Chapter 8 Cell Reproduction 8 1 Chromosomes DNA

Chapter 8 Cell Reproduction

8 -1 Chromosomes

DNA • Long, thin molecule that stores genetic information • Instructions for Life

Chromosomes • DNA coiled up • This happens during cell division

Histones • Proteins that the DNA wraps around to make chromosomes

Chromatid • Each half of a chromosome • “sister chromatids” = chromosome

Centromere • Holds chromatids together until they separate during cell division

Chromatin • Less tightly coiled DNA/protein • Happens when the cell is NOT dividing so the information can be used

Chromosome Numbers • Each species has its own number of chromosomes in each cell • Ex: Fruit Flies = 8 Carrot = 18 Cat = 32 Human = 46

Two Types of Chromosomes • 1. Sex chromosomes • 2. autosomes

Sex Chromosomes • Chromosomes that determine if an animal is a male or a female • Females = X and X • Males = X and Y

Autosome • All other chromosomes that do not determine the sex of an animal • Each cell has two copies of each autosome • Example: DNA for eye color

Homologous Chromosomes • The two copies of each autosome • Same size and shape • Carry genes for the same traits (for example blue eye color)

Karyotype • A picture of homologous chromosomes

Diploid • Cells with two sets of chromosomes

Haploid • One set of chromosomes • Ex: human sperm and egg cells

Chapter 8. 3 Meiosis

Types of Reproduction • Asexual reproduction – Produce offspring with only ONE parent – “babies” are identical to parent • Sexual reproduction – Produce offspring by meiosis and the joining of the sperm and egg – “babies” are NOT identical to parents

• Meiosis – New cells have half the number of chromosomes • Gametes – Reproductive cells (sperm and egg)

Key Points • Meiosis must happen so when the sperm and egg join to create a new organism, it has the correct number of chromosomes. • Examples: – Human = 46 – Egg = 23 – Sperm = 23 – Egg + Sperm = baby – 23 + 23 = 46

Meiosis I. (very similar to mitosis) • Prophase I. – DNA coils into chromosomes – Each chromosome lines up with its homologue (the similar chromosomes) – Synapsis: pairing of homologous chromosomes – Each pair is called a tetrad – Crossing-over: may occur where pieces of one chromosome break off and attach to the other chromosome genetic recombination

• Metaphase I. – Tetrads line up along the middle of the cell

• Anaphase I. – Each homologous chromosome of each tetrad moves to opposite ends of the cell

• Telophase I. – Cell divides creating 2 new cells each with a haploid number of chromosomes (1/2 the original number of chromosomes, but 2 copies of each)

Meiosis II. (DNA does NOT get copied first) • Prophase II. – Chromosomes begin to move

• Metaphase II. – Chromosomes move to the cell’s midline

• Anaphase II. – Chromatids separate and move to opposite ends of the cell

• Telophase II. – Four new cells form with ½ the original number of chromosomes

• **Meiosis occurs ONLY IN REPRODUCTIVE ORGANS. • See Figure 8 -12 page 155

• Spermatogenesis: – Production of sperm cells • Oogenesis: – Production of egg cells