Genetics and Molecular Biology: Chromosomes, DNA Replication, and Gene Expression

Chromosome Structure and Ploidy

Somatic and Sex Cells

Chromosomes are structures within cells that carry genetic information. The number and arrangement of chromosomes differ between somatic (body) cells and sex cells (gametes).

• Somatic cells have a chromosome condition of 2n = 46 chromosomes, organized as 23 homologous pairs.

• Sex cells (gametes) have a chromosome condition of n = 23 chromosomes, with no pairs.

• When gametes (sperm and egg) combine during fertilization, the resulting zygote has 2n = 46 chromosomes.

• Diploid refers to cells with two sets of chromosomes (2n), while haploid refers to cells with one set (n).

Homologous chromosomes are pairs that carry the same genes, possibly in different forms called alleles.

Genetic Variation and Reproduction

Genetic diversity is crucial for adaptability and survival in changing environments.

• Asexually reproducing populations can quickly fix deleterious alleles, reducing genetic diversity.

• Sexually reproducing populations use genetic shuffling mechanisms (such as crossing over and independent assortment) to reduce the frequency of deleterious alleles.

• Genetic diversity allows populations to adapt to changing environments, as seen in agricultural monocrops versus hybrid crops.

• Hybrids tend to be healthier than purebreds due to increased genetic variation.

• Hemophilia is an example of a genetic disease caused by a deleterious allele.

DNA Replication

Overview of DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division. This ensures that each daughter cell receives an identical set of genetic information.

• Replication is semi-conservative: each new DNA molecule contains one original strand and one newly synthesized strand.

• Replication occurs at the replication fork, where the DNA double helix is unwound.

Key Enzymes and Their Functions

• Helicase: Unwinds the DNA double helix at the replication fork.

• Primase: Synthesizes short RNA primers (10-12 bases) to provide a starting point for DNA synthesis.

• DNA Polymerase: Adds new DNA nucleotides to the 3' end of the primer, synthesizing DNA in the 5' to 3' direction.

• Ligase: Joins Okazaki fragments on the lagging strand by forming phosphodiester bonds.

• Single-strand binding proteins: Stabilize unwound DNA and prevent reannealing.

• Topoisomerase: Relieves tension ahead of the replication fork by cutting and rejoining DNA strands.

• Telomerase: Adds repetitive DNA sequences to the ends of chromosomes (telomeres) to prevent shortening during replication (not found in somatic cells).

Leading and Lagging Strands

DNA replication is continuous on one strand (leading) and discontinuous on the other (lagging).

• Leading strand: Synthesized continuously in the 5' to 3' direction.

• Lagging strand: Synthesized discontinuously in short fragments called Okazaki fragments.

• RNA primers are removed and replaced with DNA nucleotides by DNA polymerase I.

• DNA ligase joins the Okazaki fragments to create a continuous strand.

Accuracy and Mutation

• DNA polymerase has 3' to 5' exonuclease activity for proofreading and error correction.

• Error rate is approximately 1 in 106 to 108 bases.

• Mutations can occur if errors are not corrected, potentially leading to genetic diseases.

Central Dogma of Molecular Biology

Information Flow in Gene Expression

The Central Dogma describes the flow of genetic information from DNA to RNA to protein.

• Proposed by Francis Crick.

• Consists of two main processes:

  • Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

  • Translation: mRNA is used as a template to synthesize proteins.

• The primary structure of a protein depends on the sequence of nucleotides in DNA.

• One gene typically codes for one protein (enzyme).

Types of Mutations

Mutations in Replication and Transcription

Mutations are changes in the DNA sequence that can occur during replication or transcription.

• Point mutations: Change a single nucleotide (e.g., substitution, insertion, deletion).

• Frameshift mutations: Insertions or deletions that alter the reading frame of the gene.

• Silent mutations: Do not change the amino acid sequence of the protein.

• Missense mutations: Change one amino acid in the protein sequence.

• Nonsense mutations: Create a premature stop codon, truncating the protein.

Summary Table: Key Enzymes in DNA Replication

Enzyme	Function
Helicase	Unwinds the DNA double helix
Primase	Synthesizes RNA primers
DNA Polymerase	Adds DNA nucleotides to the growing strand
Ligase	Joins Okazaki fragments
Single-strand binding proteins	Stabilize unwound DNA
Topoisomerase	Relieves tension ahead of the fork
Telomerase	Adds telomere repeats

Key Formulas and Equations

• Ploidy: $2n$ (diploid), $n$ (haploid)

• Central Dogma: $\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Example: If a human somatic cell has 46 chromosomes ($2n$), a gamete will have 23 chromosomes ($n$). When two gametes fuse, the zygote will have 46 chromosomes ($2n$).

Additional info: These notes cover introductory genetics and molecular biology, not Precalculus. Topics include chromosome structure, DNA replication, gene expression, and mutation types, which are foundational for biology courses.