Ch. 8: Cell Reproduction

DNA Stores genetic information Found coiled as Chromosomes during cell division Found a thin threads called Chromatin between cell divisions

Chromosome structure & number differ between Eukaryotes & Prokaryotes Prokaryotes: usually ONE circular chromosome

Chromosomes in Eukaryotes Rod-shaped DNA condenses by: wrapping around proteins called histones folding, twisting, & coiling

Chromosome Structure Each chromosome has 2 identical halves called Chromatids Centromere – point of attachment between two chromatids

Chromosome Number Each species has characteristic # of chromosomes ex:HUMANS = 46

2 types of chromosomes 1. Sex Chromosomes Determine gender May also carry other traits Humans have 2 forms: X or Y Males = XY Females = XX Which parent determines gender?

2. Autosomes Other 44 chromosomes / 22 pairs Determine traits other than sex each copy in a pair is Homologous: same size, shape and carry genes for the SAME TRAITS

HUMANS 2 sex chromosomes (1 pair) ‏ + 44 autosomes (22 pairs) ‏ = 46 chromosomes Karyotype – diagram that shows chromosomes arranged in homologous pairs

Normal Male 46 XY

Normal Female 46 XX

Diploid & Haploid Cells 1. Diploid Cells (2n) ‏ have 2 sets of chromosomes 46 chromosomes: 2 sex chromosomes + 44 autosomes ALL human cells EXCEPT gametes (sex cells) are diploid

2. Haploid Cells (1n) ‏ have only 1 set of chromosomes 23 chromosomes: 1 sex chromosome, 22 autosomes In humans, only gametes are haploid Gametes are reproductive cells Males = sperm females = ova

Fertilization restores diploid # in cells: Sperm + Ovum = Zygote (fertilized egg) ‏ 1n + 1n = 2n = 46

Why do cells divide? There are limits to cell growth Surface-to-volume ratio Lg. cells have harder time diffusing materials in/out of cell DNA can only produce a limited amount RNA Cell will divide when it can’t grow any more

Uncontrolled Cell Growth Cancer: cells have lost ability to control growth. do not stop growing when they touch other cells continue to grow until the supply of nutrients is gone (death).

Cell Division Process where cells divide producing offspring cells 2 types: Mitosis Produces 2 offspring cells Offspring are genetically identical to original cell Meiosis Produces 4 gametes Offspring are genetically different from original cell

The Cell Cycle Time from the beginning of one division to the beginning of the next. During one cell cycle: the cell grows prepares for division divides to form offspring cells

Stages of the cell cycle Interphase- time in between divisions Mitosis / Meiosis- chromosome division and separation Cytokinesis- division of the cytoplasm: forms 2 cells

Interphase Longest stage of cell cycle 3 steps: G1: cell grows S: DNA replication occurs G2: cell prepares for division

Mitosis Division of the nucleus occurs for growth or to replace old cells Mitosis has four stages: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase “Please Make Anna Toast”

Prophase Longest stage Chromosomes condense & become visible Centrioles move to opposite sides of cell Nuclear envelope breaks down Spindle fibers form

Metaphase Shortest phase Chromosomes line up along equator (center) of cell

Sister chromatids separate & become individual chromosomes Chromosomes move toward opposite poles Anaphase

Telophase Chromosomes uncoil Nuclear envelope reforms around chromatin Spindle breaks down

Cytokinesis Begins as soon as nuclei reform Cytoplasm divides: -animal cells: membrane pinches in (cleavage furrow) -plant cells: cell plate forms B_m4

Meiosis Division to form reproductive cells aka: gametogenesis Produces 4 haploid gametes (sex cells) ‏

Meiosis = 2 rounds of division Meiosis I: DNA replicates & cell divides producing 2 haploid cells Meiosis II: 2 cells divide (w/o DNA replication) forming 4 haploid cells

Meiosis I Prophase I: synapsis occurs: chromosomes line up in homologous pairs called tetrads crossing over occurs: chromosomes in tetrads exchange genetic material -creates new combinations of genes

Metaphase I: Tetrads line up along equator Anaphase I: Independent assortment occurs: homologous chromosomes randomly separate to opposite poles creates genetic variation

Telophase I: Chromosomes uncoil Nuclear envelope reforms Cytokinesis I: 2 haploid cells produced Haploid b/c each has 23 chromosomes w/ 2 chromatids Genetically different

Meiosis II No Interphase between meiosis I & II – This means DNA does NOT replicate! Prophase II – Chromosomes coil, etc. Metaphase II – Chromosomes line up on equator

Anaphase II – Sister chromatids separate Telophase II – Chromosomes uncoil, etc Cytokinesis II – 2 cells split producing 4 gamete mQS_FZ0

Gender Differences o Meiosis in males (spermatogenesis) produces 4 sperm o Meiosis in females (Oogenesis) produces 1 ovum & 3 polar bodies - b/c cytokinesis is uneven - polar bodies should dissolve