Cell Division

Cell Division

Nucleic AcidsDeoxyribonucleic Acid - DNA Found primarily in the nucleus in chromosomes - stores

and transmit information to make proteins. Structure

Consists of two strands of nucleotide monomers Parts of a nucleotide Deoxyribose - 5-carbon sugar Phosphate group Nitrogen base

Nucleic Acids

Nucleic Acids

Nucleic AcidsNitrogen base Purine - nitrogen base with a double ring of

carbon and nitrogen atoms. AdenineGuanine

Pyrimidine - nitrogen base having a single ring of carbon and nitrogen atoms. ThymineCytosine

Nucleic Acids

Nucleic AcidsTwo strands of nucleotides twist around a central axis to form

a double helixFirst described by Watson and Crick in 1953. Similar to a twisted ladder Sides of alternating sugar and phosphateRungs consist of pairs of nitrogen bases - equal in length. Purine always pairs with a pyrimidine -have hydrogen bonds. Adenine bonds with thymine Guanine bonds with cytosine

Nucleic Acids

Nucleic Acids

Nucleic Acids

Nucleic Acids

Base sequence one strand is exact complement of base sequence in second strand.

1st Strand Sequence A - G - C - T - T - A - G - C

2nd Strand Sequence T - C - G - A - A - T - C - G

Nucleic Acids

Nucleic Acids

Replication

Process of duplicating the DNA molecule

Each strand serves as a template or mold for new complementary strand to be built.

Nucleic Acids

Process of Replication

DNA helicase attaches to DNA molecule - "unzips" the 2 strands - breaks hydrogen bonds between bases.

Unpaired bases of strands react with complementary bases of nucleotides in nucleus - hydrogen bonds form.

DNA polymerase catalyzes formation of sugar to phosphate bonds - connect one nucleotide to the next.

Result - two new DNA molecules - each consists of 1 "old" strand and 1 "new" strand.

Process doesn't begin at one end and proceed to the other - may occur simultaneously at many points.

Nucleic Acids

Nucleic Acids

DNA may be damaged by body heat, radiation, chemicals, etc.

Cell can "proofread" for mistakes

Can be repaired and corrected

Nucleic AcidsRibonucleic Acid – RNA

Nucleic acid that uses information stored in DNA to synthesize proteins.

StructureConsists of a single strand of nucleotide monomers.Parts of nucleotideRibose - 5-carbon sugarPhosphate groupNitrogen base

Nucleic AcidsNitrogen bases of RNA

PurinesAdenineGuanine

PyrimidinesUracilCytosine

Nucleic Acids

RNA differs from DNA in 3 ways RNA consists of only one strand of

nucleotides instead of the 2 strands of DNA. RNA contains the 5-carbon sugar, ribose,

instead of deoxyribose RNA contains the nitrogen base, uracil,

instead of thymine.

Nucleic Acids

Nucleic Acids

Nucleic AcidsTypes of RNA Messenger RNA (mRNA) - single, uncoiled strand,

transmits information from DNA for protein synthesis; acts as template for amino acid assembly during protein synthesis at ribosomes.

Transfer (tRNA) - single strand of RNA folded back on itself in hairpin fashion; allows some bases pairing; exists in 20 or more varieties each for only 1 specific type of amino acid.

Ribosomal RNA (rRNA) - globular form of RNA; major constituent of ribosomes.

Nucleic Acids Transcription

Process where mRNA is produced from the DNA - transcribed according to the information coded in the base sequence of DNA. Base sequence of mRNA complementary to sequence of DNA from which it was transcribed. Directed by enzyme, RNA polymerase

Process Enzyme binds to DNA causing strands to separate. Hydrogen bonds to form between DNA template and complementary RNA nucleotide bases. Enzyme moves to next section of DNA, bonds form between phosphate groups and ribose. RNA molecule released when enzyme reaches DNA sequence that acts as a termination signal.

Nucleic Acids

Nucleic AcidsProtein Synthesis Structural and functional characteristics of proteins are determined by the

sequence of amino acids in the protein

Sequence of amino acids in a protein encoded in DNA

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. Codon - series of three bases in mRNA that codes for a specific amino acid. Anticodon - series of three bases in tRNA, complementary to a codon; pairs with the codon during translation

Nucleic AcidsTranslation - process where protein molecules are made from

information encoded in mRNA. mRNA moves out of nucleus through a nuclear pore mRNA migrates to a group of ribosomes Amino acids in the cytoplasm are transported to the ribosomes by tRNA -

each specific for an amino acid. Assembly of polypeptide begins when A - U - G codon attaches to the

ribosome. Codon pairs with its anticodon adding specific amino acid to the growing

polypeptide chain. Process continues until a "stop" codon reaches the ribosome. mRNA is released and polypeptide is complete and released

Nucleic Acids

Nucleic AcidsGene - region of DNA that directs the

formation of a polypeptide. Proteins usually consist of more than

one polypeptide.

Several genes may direct protein synthesis

Cell DivisionProcess by which cells reproduce. Cell Theory

Cell is basic unit of life Living things are made of cells or cell fragments. All cells come form existing cells.

Reasons Cells Divide Maintenance Repair Growth Reproduction

Cell DivisionNucleus Organelle that directs the everyday metabolic activities of the cell. Composed of Chromatin

Normally - grainy mass of material During cell division appear as bodies called chromosomes. Composed of DNA and protein Carries the "genetic code" Individual heredity units are genes - determine cell characteristics and how cell functions - by proteins that are synthesized.

Each time the cell divides the genetic material must be replicated – makes exact copy Must then be distributed to the new cell. Replication occurs during interphase.

Cell Division

Cell DivisionChromosome Number The number of chromosomes found in the nucleus.

Varies from one organism to another. Man - 46 Fruit Fly - 8 Bullfrog - 26

All cells of an organism will have the same number of chromosomes.

Exception is the sex cells or Gametes Egg - female Sperm - male

Body cells called Somatic Cells or Somates.

Cell DivisionHumans 23 pairs of chromosomes - total of 46 chromosomes. Pairs of chromosomes are known as being Homologous.

Individual members of the pair are called Homologs. Get one homolog of each pair from each parent Each homolog of pair has same size and shape.

Number of pairs of chromosomes in a somate is expressed as "n" - total number of chromosomes expressed as "2n" Humans: n = 23; 2n = 46

Total number of chromosomes is Diploid Number = 2n Total number of different pairs in cell is its Haploid Number

(Monoploid) = n Features of the cell's chromosomes including size and number make

up cell's Karyotype - arranged by size and shape.

Cell DivisionActually talking about division of nuclear

material

Two types of nuclear division MitosisMeiosis

Cytokinesis - division of the cytoplasm

MitosisProcess by which the nucleus divides to produce 2 new nuclei, each with the same

number of chromosomes as the parent nucleus. From the Greek - Mitos - Thread Occurs in somatic cells.

Requirements Precise replication of the genetic material Distribution of a complete set of chromosomes to each new cell - called Daughter Cells

Five Phases of Cell Cycle - continuous Interphase Prophase Metaphase Anaphase Telophase

MitosisInterphase – not really a part of mitosis

Once called the "resting" stage Cell performing various metabolic activities Occupies about 2/3 of cell's life cycle Genetic material replicates during phase. Nucleus clearly defined by membrane Chromosomes not visible - chromatin appears grainy. Consists of 3 subphases

G1 Phase - first phase of interphase; cell doubles in size; enzymes and organelles,roughly double in size. S Phase - DNA in the chromatin replicates G2 Phase - Cell undergoes rapid growth that prepares it for mitosis, synthesizing necessary enzymes and structures.

Cell Division

MitosisProphase

Early prophase - chromatin coils; forms chromosomes. Chromosomes appear as rod-like structures. Nucleolus and nuclear membrane breakdown and disappear. Centrioles appear next to the disappearing nucleus - move to

opposite ends or poles of cell (not plants). Spindle fibers form - asters radiate from centrioles in animal

cells. Chromosomes appear as double stranded structure

Each strand is a Chromatid Two chromatids are joined at a Centromere.

Mitosis

MitosisMetaphase

Chromosomes arrange themselves on equator of spindle

Chromosomes attach to spindle fibers at the centromere.

Centromeres aligned on the equator.

Mitosis

MitosisAnaphase Centromere divides - one chromatid

moves toward 1 pole of spindle while other chromatid moves to the opposite pole.

Mitosis

MitosisTelophase

Two identical sets of chromatids are clustered at opposite ends of the cell.

Centrioles and spindle fibers disappear.

Chromatids unwind and elongate into chromatin - chromosomes disappear

Nuclear membrane and nucleolus reappear.

Mitosis

Mitosis Process completed with cytokinesis - get two

distinct cells Animals – Cleavage Furrow forms from

outside toward the center – pinches cell in two.

Plants – Cell Plate forms from center and grows to the outside.

No reduction in chromosome number Parent Cell = 2n ----> Each Daughter Cell =

2n

Mitosis

MeiosisProcess by which the cell nucleus divides resulting in a reduction of

chromosome number from the diploid number (2n) to the haploid number (n)

Involves 2 divisions Chromosome number reduced in first division - Meiosis I Second division - mitotic - Meiosis II

Cell division that results in formation of gametes or sex cells. Occurs in the Gonads (sex organs) Ovary - female Testes - male Process in males - called Spermatogenesis Process in females - called Oogenesis

MeiosisPhases

Interphase Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II

Meiosis

Interphase - same as in mitosis

Chromosomes replicate

MeiosisProphase I DNA strands coil, shorten, and thicken- Chromosomes become

visible. Nuclear membrane disappears, spindle fibers appear. Homologous pairs of chromosomes move together - Homologs

pair Pairing movement is called Synapsis Two double stranded chromosomes are so close they appear to be joined at their centromeres. Looks like 4 chromatids connected – group called a Tetrad. Crossing Over of chromatids may occur during synapsis

Meiosis

Meiosis

Metaphase I Tetrads align on the equator of the cell

Have 2 chromosomes with 4 chromatids attached to single spindle fiber.

Meiosis

Anaphase I Homologous pairs of chromosomes separate -

one chromosome of the pair goes to each pole - centromeres do not divide.

Each chromosome still consists of 2 chromatids joined by a centromere.

Meiosis

Telophase I Cytoplasm divides to form 2 daughter cells. Nuclear membrane begins to reappear, double

stranded chromosomes disappear; spindle may disappear.

Each daughter cell contains one half the original number of chromosomes that parent cell had.

Cells may enter resting state – Interkinesis

Meiosis

Prophase II No further replication of DNA occurs

Spindle reappears; chromosomes reappear

Meiosis

Metaphase II Chromosomes move to cell's equator. Centromere attaches to spindle fiber. Each chromosome composed of sister

chromatids joined at centromere.

Meiosis

Anaphase II Centromere joining chromatids divides. Each chromatid moves toward the

opposite pole.

Meiosis

Telophase II Spindle disappears Nuclear membrane forms around

chromosomes of each daughter cell. Each of the 4 cells formed from the original

parent cell has 1/2 the number of chromosomes of the parent.

Meiosis

Summary First Meiotic Division - produces 2 haploid

cells; double stranded chromosomes. Second Meiotic Division - 2 haploid cells

divide to produce 4 haploid cells. 4 haploid cells can develop into gametes.

Spermatogenesis and Oogenesis

Spermatogenesis - Production of sperm Oogenesis - Production of egg


Cells at the end of Prophase I

Male - Primary Spermatocyte – at puberty Female - Primary Oocyte - formed as early as

3rd month of development - meiosis stops until sexual maturity


Two cells at the end of Telophase I Male - secondary spermatocyte

Female - cells unequal - 1 secondary oocyte, 1 polar body


Cells at end of Telophase II Male - 4 haploid cells – Spermatids

Female - 1 Ootid and 3 Polar Bodies (die) - ootid has most of the cytoplasm.


Spermatids - mature to form sperm

Ootid - matures to form egg.


Importance

Ensures that chromosome number remains constant in sexual reproduction

Egg (n) + Sperm (n) -- Fertilization --> Zygote (2n)

Chromosome number of zygote is thus the same as the two parents.

Types of Reproduction

Asexual - production of offspring from one parent

No fusion of gametes Offspring is identical to parent. Occurs due to mitosis. No genetic variation between parent

and offspring

Types of Reproduction

Sexual - production of offspring through meiosis and subsequent fusion of gametes from two parents

Offspring is different from either parent Half of genetic material has come from one

parent; other half has come from the other parent.

Introduces variation between the parents and offspring.

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Cell Division