diabetes, and obesity. These risks were also passed on to subsequent generations, demonstrating the heritability of epigenetic changes.

• Epigenetic inheritance: Health risks passed to children and grandchildren

Additional info:

• Epigenetic changes can be reversible and are a focus of current research in disease prevention and treatment.

• Gene regulation is essential for cell differentiation, allowing cells to perform specialized functions.