Science Unit A Section 3

DNA Structure

• DNA (deoxyribonucleic acid) is a molecule that carries the genetic instructions for life.

• Structure:

  • Double helix: Two strands twisted around each other.

  • Comprised of nucleotides: Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and a nitrogenous base.

  • Four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

Genetic Code and Genes

• Genetic code: The sequence of nitrogenous bases that determines the traits of an organism.

• Genes: Segments of DNA that code for proteins and influence traits.

Nucleotides and Nitrogenous Bases

• Nucleotides: The building blocks of DNA.

• Nitrogenous bases pair specifically:

  • Adenine pairs with Thymine (A-T)

  • Cytosine pairs with Guanine (C-G)

Mutations

• Changes in the DNA sequence that may result in altered traits. They can be caused by environmental factors or occur randomly.

• Types:

  • Point mutations (substitution of a single nucleotide)

  • Frameshift mutations (insertion or deletion of nucleotides)

Chromosomes

• Structures within cells that contain genes.

• Humans have 23 pairs of chromosomes (46 total).

• Chromosomal abnormalities can lead to disorders.

Mitosis and Meiosis

• Mitosis: Process of cell division resulting in two identical daughter cells; occurs in somatic (body) cells.

• Meiosis: Process of cell division that reduces the chromosome number by half to produce gametes (sperm and eggs).

Gametes

• Reproductive cells produced during meiosis; contain half the number of chromosomes, essential for sexual reproduction.

Trisomy Disorders

• Trisomy: A condition where an individual has an extra chromosome.

  • Down syndrome (Trisomy 21): Caused by having three copies of chromosome 21, leading to developmental delays and characteristic physical features.

  • Klinefelter syndrome (XXY): Males have an extra X chromosome, leading to physical and hormonal changes.

Dominant and Recessive Traits

• Dominant traits: Traits that will appear in the offspring if one of the parents contributes them. Represented by a capital letter (e.g., A).

• Recessive traits: Traits that will only appear if both parents contribute them. Represented by a lowercase letter (e.g., a).

Codominance and Incomplete Dominance

• Codominance: Both alleles are expressed equally in the phenotype (e.g., AB blood type).

• Incomplete dominance: A blending of traits occurs (e.g., red and white flowers producing pink flowers).

Hybrid and Purebred

• Hybrid: Offspring resulting from the cross of two different purebred varieties (e.g., Aa).

• Purebred: Offspring from parents of the same genetic type (homozygous, e.g., AA or aa).