Cell Division and DNA Structure Quiz

Cell Division: Mitosis

• Mitosis is a process where a single cell divides into two identical daughter cells
• The genetic material (DNA) is replicated before mitosis begins
• DNA is Deoxyribonucleic acid
• Eukaryotic DNA is composed of multiple molecules of double-stranded linear DNA, while prokaryotic DNA is double-stranded and circular

DNA Function

• DNA carries and stores genetic information passed from one generation to the next through chromosomes
• DNA is a large molecule composed of nucleotides

DNA Structure

• DNA is structured as a large number of nucleotides arranged in a single strand
• Nucleotides consist of a sugar (5 carbon sugar), a phosphate group, and nitrogen bases (purine or pyrimidine)

Nitrogen Bases

• Purines: Adenine (A) and Guanine (G), with double-cycle structures
• Pyrimidines: Thymine (T) and Cytosine (C), with single-cycle structures
• Two strands coil around each other to form a double helix
• The two strands are held together by hydrogen bonds between complementary base pairs (A with T, and G with C)

Chromosome Number

• The number of chromosomes exists as pairs except in germ cells (sex cells)
• Somatic cells (body cells) contain two sets of chromosomes, also called diploid (2n; 46 in humans)
• Reproductive cells (gametes; sperm and ova) contain a single set of chromosomes, or haploid (n; 23 in humans)
• Chromatin consists of a single, long DNA double helix, plus histone and nonhistone proteins (chromosomal proteins) scattered in the nucleus

Cell Division Purposes

• Cell growth
• Reproduction
• Tissue repair

Mitosis Phases

• Interphase: The period between cell divisions, consisting of G1, S (DNA synthesis and replication), and G2 sub-phases. Time varies
• Prophase: Replicated chromosomes condense, nuclear envelope breaks down, nucleolus disappears, mitotic apparatus (spindle) forms
• Metaphase: Replicated chromosomes align at the cell's equator (middle) attached to the microtubules of the mitotic spindle
• Anaphase: Duplicated chromatids separate at the centromere, and move to opposite poles of the mitotic spindle
• Telophase: Chromosomes reach the poles of the mitotic spindle, the nuclear envelope reforms, and the cell completes division
• Cytokinesis: Cytoplasm divides, creating two new diploid cells, following telophase. In animal cells, cytokinesis begins during early anaphase.

Meiosis

• Meiosis is a specialized cell division occurring in sex organs to produce gametes (sex cells; e.g., oocyte and spermatocyte).
• Meiosis reduces the chromosome number from diploid (2n) to haploid(n)
• Meiosis has two steps:
• Reductional division (Meiosis I): Homologous chromosomes separate, producing two haploid daughter cells
• Equational division (Meiosis II): Sister chromatids separate, producing four haploid daughter cells

Meiosis I: Prophase I

• Prophase I is the longest stage taking a day to complete
• The nucleolus and nuclear membrane disappear
• Chromatin material replicates to form chromosomes
• Synapsis and tetrad formation: Homologous chromosome pairs come together and may exchange portions of chromatids called crossing over

Meiosis I: Metaphase I

• All tetrad chromosomes line up at the equator of spindles

Meiosis I: Anaphase I

• Homologous chromosomes move to the opposite poles.
• Sister chromatids do not separate

Meiosis I: Telophase I

• Chromosomes reach the poles
• Nuclear envelope reforms
• Cytoplasm divides
• Four haploid daughter cells are produced

Meiosis II: Prophase II

• DNA does not replicate
• A new spindle forms in the two cells

Meiosis II: Metaphase II

• Chromosomes align along the equator of the spindles

Meiosis II: Anaphase II

• Sister chromatids separate and move apart to opposite poles

Meiosis II: Telophase II

• Nuclear envelopes form
• Cytokinesis occurs
• Four haploid nuclei (n=23 chromosomes) form.