CELL DIVISION

Chromosomes

Made of DNA Carries cell genetic

information 23 pair

Differs for different organisms

fruit fly: 8 Carrot: 18

chromosomes

Before a cell can divide it must duplicate DNA

Each chromosome consists of identical sister chromatids

Since they are identical, where will each one go?

One goes to each new cell Each pair of chromatids are attached at

centromere Usually located near the middle of

chromosome

Cell cycle

Series of events that cells undergo as they grow and divide

During cell cycle the cell:

Grows Prepares for division Divides to form 2

daughter cells, each of which will re-enter into cell cycle

Cell cycle

Interphase: “in-between” period of growth Time spent Between divisions

Cycle consists of 3 phases:1. G12. S3. G2

Cell cycle: G1 (growth)

Most of the growing Synthesize proteins

Cell cycle: S phase

Chromosomes get replicated

Synthesis of DNA When completed,

cell completes remainder of cycle

Cell cycle: G2

Shortest of phases Cells prepare for

division Organelles

reproduced Molecules needed

for replication created

Mitosis

Mitosis: process in cell division where nucleus divides

Divided into 4 stages1. Prophase2. Metaphase3. Anaphase4. Telophase

prophase

First and longest phase1. Chromosomes become visible2. Centrioles separate and move to poles3. Centrioles help organize mitotic spindle

(spindle apparatus): fanlike microtubule structure that helps separate chromosomes

4. Chromosomes attached to fibers in spindle at the centromere

5. Nuclear envelope breaks down

metaphase

1. Chromosomes line up in middle of cell

anaphase

1. Each sister chromatid separates and becomes an individual chromosome

Centromeres split Chromosomes continue to move until they

are in opposite poles

telophase

1. Chromosomes uncondense2. Spindle breaks apart3. Nuclear envelope reforms Essentially, the opposite of prophase

cytokinesis

Separation of cytoplasm and its contents In animal cells, membrane is squeezed

in the middle until 2 cells are formed Cleavage furrow

In plant cells, a cell plate will form This will eventually become the cell wall

Uncontrolled cell growth

Cancer Cancerous cells do not respond to signals

that regulate growth of most cells Divide uncontrollably tumor Cells can break lose and travel to other

parts of body Metastizing

Causes can be genetic or environmental Effects depend on affected tissue

Chromosome number review Humans have how many chromosomes? 23 pairs of homologous chromosomes We get half from mom and the other half

from dad 23 + 23 =46 Diploid cell: contains 2 sets of

chromosomes Represented as 2n (n=number of

chromosomes of organism) Diploid cells contain 2 complete sets of

genes

Chromosome number

Haploid cells: contain only a single set of genes

Represented as n (n= number of chromosomes)

Diploid number for humans? 2n=?... 2n=46 Haploid number in humans? N If 2n=46, then n= n=23

Haploid cells

Haploid cells are found in sexually reproducing organisms

Each parent offers on set of their own DNA (n)

Combination of gametes (sex cells) n + n = 2n

In humans: Sperm (n) has 23 chromosomes Egg (n) has 23 chromosomes They combine to form a diploid zygote

(2n)

Section 10-3 Meiosis

Meiosis: process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

***Very similar to mitosis only with an extra set of phases

Meiosis

Composed of two distinct divisions Meiosis I Meiosis II

Begins with a single diploid cell Ends with 4 haploid cells

Homologous chromosomes

Homologous chromosomes: pairs of corresponding chromosomes from each parent Ex. # 12 from mom matched up with # 12

from dad

Meiosis 1: Prior: chromosomes replicated Meiosis I: division of Homologous

chromosomes Results in 2 haploid daughter cells Prophase I, Metaphase I, Anaphase I,

Telophase I and cytokinesis

Prophase I: Homologous

chromosomes pair up (one from each parent) to form a tetrad (structure formed when homologues come together)

Crossing over occurs: results in the exchange of genetic information between homologous chromosomes

Produces new combinations of genes

Crossing over

Occurs during Prophase I

Overlapping arms of homologues have same information and can swap info at any point along their length

*****increases genetic variation*****

Metaphase I

Spindle fibers attach to centromere

Tetrads (pair of homologous chromosomes) line up in middle of cell independent of each other

Anaphase I

Spindle Fibers pull homologous chromosomes toward opposite ends of the cell

Telophase I and Cytokinesis

Nuclear membranes reform

Cell separates to become two cells

*Cells produced have different genetic composition than the parent cell

Meiosis II Meiosis II: paired chromatids divide Each cell now has 2 chromatids Almost exactly like mitosis!!

Prophase II

Nothing really happens

Chromosomes are already condensed and compacted

Nuclear envelope goes away

Metaphase II

Chromosomes line up similar to mitosis

Anaphase II

Paired chromatids separated

Telophase II and cytokinesis Four haploid cells

have been created

overview

2n

n

nMeiosis II

Meiosis I

Gamete formation Gamete: cell

designed for sexual reproduction

Example: sperm and egg

Males make four sperm while females make one