Chapter 6Chromosomes and Cell

Reproduction

I. Formation of New Cells by Cell Division• About 2 trillion cells are produced by an adult

human body every day (about 25 million a second!)

• These new cells are formed when older cells divide in a process called cell division

http://www.youtube.com/watch?v=VGV3fv-uZYI&feature=watch_response

• All of the information stored in the molecule DNA must be present in each of the resulting cells

• Information stored in DNA directs a cell’s activities and determines its characteristics

A. Prokaryotic Cell Reproduction• A prokaryote has circular DNA• Prokaryotes reproduce by a type of cell

division called binary fission• Binary fission is a form of asexual

reproduction that produces identical offspring with exact copies of DNA

• 2 stages1. DNA is copied2. Cell divides into two

• Results in two complete functioning prokaryotes each with its own copy of circular DNA

B. Eukaryotic Cell Reproduction• DNA is organized into units called genes• A gene codes for a protein or RNA molecule• A single DNA molecule has thousands of

genes

II. How Chromosome Number and Structure Affect Development

A. Sets of Chromosomes• Humans have trillions of somatic cells, or

body cells (cells other than gametes)• Gametes are reproductive cells (eggs or

sperm cells)• Each somatic cell has 23 pairs of differing

chromosomes (total of 46 chromosomes)- One set comes form your mother- The other set comes from your father

• Each pair of chromosomes consists of two homologous chromosomes that are similar in size, shape, and genetic content

• When a cell, such as a human somatic cell, contains two sets of chromosomes, it is said to be diploid, 2n

• Human gametes only have one set of chromosomes and are said to be haploid, n

• The fusion of two haploid gametes (n=23) – a process called fertilization – forms a diploid zygote (2n = 23 or n = 46)

• A zygote is a fertilized egg cell, the first cell of an individual

• Each organism has a characteristic number of chromosomes

B. Sex Chromosomes• 22 pairs of human

chromosomes are called autosomes, chromosomes that are not directly involved in determining the sex of an individual

• One pair of chromosomes, the sex chromosomes, contain genes that will determine the sex of an individual

- Referred to as the X and Y chromosomes in a human

- The genes that cause a fertilized egg to develop into a male are located on the Y chromosome

- Thus the sex of an individual is determined by the male

C. Change in Chromosome Number• The presence of all 46 chromosomes is

essential for normal development and function

• Humans who are missing even one of the 46 chromosomes do not survive

• Humans with an extra chromosome, a condition called trisomy, will not develop properly

• Abnormalities in chromosome number can be detected by analyzing a karyotype, a photo of chromosomes arranged by size

• Trisomy 21, or down syndrome, results in an extra copy of chromosome 21

D. Change in Chromosome Structure• Mutations are changes in an organism’s

chromosome structure1. Deletion – a piece of chromosome is

missing2. Duplication – two copies of a certain set

of genes3. Inversion – chromosomes reattach in a

reverse order4. Translocation – two different

chromosome pieces attach together

III. The Life of a Eukaryotic CellA. The Cell Cycle• A repeating sequence

of cellular growth and division during the life of an organism

• Allows organisms to grow, replace damaged tissues, and, in some organisms, reproduce asexually

• Some somatic cells, such as most muscle and nerve cells, never divide- If these cells die, the body cannot replace

them

1. Interphasea. First growth (G1) phase• A cell grows and carries out its routine

functions

b. Synthesis (S) phase• A cell’s DNA is copied

- Each chromosome consists of two chromatids attached at the centromere

c. Second growth (G2) phase• Preparations are made for the nucleus to

divide• Microtubules are assembled

- Used to move chromatids during mitosis

2. Mitosis• The nucleus of a cell is divided into two

nuclei• Each nucleus ends up with the same

number and kinds of chromosomes as the original cell

3. Cytokinesis• Cytoplasm divides

IV. Control of the Cell Cycle• Feedback signals from the cell can trigger

the next phase of the cell cycle• Other feedback signals can delay the next

phase to allow completion of the current phase

A. Cell growth (G1) checkpoint• Makes the decision of whether the cell will

divide• Proteins will stimulate the cell to begin the

synthesis phase if conditions are favorable

B. DNA synthesis (G2) checkpoint• DNA repair enzymes check DNA replication

before the cell can enter into mitosis

C. Mitosis checkpoint• Triggers the exit from mitosis

D. When Control is Lost: Cancer• Proteins that regulate these checkpoints are

made from information contained in genes• If one of these genes is mutated, the

protein may not function to regulate these checkpoints

• Cancer, the uncontrolled growth of cells, may result

V. Chromatid Separation in MitosisA. Forming the Spindles• Spindles are made up of both centrioles

and microtubule fibers

• Microtubule fibers are involved in moving chromatids during cell division to opposite sides of the dividing cell

• Animal cells contain one pair of centrioles until they are replicated during the G2 phase

• Each pair moves to opposite sides of the cell• Spindle fibers made of microtubules stretch

between the two pairs of centrioles• Plant cells do not have centrioles

B. Separation of Chromatids by Attaching Spindle Fibers• The ends of the spindle fibers are broken

down bit by bit at each of the “poles” near the centrioles

• As the fibers become shorter, the chromatids move closer and closer to the poles

• As soon as the chromatids separate from each other they are called chromosomes

VI. Mitosis and CytokinesisA. Mitosis

1. Prophase• Chromosomes condense• Nuclear envelope dissolves• Spindle forms

Condensed chromatids

2. Metaphase• Chromosomes line

up along equator

Cell equator

3. Anaphase• Chromatids move toward opposite poles as

the spindle fibers attached to them shorten

4. Telophase• Nuclear envelope forms at each pole• Chromosomes uncoil• Spindle dissolves

B. Cytokinesis• The cytoplasm of the cell is divided in half• The cell membrane grows to enclose each

cell, forming two separate cells• Each resulting cell is identical to the existing

cell

http://www.youtube.com/watch?v=VGV3fv-uZYI&feature=watch_response

• In plant cells, vesicles formed by the Golgi apparatus fuse and form a cell plate

• A new cell wall forms on both sides of the cell plate and separates the plant cell into two new plant cells