UNIT 6
CELL DIVISION
READING:
Chapter 6
Chromosomes and Cell Reproduction
Chapter 7
Meiosis and Sexual Reproduction
Chapter 9
DNA: The Genetic Material
Chapter 10
How Proteins are Made
OBJECTIVES
At the end of this unit of study the student will be able to:
1.
List the importance of DNA and describe the basic structure of a DNA molecule.
2.
Summarize the process of replication.
3.
Describe the structure of a RNA nucleotide.
4.
Compare and contrast the structure of RNA with that of DNA.
5.
Name and describe the three types of RNA.
6.
Summarize the process of transcription.
7.
Distinguish between a codon and an anticodon and state where each is found.
8.
Summarize the process of translation as related to protein synthesis.
9.
Define chromosome number and distinguish between haploid and diploid number.
10. Define mitosis and list, draw, and summarize the phases of mitosis.
11. Define meiosis and list, draw, and summarize the phases of meiosis.
12. Describe spermatogenesis and oogenesis.
13. Distinguish between sexual and asexual reproduction.
ASSIGNMENTS:
1.
2.
3.
4.
5.
6.
CELL DIVISION
I.
UNIT 6
Nucleic Acids
A. Deoxyribonucleic Acid - DNA
1. Found primarily in the nucleus in chromosomes - store and transmit information to make proteins.
2. Structure
a. Consists of two strands of nucleotide monomers
b. Parts of a nucleotide
1) Deoxyribose - 5-carbon sugar
2) Phosphate group
3) Nitrogen base
a) PURINE - nitrogen base with a double ring of carbon and nitrogen atoms.
(1) Adenine
(2) Guanine
b) PYRIMIDINE - nitrogen base having a single ring of carbon and nitrogen atoms.
(1) Thymine
(2) Cytosine
c. Two strands of nucleotides twist around a central axis to form a double helix - first described
by James Watson and Francis Crick in 1953.
1) Similar to a twisted ladder
a) Sides formed by alternating sugar and phosphate units.
b) Rungs consist of bonded pairs of nitrogen bases - equal in length.
2) Purine always pairs with a pyrimidine - held together by hydrogen bonds.
a) Adenine always bonds with thymine
b) Guanine always bonds with cytosine
d. Sequence of bases in one strand is exact complement of base arrangement in second strand.
1) 1st Strand Sequence A - G - C - T - T - A - G - C
2) 2nd Strand Sequence T - C - G - A - A - T - C - G
3. REPLICATION
a. Process of duplicating the DNA molecule
b. Each strand can serve as a template or mold on which new complementary strand can be built.
c. Process of Replication
1) DNA helicase attaches to DNA molecule - "unzips" the 2 strands - breaks the hydrogen
bonds between the bases.
2) Unpaired bases of strands react with complementary bases of nucleotides floating in nucleus –
new set of hydrogen bonds form.
3) DNA polymerase catalyzes formation of sugar to phosphate bonds that connect one nucleotide
to the next.
4) Result - two new DNA molecules - each consists of 1 "old" strand and 1 "new" strand.
5) Process doesn't begin at one end and proceed to the other - may occur simultaneously at
many points.
4. DNA may be damaged by body heat, radiation, chemicals, etc.
a. Cell can "proofread" for mistakes
b. Can be repaired and corrected
B. Ribonucleic Acid - RNA
1. Nucleic acid that uses information stored in DNA to synthesize proteins.
2. Structure
a. Consists of a single strand of nucleotide monomers.
b. Parts of nucleotide
1) Ribose - 5-carbon sugar
2) Phosphate group
3) Nitrogen base
a) Purines
(1) Adenine
(2) Guanine
b) Pyrimidines
(1)
Uracil
(2) Cytosine
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3. RNA differs from DNA in 3 ways
a. RNA consists of only one strand of nucleotides instead of the 2 strands found in DNA.
b. RNA contains the 5-carbon sugar, ribose, instead of deoxyribose
c. RNA contains the nitrogen base, uracil, instead of thymine.
4. Types of RNA
a. MESSENGER RNA (mRNA) - single, uncoiled strand,transmits information from DNA for
protein synthesis; acts as template for the assembly of amino acids during protein synthesis.
b. TRANSFER RNA (tRNA) - single strand of RNA folded back on itself in hairpin fashion;
allows some complementary bases to pair; exists in 20 or more varieties each with the ability
to bond to only 1 specific type of amino acid.
c. RIBOSOMAL RNA (rRNA) - globular form of RNA; major constituent of ribosomes.
5. TRANSCRIPTION
a. Process whereby RNA is produced from the the DNA - transcribed according to the information
encoded in the base sequence of DNA.
1) Base sequence of RNA complementary to the sequence of DNA from which it was transcribed.
2) Process directed by enzyme, RNA polymerase
b. Process
1) Enzyme binds to DNA causing strands to separate.
2) Hydrogen bonds to form between DNA template and complementary RNA nucleotide bases.
3) Enzyme moves to next section of DNA, bonds form between phosphate groups and ribose.
4) RNA molecule released when enzyme reaches DNA sequence that acts as a termination signal.
C. Protein Synthesis
1. Structural and functional characteristics of proteins are determined by the sequence of amino acids
in the protein
2. Sequence of amino acids in a protein encoded in DNA
3. GENETIC CODE - System that contains information needed by cells for proper functioning; built
into the arrangement of nitrogen bases in a particular sequence of DNA.
a. CODON - series of three bases in mRNA that codes for a specific amino acid.
b. ANTICODON - series of three bases in tRNA, complementary to a codon; pairs with the codon
during translation
4. TRANSLATION - process by which protein molecules are made from information encoded in mRNA.
a. mRNA moves out of nucleus through a nuclear pore
b. mRNA migrates to a group of ribosomes (site of protein synthesis)
c. Free floating amino acids in the cytoplasm are transported to the ribosomes by tRNA - each one
specific for an amino acid.
d. Assembly of polypeptide begins when A - U - G codon attaches to the ribosome.
e. Codon pairs with its anticodon adding specific amino acid to the growing polypeptide chain.
f. Process continues until a "stop" codon reaches the ribosome.
g. mRNA is released and polypeptide is complete.
5. GENE - region of DNA that directs the formation of a polypeptide.
a. Proteins usually consist of more than one polypeptide.
b. Several genes may direct protein synthesis
II. CELL DIVISION
A. Process by which cells reproduce.
1. Cell Theory
a. Cell is basic unit of life
b. All living things are made of cells or cell fragments.
c. All cells come form existing cells.
2. Reasons Cells Divide
a. Maintenance
b. Repair
c. Growth
d. Reproduction
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B. NUCLEUS
1. Organelle that directs the everyday metabolic activities of the cell.
2. Composed of material known as CHROMATIN
a. Normal condition - grainy mass of material
b. During cell division chromatin appears as bodies known as chromosomes.
1) Composed of DNA and protein
2) Carries the "genetic code"
3) Individual units of heredity are known as genes - determine cell characteristics and how cell
functions - by proteins that are synthesized.
3. Each time the cell divides the genetic material must be REPLICATED - produces an exact copy
a. Must then be distributed to the new cell.
b. Replication occurs during interphase.
C. CHROMOSOME NUMBER
1. The number of chromosomes found in the nucleus.
a. Varies from one organism to another.
1) Man - 46
2) Fruit Fly - 8
3) Bullfrog - 26
2. All cells of an organism will have the same number of chromosomes.
a. Only exception is the sex cells or GAMETES
1) Egg - female
2) Sperm - male
b. Body cells except gametes are called SOMATIC CELLS or SOMATES.
3. In humans, somates have 23 pairs of chromosomes - total of 46 chromosomes.
4. Chromosomes which makeup the pair are known as being HOMOLOGOUS.
a. Individual members of the pair are called HOMOLOGS.
b. Get one homolog from each parent - 1/2 from mother; 1/2 from father
c. Each chromosome of pair has same size and shape as its homolog.
5. Number of pairs of chromosomes in a somate is expressed as "n" - the total number of chromosomes
is expressed as "2n"
a. Humans: n = 23; 2n = 46
6. Total number of chromosomes is the cell's DIPLOID NUMBER = 2n
Total number of different pairs in cell is its HAPLOID NUMBER(MONOPLOID NUMBER) = n
7. All the features of the cell's chromosomes including size and number make up a cell's KARYOTYPE
- arranged by size and shape.
D. Cell Division
1. Actually talking about division of nuclear material
2. Two types of nuclear division
a. Mitosis
b. Meiosis
3. CYTOKINESIS - division of the cytoplasm
E. MITOSIS
1. Process by which the nucleus divides to produce 2 new nuclei, each with the same number of
chromosomes as the parent nucleus.
a. From the Greek - Mitos - Thread
b. Occurs in somatic cells.
2. Requirements
a. Precise replication of the genetic material
b. Distribution of a complete set of chromosomes to each new cell - called DAUGHTER CELLS
3. Five Phases of Cell Cycle - continuous
a. Interphase
b. Prophase
c. Metaphase
d. Anaphase
e. Telophase
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4. INTERPHASE
a. Once called the "resting" stage
b. Cell performing various metabolic activities - respiration, protein synthesis, etc.
c. Occupies about 2/3 of cell's life cycle
d. Genetic material replicates during phase.
e. Nucleus clearly defined by membrane
f. Chromosomes not visible - chromatin appears grainy.
g. Consists of 3 subphases
1) G1 Phase - first phase of interphase; cell doubles in size, and enzymes and organelles, such
as mitochondria and ribosomes, roughly double in size.
2) S Phase - Phase during which the DNA in the chromatin replicates
3) G2 Phase - Cell undergoes rapid growth that prepares it for mitosis, synthesizing necessary
enzymes and structures.
5. PROPHASE
a. Early prophase - chromatin coils and forms chromosomes.
b. Chromosomes appear as rod-like structures.
c. Nucleolus and nuclear membrane breakdown and disappear.
d. In all but plant cells, centrioles appear next to the disappearing nucleus - move to opposite ends
or poles of cell.
e. Spindle fibers form - asters radiate from centrioles in animal cells.
f. Chromosomes appear as double stranded structure
1) Each strand is a CHROMATID
2) Two chromatids are joined at a CENTROMERE.
Chromatid
Centromere
6. METAPHASE
a. Chromosomes arrange themselves on equator of spindle
b. Chromosomes attach to spindle fibers at the centromere.
c. Centromeres are aligned on the equator.
7. ANAPHASE
a. Centromere divides taking one chromatid toward 1 pole of spindle other chromatid moves to the
opposite pole.
8. TELOPHASE
a. Two identical sets of chromatids are clustered at opposite ends of the cell.
b. Centrioles and spindle fibers disappear.
c. Chromatids unwind and elongate into chromatin - chromosomes are no longer visible.
d. Nuclear membrane and nucleolus reappear.
9. Process completed with cytokinesis - get two distinct cells
a. Animals - CLEAVAGE FURROW forms from outside toward the center - pinches cell in two.
b. Plants - CELL PLATE forms from center and moves to the outside.
10. No reduction in chromosome number
Parent Cell = 2n ----> Each Daughter Cell = 2n
F. MEIOSIS
1. Process by which the cell nucleus divides resulting in a reduction of chromosome number from the
diploid number (2n) to the haploid number (n)
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2. Involves 2 divisions
a. Chromosome number reduced in first division - Meiosis I
b. Second division - mitotic - Meiosis II
3. Cell division that results in formation of gametes or sex cells.
a. Occurs in the GONADS (sex organs)
1) OVARY - female
2) TESTES - male
b. Process in males - called SPERMATOGENESIS
c. Process in females - called OOGENESIS
4. Phases
a. Interphase
b. Prophase I
f. Prophase II
c. Metaphase I
g. Metaphase II
d. Anaphase I
h. Anaphase II
e. Telophase I
i. Telophase II
5. INTERPHASE - same as in mitosis
a. Chromosomes replicate
6. PROPHASE I
a. DNA strands coil, shorten, and thickens - Chromosomes become visible.
b. Nuclear membrane disappears, spindle fibers appear.
c. Homologous pairs of chromosomes move together - Homologs pair
1) Pairing movement is called SYNAPSIS
2) Two double stranded chromosomes are so close they appear to be joined at their centromeres.
3) Looks like 4 chromatids connected - group is called a TETRAD.
4) CROSSING OVER of chromatids may occur during synapsis.
7. METAPHASE I
a. Tetrads align on the equator of the cell
b. Have 2 chromosomes with 4 chromatids attached to single spindle fiber.
8. ANAPHASE I
a. Homologous pairs of chromosomes separate - one chromosome of the pair goes to each pole centromeres do not divide.
b. Each chromosome still consists of 2 chromatids joined by a centromere.
9. TELOPHASE I
a. Cytoplasm divides to form 2 daughter cells.
b. Nuclear membrane begins to reappear, double stranded chromosomes disappear; spindle may
disappear.
c. Each daughter cell contains one half the original number of chromosomes that parent cell had.
d. Cells may enter resting state - INTERKINESIS
10. PROPHASE II
a. No further replication of DNA occurs
b. Spindle reappears; chromosomes reappear
11. METAPHASE II
a. Chromosomes move to cell's equator.
b. Centromere attaches to spindle fiber.
c. Each chromosome composed of sister chromatids joined at centromere.
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12. ANAPHASE II
a. Centromere joining chromatids divides.
b. Each chromatid moves toward the opposite pole.
13. TELOPHASE II
a. Spindle disappears
b. Nuclear membrane forms around chromosomes of each daughter cell.
c. Each of the 4 cells formed from the original parent cell has 1/2 the number of chromosomes of
the parent.
14. Summary
a. First Meiotic Division - produces 2 haploid cells; double stranded chromosomes.
b. Second Meiotic Division - 2 haploid cells divide to produce 4 haploid cells.
c. 4 haploid cells can develop into gametes.
G. Spermatogenesis and Oogenesis
1. SPERMATOGENESIS - Production of sperm
2. OOGENESIS - Production of egg
3. Cells at the end of Prophase I
a. Male - Primary Spermatocyte b. Female - Primary Oocyte - formed as early as 3rd month of development - meiosis stops until
sexual maturity
4. Two cells at the end of Telophase I
a. Male - secondary spermatocyte
b. Female - cells unequal - 1 secondary oocyte, 1 polar body
5. Cells at end of Telophase II
a. Male - 4 haploid cells - SPERMATIDS
b. Female - 1 OOTID and 3 POLAR BODIES (die) - ootid has most of the cytoplasm.
6. SPERMATIDS - mature to form sperm; OOTID - matures to form egg.
7. Meiosis important in that chromosome number remains constant in sexual reproduction
a. Egg (n) + Sperm (n) -- Fertilization --> Zygote (2n)
b. Chromosome number of zygote is thus the same as the two parents.
H. Types of Reproduction
1. ASEXUAL - production of offspring from one parent
a. No fusion of gametes
b. Offspring is identical to parent.
c. Occurs due to mitosis.
d. No genetic variation between parent and offspring
2. SEXUAL - production of offspring through meiosis and subsequent fusion of gametes from two
parents
a. Offspring is different from either parent
b. Half of genetic material has come from one parent; other half has come from the other parent.
c. Introduces variation between the parents and offspring.
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