Key Concepts in DNA, Genetics, and Cell Division

DNA (Deoxyribonucleic Acid)

A double-stranded molecule that carries genetic instructions for the development and functioning of living organisms.

RNA (Ribonucleic Acid)

A single-stranded molecule that helps in protein synthesis and gene expression.

Nucleotide

The basic building block of DNA and RNA, consisting of a sugar, phosphate group, and nitrogenous base.

Nitrogenous Base

A component of nucleotides; includes adenine (A), thymine (T), cytosine (C), guanine (G) in DNA, and uracil (U) in RNA.

Ribose

The sugar found in RNA nucleotides.

Deoxyribose

The sugar found in DNA nucleotides, lacking one oxygen atom compared to ribose.

Thymine (T)

A nitrogenous base in DNA that pairs with adenine (A).

Cytosine (C)

A nitrogenous base that pairs with guanine (G).

Adenine (A)

A nitrogenous base that pairs with thymine (T) in DNA and uracil (U) in RNA.

Guanine (G)

A nitrogenous base that pairs with cytosine (C).

DNA Replication

The process by which DNA makes an identical copy of itself before cell division.

Base Pairing

The specific hydrogen bonding between adenine-thymine (A-T) and cytosine-guanine (C-G) in DNA.

DNA Polymerase

An enzyme that synthesizes new DNA strands by adding complementary nucleotides during replication.

Chromosome

A structure made of DNA and proteins that contains genetic information.

Sister Chromatids

Identical copies of a chromosome connected by a centromere.

Homologous Chromosomes

A pair of chromosomes, one from each parent, that carry the same genes.

Cell Cycle

The series of events in a cell's life, including growth, DNA replication, and division.

Interphase

The longest phase of the cell cycle, where the cell grows and replicates DNA.

Mitosis

A type of cell division that produces two genetically identical daughter cells.

Meiosis

A type of cell division that produces four genetically different gametes (sperm or eggs).

Cytokinesis

The division of the cytoplasm, resulting in two separate daughter cells.

Diploid (2n)

A cell that contains two sets of chromosomes (one from each parent).

Haploid (n)

A cell that contains only one set of chromosomes (e.g., gametes).

Gamete

A reproductive cell (sperm or egg) that carries half the number of chromosomes.

Crossing Over

The exchange of genetic material between homologous chromosomes during meiosis, increasing genetic variation.

Genetics

The study of heredity and the variation of inherited traits.

Genes

Segments of DNA that code for proteins and determine traits.

Traits

Characteristics inherited from parents, such as eye color or height.

Genotype

The genetic makeup of an organism (e.g., AA, Aa, or aa).

Phenotype

The physical expression of a trait (e.g., blue eyes, brown hair).

Allele

A version of a gene (e.g., dominant or recessive).

Dominant Allele

An allele that expresses its trait even if only one copy is present (e.g., A).

Recessive Allele

An allele that is only expressed when two copies are present (e.g., aa).

Homozygous

Having two identical alleles for a trait (AA or aa).

Heterozygous

Having two different alleles for a trait (Aa).

Monohybrid Cross

A genetic cross analyzing one trait.

Dihybrid Cross

A genetic cross analyzing two traits simultaneously.

Punnett Square

A tool used to predict genetic outcomes of a cross.

Incomplete Dominance

A form of inheritance where the heterozygous phenotype is a blend of both alleles (e.g., red + white = pink flowers).

Codominance

A form of inheritance where both alleles are fully expressed (e.g., blood type AB).

Polygenic Inheritance

A trait controlled by multiple genes, leading to a wide range of phenotypes (e.g., skin color, height).

Linked Genes

Genes located close together on the same chromosome that tend to be inherited together.

Sex Chromosomes

Chromosomes that determine biological sex (XX = female, XY = male).

Sex-Linked Genes

Genes located on the sex chromosomes, often affecting males more due to the single X chromosome (e.g., hemophilia, color blindness).

What is the overall shape of a DNA molecule?

A double helix, with two strands twisted around each other like a spiral staircase.

What is a nucleotide, and what are its components?

A nucleotide is the building block of DNA and RNA. It consists of a phosphate group, a sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base

What are the four nucleotide bases in DNA, and how do they pair?

Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G).

What is the structure of the sugar-phosphate backbone?

It consists of alternating sugar and phosphate groups, forming the outer “rails” of the DNA double helix.

How does RNA differ from DNA?

RNA is single-stranded, contains ribose sugar instead of deoxyribose, and uses uracil (U) instead of thymine (T).

What is the central dogma of molecular biology?

It describes the flow of genetic information: DNA → RNA → Protein.

What is DNA replication?

The process by which DNA makes an identical copy of itself before cell division.

How does DNA replication work?

DNA polymerase helps unzip the double helix and adds complementary nucleotides to form two identical strands.

Why does DNA replication occur on both strands and in both directions?

This makes replication faster and more efficient.

Why is the fidelity (accuracy) of DNA replication important?

Errors in replication can lead to mutations, which may cause genetic diseases or cancer.

What does "semi-conservative" DNA replication mean?

Each new DNA molecule consists of one original strand and one newly synthesized strand.

What happens in the G1, S, and G2 phases of the cell cycle?

• G1: Cell grows and performs normal functions.

• S: DNA replication occurs.

• G2: Cell prepares for division.

What types of cells undergo mitosis and meiosis?

• Mitosis: Somatic (body) cells

• Meiosis: Gametes (sperm and egg cells

What are the stages of mitosis?

Prophase, Metaphase, Anaphase, Telophase (PMAT), followed by cytokinesis.

What happens in each stage of mitosis?

• Prophase: Chromosomes condense, spindle fibers form.

• Metaphase: Chromosomes line up at the center.

• Anaphase: Sister chromatids separate.

• Telophase: Nuclear membranes form around two sets of chromosomes.

What are the stages of meiosis?

Meiosis has two rounds:

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate (like mitosis).

How many cells are produced in mitosis vs. meiosis, and are they identical?

• Mitosis: 2 identical diploid cells.

• Meiosis: 4 genetically different haploid cells.

How does mitosis differ from meiosis?

• Mitosis: Produces 2 identical diploid cells.

• Meiosis: Produces 4 genetically unique haploid gametes.

What are the three ways genetic variation occurs in offspring?

• Crossing over (Meiosis I).

• Independent assortment of chromosomes.

• Random fertilization.

What is a gene?

A segment of DNA that codes for a specific protein or trait.

What is an allele?

A version of a gene (e.g., dominant or recessive).

What is the difference between dominant and recessive traits?

• Dominant: Only one copy needed to express the trait.

• Recessive: Two copies needed to express the trait.

What is the difference between genotype and phenotype?

• Genotype: The genetic makeup (AA, Aa, or aa).

• Phenotype: The physical expression of a trait.

How is a monohybrid cross used in genetics?

It shows the inheritance of a single trait using a Punnett square.

What is a dihybrid cross?

A cross between individuals for two different traits (e.g., AaBb × AaBb).

What is Mendel’s Law of Segregation?

Each individual has two alleles for a trait, and these alleles separate during gamete formation.

What is Mendel’s Law of Independent Assortment?

Genes for different traits assort independently of each other during meiosis.

What is incomplete dominance?

A blending of traits (e.g., red + white = pink flowers).

What is codominance?

Both alleles are fully expressed (e.g., AB blood type).

What is polygenic inheritance?

A trait controlled by multiple genes, such as height or skin color.

What are linked genes?

Genes located close together on the same chromosome that tend to be inherited together.

How do chromosomes determine gender?

The presence of XX results in a female, and XY results in a male.

What is sex-linked inheritance?

Traits controlled by genes on the X chromosome (e.g., color blindness, hemophilia).