Cell Division

When a cell reaches its maximum size, the nucleus gives the signal for cell division.

  1. Cell Division

Needed for 3 major life processes:

  1. growth: increased number of cells
  2. repair: mending skin, blood vessels, etc.
  3. reproduction

There are 2 types of cell division

  1. mitosis: used for growth and repair, mitosis produces cells that are identical to the parent cell; parent and offspring cells have identical DNA (genetic material)
  1. used for asexual reproduction: process by which an organism simply duplicates its genetic material and splits into 2 separate, identical organisms
  1. meiosis: special form of cell division; produces cells that are NOT identical to the parent cell
  1. used in sexual reproduction, which produces

an offspring that has a combination of genetic

material from 2 parent organisms

  1. the products of meiosis have only HALF the

amount of DNA (genetic material) found in

each parent cell

  1. Mitosis

 process of cell division that gives 2 identical cells from

one parent cell

  1. Cell Cycle
  1. sequence of phases in the life cycle of a cell
  2. covers the period of time from the beginning of one cell division to the beginning of the next cell division
  3. the cell cycle has two parts:
  1. interphase (growth and preparation)
  2. cell division (mitosis)
  1. Interphase:
  1. occurs between divisions
  2. during interphase, the cell produces all of the material necessary for cell growth and for cell division
  3. Interphase – longest part of the cell cycle (90%)
  4. Includes replication (copying) of the genetic material before cell division
  1. DNA is in the form of chromatin (thin, fibrous strands) while it is being copied
  1. Stages of Mitosis: Prophase, Metaphase, Anaphase and Telophase
  1. Prophase: chromatin condenses to form chromosomes (which are made of DNA)
  1. each species of organism’s chromosomes are unique – in size, shape, number, etc.
  1. chromosomes usually occur in pairs

examples:

all humans have 23 pairs of chromosomes

fruit flies have 4 pairs

some plants have hundreds of pairs

some molds have thousands of pairs

  1. each chromosome has been copied (during interphase) and the IDENTICAL chromosomes are called sister chromatids
  2. the sister chromatids are joined together at a point called the centromere

sister chromatid

(each half of

the “X” is one)

chromatinchromosome centromere

(interphase)(prophase)

  1. also during prophase: nuclear membrane (around nucleus) and the nucleolus disappear; mitotic spindle forms (fibrous scaffolding inside the cell)
  1. Metaphase:
  1. chromosomes are pulled to the center of the cell and begin to line up in the middle

(if the cell was a globe, then chromosomes would line up on the equator)

  1. Anaphase
  1. centromeres divide
  2. spindle fibers start to pull the sister chromatids apart
  1. one of each pair goes to the poles of the cell
  2. complete set of chromosomes at each end

Think of chromosomes being pulled from the

equator to the North and South Poles

4. Telophase

  1. 2 nuclei are formed
  2. nuclear membranes form around each set of chromosomes
  3. chromosomes uncoil  chromatin
  4. mitotic spindle disassembles
  1. Cytokinesis – split cytoplasm
  1. animal cells: cell membrane of the parent cell starts to fold inward to the center until 2 cells are formed from pinching the parent cell in half
  1. plant cells: a cell plate forms in the middle of the cell
  1. cell plate consists of a double membrane
  2. a new cell wall forms between the double membrane of the cell plate
  3. the double membrane remains and forms part of the cell membrane of the 2 new cells

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Vocabulary for Meiosis

Gene: section of DNA that controls a particular trait or characteristic (ex. eye color)

Chromosome: lots of DNA condensed; each chromosome contains lots of genes

Homologous chromosomes: same length, size, and shape; code for the same traits but might have different versions of the genes for these traits

Example: 2 homologous chromosomes carry the code for

eye color

 one chromosome might have the gene for blue

eyes while the other chromosome has the gene

for brown eyes

 same traits (eye color), different versions (blue, brown)

Our 23 pairs of chromosomes are 23 homologous pairs (46 chromosomes).

Diploid: di=2 ; genes are in pairs and therefore, chromosomes are in pairs (2 sets of chromosomes)

Human cells are normally diploid: chromosomes are in pairs (23 pairs).

Haploid: half of the normal number of chromosomes

Humans: diploid 23 pairs  46 chromosomes

haploid not pairs  23 chromosomes

Symbols:

n = number of sets of chromosomes

2n = diploid, 2 sets of chromosomes

1n = 1 set of chromosomes

somatic cells: normal body cells of an organism; usually diploid; made through mitosis

gametes: sex cells of an organism; haploid; made through meiosis

Meiosis

  1. Called reduction division because it reduces the number of chromosomes by half

2n meiosis 1n 4n meiosis 2n

(diploid) (haploid) (tetraploid)(haploid)

6n meiosis 3n

  1. Meiosis is used in sexual reproduction.
  1. Results in 4 new haploid cells, called gametes.
  1. Used for gametogenesis (process of making gametes)

Two types of gametogenesis:

  1. spermatogenesis : process of making sperm
  2. oogenesis: process of making eggs

Homologous Chromosomes

Further Explanation

  • Homologous chromosomes are the same length, size, shape, and code for the same TRAITS.
  • The location of these traits is the same. On the diagram below, locations are indicated with a solid black line on the chromosome.
  • However, the specific GENES for these traits can be different or it can be the same.
  • Homologous chromosomes are NOT IDENTICAL. Sister chromatids are identical.
  • In the diagram below, notice that the TRAITSare written in green, while the GENES for those traits are written in blue.