Genes and Chromosomes

• Humans have 46 chromosomes in all body cells (not sex cells)

---called SOMATIC CELLS

• You have 23 pairs of chromosomes ---each pair is called HOMOLOGOUS

• Each chromosome in the homologous pair contains genes that affect the same characteristic (example eye color, skin tone etc.)

Chromosome NumbersPlants Mammals Reptiles14- cucumber 40- beaver 36- boa constrictor40- peanut 38- domestic cat 22-

American Toad24- pine tree 78- dog48- potato 48- chimpanzee Fish80- sugar cane 56- elephant 46- small-mouth bass

30- giraffe 84- troutBirds 64- horse80- canary 60- cattle Insects68- dove 32- walrus 6- mosquito80- duck 44- killer whale 32- honey bee78- raven 8- drosophila (fruit fly)

446- butterfly

Anatomy of a Chromosome

How does 6 feet of DNA fit into each nucleus of each cell??

• Video clip: How DNA is Packaged

Check your Understanding• How many chromosomes do human cells have?• How many pairs of chromosomes do human

cells have?• What are somatic cells?• You have one gene for blue eyes and one gene for

brown eyes. – Where did these genes come from?

• What are the two halves of a single chromosome called?

• What structure holds the chromatids together?• What is a gene?

Cell Cycle• Cell spends…

– 90% of time in INTERPHASE – doing the job it is programed for• DNA is stretched out into long, thin

chromatin strands so it can be utilized– 10 % of time doing cell DIVISION

• Before division, each chromosome is copied exactly—S phase replication

• Nucleus divides and each daughter cell gets a full set of chromosomes

video clip: eukaryotic cell cycle

cell cycle animation

Lets look at one way cells divide

Mitosis: Nuclear division resulting in

two identical sets of

chromosomes

Phases of Mitosis

1- Prophase2- Metaphase3- Anaphase4- Telophase

Stages of Mitosis• Prophase

1. Chromosomes become visible (coil up and get shorter and thicker) CHROMATINCHROMATID

2. Nuclear membrane disintegrates (disappears)

3. Centrioles move to opposite sides of the cell

4. Spindle fibers appear and shoot out from centrioles

(longest phase)

• Metaphase

1. Chromosomes migrate to equator• Centromeres align along center

2. Spindle fibers attach to each chromosome

• Anaphase

1. Sister chromatids are pulled apart

2. Separate chromatids are pulled to opposite sides of the cell by spindle fibers

• Telophase1. Nuclear membrane appears

around each set of chromosomes

2. Chromosomes unwind and go back to being chromatin

CYTOKINESIS

• Entire cell divides in half• Each daughter cell has…

– an exact duplicate of the DNA– 46 chromosomes– 23 pairs of chromosomes (diploid)

Check your Understanding• How are the terms DNA, chromatin,

chromosome, and genes related?• What is the name of the process by which DNA

is copied?• Name, in order, the 6 stages of the cell cycle,

starting with the phase cells are in most of the time

• Name the phase of the cell cycle where:– Chromosomes line up in the middle of the cell– Chromosomes first become visible– Spindle fibers first appear– Spindle fibers attach to sister chromatids– Centrioles move to opposite sides of the cell– Sister chromatids get pulled apart

Still checking…– Nuclear membrane disappears– Nuclear membrane reappears– Entire cell divides in half– No chromosomes are visible

• How many daughter cells are created from one parent cell?– How many chromosomes does each

daughter cell have?– How many pairs of chromosomes does

each daughter cell have?– What is the ploidy of the daughter cells?

1: interphase2: late interphase /

beginning prophase3: early prophase4: mid prophase5: late prophase

6: metaphase7: late metaphase / early anaphase8: anaphase9: early telophase10: late telophase / cytokinesis

VOCABULARY TERMS

Interphase

Chromatin

Replication

Daughter cells

Mitosis

Cytokinesis

SomaticHomologousDiploidCentromereGeneChromatidChromosome