Stages of an Organisms Life Cycle:

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• Introduction
• Stages of an Organisms Life Cycle
• Development All changes that occur from a
  fertilized egg or an initial cell to an adult
  organism.
• Reproduction Production of offspring that carry
  genetic information in the form of DNA, from
  their parents.
• Two types of reproduction
• 1. Sexual Reproduction
• 2. Asexual Reproduction

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• Lecture 10
• The Cellular Basis of Reproduction and Inheritance

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1. Sexual Reproduction

• Most common type of animal reproduction.
• Male and female gametes or sex cells (sperm and
  egg cell) join together to create a fertilized
  egg or zygote.
• The offspring has genetic information from both
  parents.
• Offspring are genetically different from each
  parents and their siblings.
• Advantages
• Ensures genetic diversity of offspring.
• Population more likely to survive changing
  environment.
• Disadvantages
• Cannot reproduce without a partner of opposite
  sex.
• Considerable time, energy, and resources spent to
  find a suitable mate.
• Parents only pass on 1/2 (50) of their genetic
  information to each offspring.

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2. Asexual Reproduction

• Production of offspring by a single parent
  through
• Splitting Binary fission in bacteria.
• Budding Yeasts, plants
• Offspring inherit DNA form one parent only.
• Offspring are genetically identical to parent and
  siblings, unless
• mutations occur.
• Advantages
• Can reproduce without a partner of opposite sex.
• Dont spend time, energy, and resources to find a
  suitable mate.
• Parents pass on 100 of their genetic information
  to each offspring.
• Disadvantage
• No genetic diversity of offspring.
• Population less likely to survive changing
  environment.

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Cells Only Arise from Preexisting Cells

• New cells are made through cell division
• Unicellular organisms (Bacteria, protozoa)
  Division of one cell into two new organisms
  through binary fission or mitosis.
• Multicellular organisms (Plants, animals)
• 1. Growth and development from zygote or
  fertilized egg. Original cell divides by mitosis
  to produce many cells, that are genetically
  identical to first cell. Cells later develop
  specific functions (differentiation).
• 2. Reproduction requires Meiosis Special type
  of cell division that will generate gametes or
  sex cells, with 50 of individuals genetic
  material.

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Bacteria (Prokarytoes) Reproduce Asexually by
Binary Fission

• Features of Bacterial DNA
• Single, relatively small circular chromosome
• About 3-5 million nucleotide base pairs Contains
  only about 5-10,000 genes
• Binary fission
• Single circular DNA is replicated
• Bacterium grows to twice normal size
• Cell divides into two daughter cells
• Each daughter cell with an identical copy of DNA
  Rapid process, as little as 20 minutes.

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Bacteria Reproduce Asexually by Binary Fission
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Eukaryotic Cell Division

• Eukaryotic cell division is more complex and time
  consuming process than binary fission
• Features of Eukaryotic DNA
• 1. DNA is in multiple linear chromosomes. Unique
  number for each species Humans have 46
  chromosomes. Cabbage has 20, mosquito 6, and fern
  over 1000.
• 2. Large Genome Up to 3 billion base pairs
  (humans) Contains up to 50,000-150,000 genes
  Human genome project is determining the sequence
  of entire human DNA.
• 3. DNA is enclosed by nuclear membrane. Correct
  distribution of multiple chromosomes in each
  daughter cell requires a much more elaborate
  process than binary fission.

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Human Body Cells Have 46 Chromosomes
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• DNA Found as Chromosomes or Chromatin
• Chromosomes Chromatin
• Found only during cell division Found throughout
  cell cycle
• Tightly packaged DNA Unwound
  DNA
• DNA is not being used DNA is being used
• for macromolecule synthesis. for macromolecule
  synthesis.

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Eucaryotic Chromosomes Duplicate Before Each Cell
Division
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Cell Cycle of Eucaryotic Cells

• The Cell Cycle is the sequence of events from
  the time a cell is formed, until the cell divides
  once again.
• Before cell division, the cell must
• a) precisely copy genetic material (DNA)
• b) roughly double its cytoplasm
• c) synthesize organelles, membranes, proteins,
  and other molecules.
• The Cell cycle is divided into two main phases
  Interphase Stage between cell divisions
• Mitotic Phase Stage when cell is dividing

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Eucaryotic Cell Cycle Interphase Mitotic Phase
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The Life Cycle of a Eucaryotic Cell

• Interphase Time between cell divisions. Most
  cells spend about 90 of their time in
  interphase. Cells actively synthesize materials
  they need to grow. Chromosomes are duplicated.
• Interphase can be divided into three stages
• 1. G1 phase Just after cell division. Cell
  grows in size, increases number of organelles,
  and makes proteins needed for DNA synthesis.
• 2. S phase DNA replication. Single chromosomes
  are duplicated so they contain two sister
  chromatids.
• 3. G2 phase Just before cell division. Protein
  synthesis increases in preparation for cell
  division.

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Duplication of Chromosomes During S stage of
Interphase
DNA replication during S stage of Interphase
Single chromosome
Two identical sister chromatids joined by a
centromere ( )
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The Life Cycle of a Eukaryotic Cell

• Mitosis The process of eucaryotic cell division.
  Most cells spend less than 10 of time in
  mitosis.
• Mitosis is divided into four stages
• 1. Prophase Cell prepares for division.
• 2. Metaphase Chromosomes line up in middle of
  cell.
• 3. Anaphase Sister chromatids split and migrate
  to opposite sides of the cell.
• 4. Telophase DNA is equally divided into two new
  daughter cells. Cytokinesis usually occurs.
  Cytokinesis Division of cytoplasm.
• Mitotic Phase Mitosis Cytokinesis

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Mitotic Phase Mitosis Cytokinesis
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Mitotic Phase Mitosis Cytokinesis

• Cytokinesis The division of cytoplasm to produce
  two daughter cells.
• Usually begins during telophase.
• In animal cells Division is accomplished by a
  cleavage furrow that encircles the cell like a
  ring in the equator region.
• In plant cells Division is accomplished by the
  formation of a cell plate between the daughter
  cells. Each cell produces a plasma membrane and
  a cell wall on its side of the plate.

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Mitosis
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Mitotic Phase Mitosis Cytokinesis

• Cytokinesis The division of cytoplasm to produce
  two daughter cells.
• Usually begins during telophase.
• In animal cells Division is accomplished by a
  cleavage furrow that encircles the cell like a
  ring in the equator region.
• In plant cells Division is accomplished by the
  formation of a cell plate between the daughter
  cells. Each cell produces a plasma membrane and
  a cell wall on its side of the plate.

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Cytokinesis in Animal and Plant Cells
Animal Cell Plant Cell
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External Factors Control Mitosis

• 1. Anchorage Most cells cannot divide unless they
  are attached to a solid surface. May prevent
  inappropriate growth of detached cells
• 2. Nutrients and growth factors Lack of nutrients
  can limit mitosis Growth factors Proteins that
  stimulate cell division.
• 3. Cell density Density-dependent inhibition
  Cultured cells will stop dividing after a single
  layer covers the petri dish. Mitosis is
  inhibited by high cell density. Cancer cells do
  not demonstrate density inhibition

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Cell-Cycle Control System

• There are three critical points at which the cell
  cycle is controlled
• 1. G1 Checkpoint Prevents cell from entering S
  phase and duplicating DNA. Most important
  checkpoint. Amitotic cells (muscle and nerve
  cells) are frozen here.
• 2. G2 Checkpoint Prevents cell from entering
  mitosis.
• 3. M Checkpoint Prevents cell from entering
  cytokinesis.
• Cells must have proper growth factors to get
  through each checkpoint.

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Cell Division is Controlled at Three Key Stages
Growth factors are required to pass each
checkpoint
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Cancer is a Disease of the Cell Cycle

• Cancer kills 1 in 5 people in the United States.
• Cancer cells divide excessively and invade other
  body tissues.
• Tumor Abnormal mass of cells that originates
  from uncontrolled mitosis of a single cell.
• Benign tumor Cancer cells remain in original
  site. Can easily be removed or treated .
• Malignant tumor Cancer cells have ability to
  detach from tumor and spread to other organs or
  tissues
• Metastasis Spread of cancer cells form site of
  origin to another organ or tissue. Tumor cells
  travel through blood vessels or lymph nodes.

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Metastasis Cancer Cells Spread Throughout Body
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Functions of Mitosis in Eukaryotes

• Growth All cells that originate after a new
  individual is created are made by mitosis.
• Cell replacement Cells that are damaged or
  destroyed due to disease or injury are replaced
  through mitosis.
• Asexual Reproduction Mitosis is used by
  organisms that reproduce asexually to make
  offspring.

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Mitosis Replaces Dead Skin Cells
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Chromosome -1

• Chromosomes are structures that contain
  information
• Chromosomes come in pairs.
• Normal humans have 46 chromosomes in 23 pairs.
• One chromosome of each pair comes from an
  individuals mother, the other comes from the
  father.
• Homologous chromosomes carry genes that control
  the same characteristics. Examples Eye color,
  blood type, flower color, or height
• Locus Physical site on a chromosomes where a
  given gene is located.
• Allele Different forms of the same gene.
  Example Alleles for blood types A, B, or O.

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Homologous Chromosomes Code for the Same
Genetic Traits, but Have Different Alleles
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Chromosomes-2

• There are two types of chromosomes
• Autosomes - Found in both males and females. In
  humans there are 22 pairs of autosomes. Autosomes
  are of the same size and are homologous.
• Sex Chromosomes (X and Y)- Determine an
  individuals gender.
• The X and Y chromosomes are not homologous.
• The X chromosome is much larger than the Y
  chromosome and contains many genes.
• The Y chromosome has a small number of genes. In
  Humans and other mammals females are XX and males
  are XY.

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Normal Genetic Complement of Humans

• Females 44 autosomes (22 pairs) XX
• Males 44 autosomes (22 pairs) XY
• Note In most cases, having additional or
  missing chromosomes is usually fatal or causes
  serious defects.
• E.g Downs Syndrome Trisomy 21. Individuals
  with an extra chromosome 21. Most common
  chromosomal defect (1 in 700 births in U.S.).
  Mental retardation, mongoloid facial features,
  heart defects, etc.

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Chromosomes-3

• Humans have two sets of chromosomes.
• One inherited from each parent.
• Diploid Cells Cells whose nuclei contain two
  homologous sets of chromosomes (2n). Almost all
  cells in our body are diploid . In humans the
  diploid number (2n) is 46.
• Haploid Cells Cells whose nuclei contain a
  single set of chromosomes (n). Egg and sperm
  cells are haploid. In humans the haploid number
  (n) is 23.
• Fertilization Haploid egg fuses with a haploid
  sperm to form a diploid zygote (fertilized egg).

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Mitosis versus Meiosis

• Mitosis Meiosis
• One cell division Two successive cell
  divisions
• Produces two (2) cells Produces four (4) cells
• Produces diploid cells Produces haploid gametes
• Daughter cells are genetically Cells are
  genetically
• identical to mother cell different from mother
  cell and each other
• No crossing over Crossing over
• Functions Growth, Functions Reproduction
• cell replacement
• Crossing over Exchange of DNA between
  homologous chromosomes.

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Meiosis Generates haploid gametes

• Reduces the number of chromosomes by half,
  producing haploid cells from diploid cells.
• Also produces genetic variability, each gamete is
  different, ensuring that two offspring from the
  same parents are never identical.
• Two divisions Meiosis I and meiosis II.
  Chromosomes are duplicated in interphase prior to
  Meiosis I.
• Meiosis I Separates the members of each
  homologous pair of chromosomes. Reductive
  division.
• Meiosis II Separates chromatids into individual
  chromosomes.

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STAGES OF MEIOSIS
Interphase Chromosomes replicate
Meiosis I Reductive division. Homologous
chromosomes separate
Meiosis II Sister chromatids separate
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Meiosis I Separation of Homologous Chromosomes

• Prophase I (90 of time) Chromatin condenses
  into chromosomes. Nuclear membrane and nucleoli
  disappear. Centrosomes move to opposite poles of
  cell and microtubules attach to chromatids.
  Synapsis Homologous chromosomes pair up and
  form a tetrad of 4 sister chromatids. Crossing
  over DNA is exchanged between homologous
  chromosomes, resulting in genetic recombination.
  Unique to meiosis. Chiasmata Sites of DNA
  exchange.
• Metaphase I Chromosome tetrads (homologous
  chromosomes) line up in the middle of the cell.
  Each homologous chromosome faces opposite poles
  of the cell.

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Meiosis I
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Significance of Meiosis

• Sexual Reproduction if meiosis did not occur
  the fusion of gametes would lead to double the
  number if chromosomes, each generation.
• Genetic Variation Opportunity for new
  combination of genes in gametes.