Chapter 6 Chromosomes & Cell Reproduction

6.1 Chromosomes

New Cells formed by cell divisionHas anyone ever fallen and scraped their knee?

What happened next?

What happens about a week later?

New Cells

• How do we get these new cells? Where do they come from?

• Cells are made by existing cells in a process called cell division.

• Take a guess on how many cells are produced by a human body everyday.

2 Trillion• That makes 25 million every second.

Why Cell Division?• Eukaryotic organisms undergo cell division for

many different reasons.GrowthDevelopmentRepairProduction of new cells

• Sex cells, or gametes, undergo a different type of cell division which gives half of the amount of genetic material.

• Gamete – Reproductive cells (Sperm & Egg)

Division of Bacterial Cells• Bacterial cells divide differently than

eukaryotic cells.• Prokaryotic cells undergo binary fission

which is a form of asexual reproduction that produces an identical offspring.

• The parent passes an exact copy of its DNA to it offspring… like a clone.

Binary Fission

• Step 1: DNA is copied

• Step 2: Cell divides. Each cell has a copy of DNA.

Division of Eukaryotic Cells

• Eukaryotic cell division is quite a bit different.

• Eukaryotic cells are very complex and a vast amount of genetic information is encoded in its DNA.

• Because there is so much genetic information that all needs to be divided up properly, eukaryotic cells bunch their DNA into nice organized structures called chromosomes.

Chromosomes

• Chromosome – A structure made of DNA coiled around proteins.

Chromosomes Cont.

• Chromosomes are made of two chromatids that are attached at a midpoint called a centromere.

• Humans have 23 pair of chromosomes.• Each pair are called homologous

chromosomes, chromosomes that are similar in shape, size & genetic content.

• One set of chromatids come from the sperm & the other from the egg.

Chromosome numbers• All of the cells other than the sex cells

are called diploid. A cell that contains two sets of chromatids.

• Sex cells are called haploid cells. Cells that contain only one set of chromatids.

Haploid = 1nDiploid = 2n

n = a variable, for instance in humans n = 23

The romance of gametes• When two gametes or haploid cells

come together a zygote is formed. Zygote – a fertilized egg cell. The first cell of a new individual.

• 1n + 1n = total chromosome number• Eg. 23 + 23 = 46 chromosomes• Females send an X chromosome &

males send either an X or a Y

Chromosome numbers

• Do you think that every organism has the same number of chromosomes?

• Eg. A fruit fly only has 4 chromosomes.

• Why so little?

Karyotyping

• Karyotype – Picture of the chromosomes found in an individual’s cells arranged in order of size & shape.

• With a karyotype it is easy to see if a certain chromosome is added, has a mutation or is missing all together.

Classwork!• Summarize how bacterial cells divide

by binary fission.• What is the difference between a

haploid and a diploid cell?• What is the difference between a

normal karyotype and one from a person whom has Down syndrome?