The Cellular Basis of Reproduction and Inheritance

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The Cellular Basis of Reproduction and Inheritance

• Chapter 8

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Reproduction

• Entails the creation of offspring carrying
  genetic information from their parents
• Two types
• Sexual- involves the union of a sperm and an egg
• Offspring inherit traits from both parents
• Asexual- production of offspring without the
  participation of sperm and egg
• Offspring inherit traits from only one parent

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The Life Cycle

• The life cycle of a multicellular organism
  includes
• Development- the sequence of life stages leading
  from the adults of one generation to the adults
  of the next
• Reproduction- formation of new individuals from
  preexisting ones

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Asexual Reproduction

• Parent cell divides and two daughter cells are
  created
• Chromosomes and DNA are duplicated
• 2 daughter cells are identical to each other and
  to the parent

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Sexual Reproduction

• Offspring produced generally resemble the parent
  but are not identical to the parents or to each
  other
• Each offspring inherits a unique set of genes
  from the parent
• Highly varied

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Cells arise only from preexisting cells

• All cells come from cells
• Cellular reproduction is called cell division
• Cell division allows an embryo to develop into an
  adult
• It also ensures the continuity of life from one
  generation to the next

http//www.microscopy-uk.org.uk/micropolitan/fresh
/desmid/frame2.html
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Cells arise only from preexisting cells

• Roles of cell division
• Asexual reproduction
• Reproduction of an entire single-celled organism
• Growth of a multicellular organism
• Growth from a fertilized egg into an adult
• Repair and replacement of cells in an adult
• Sexual reproduction
• Sperm and egg production

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Binary Fission

• Prokaryotes reproduce by binary fission, or
  dividing in half
• These cells possess a single chromosome,
  containing genes
• The chromosome is replicated
• The cell then divides into two cells, a process
  called binary fission

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Eukaryotic Cell Division

• A eukaryotic cell has many more genes than a
  prokaryotic cell
• The genes are grouped into multiple chromosomes,
  found in the nucleus
• Chromosomes contain a very long DNA molecule with
  thousands of genes

http//www.botany.org/PlantImages/ImageData.asp?ID
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Chromosomes

• Individual chromosomes are only visibleduring
  cell division
• They are packaged as chromatin
• Before a cell starts dividing, the chromosomes
  are duplicated
• This process produces sister chromatids

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Sister Chromatids

• When the cell divides, the sister chromatids
  separate
• Two daughter cells are produced
• Each has a complete and identical set of
  chromosomes

http//faculty.uca.edu/benw/biol1400/notes14.htm
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The Cell Cycle

• Orderly sequence of events which consists of two
  major phases

• Interphase, where chromosomes duplicate and cell
  parts are made
• The mitotic phase, when cell division occurs

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Interphase

• Majority of the cells time is spent in interphase
• Cells activity is very high
• Various metabolic activities
• Duplicates chromosomes
• Cell parts made, proteins, organelles
• Preparation for mitotic division
• 3 phases G1(Gap 1), S (DNA Synthesis), and G2

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Mitotic Phase

• 2 processes
• Mitosis- nucleus and its contents divide and are
  evenly distributed to form two daughter nuclei
• Cytokinesis- division of the cytoplasm into two
  daughter cells

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The Four Stages of Mitosis

• Prophase
• Chromatin coils into distinct chromosomes
• Sister chromosomes pair and move towards center
  of cell
• Nucleolus disappears, nuclear envelope fragments
• Mitotic spindle begins to form

Mitotic Spindle
Nuclear membrane
Sister Chromosomes
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Prometaphase

• Spindle microtubules reach chromosomes and attach
• Move chromosomes to center
• Nuclear envelope disappears

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The Four Stages of Mitosis

• Metaphase-
• The sister chromatids line up in the center of
  the cell
• The spindle fibers form and attach in the center
  of the chromatids in the centromere

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The Four Stages of Mitosis

• Anaphase-
• The sister chromatids then separate and move to
  opposite poles of the cell
• The spindle fibers from the mitotic spindle pull
  them apart

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The Four Stages of Mitosis

• Telophase-
• Spindle fibers disintegrate
• Chromosomes unwind
• Nuclear envelope reforms, nucleus reforms
• Cytokinesis splits the cytoplasm
• In plants, new cell wall is formed

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Cytokinesis

• In animals, cytokinesis occurs by cleavage
• Ring of microfiliments forms around the
  circumference of the cell
• The ring then contracts
• This process pinches the cell apart

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Cytokinesis

• In plants, a membranous cell plate splits the
  cell in two
• Vesicles from the golgi deposit cell wall
  material into the center
• The vesicles then fuse into a cell plate which
  spans the cell

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Cell Growth Factors

• Cells must be able to control growth and
  development in order for an organism to grow
  normally
• In laboratory cultures, most normal cells divide
  only when attached to a surface
• They are anchorage dependent, this keeps cells
  from dividing in the body while detached
• Cells continue dividing until they touch one
  another
• This is called density-dependent inhibition, it
  keeps cells from overgrowing their organs

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Growth Factors

• Inadequate supplies of certain growth factor
  proteins may be the cause of density-dependant
  inhibition
• A growth factor is a protein secreted by certain
  body cells that stimulate cells in the vicinity
  to divide
• These signals affect critical checkpoints
  determine whether the cell will go through a
  complete cycle and divide

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Growth Factors

• The binding of growth factors to specific
  receptors on the plasma membrane is usually
  necessary for cell division

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Cancer Cells

• Cancer cells have abnormal cell cycles
• They divide excessively and can form abnormal
  masses called tumors
• Radiation and chemotherapy are effective as
  cancer treatments because they interfere with
  cell division
• Malignant tumors can invade other tissues and may
  kill the organism

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Functions of Mitosis

• Growth-
• Roots continue to grow in soil
• Hair continues to grow on your head
• New leaves develop on trees in the fall
• Seeds and embryos develop into mature beings

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Functions of Mitosis

• Cell replacement
• Skin replacement
• Healing and scarring
• Starfish
• Asexual Reproduction
• Cuttings
• Runners
• Amoebas
• Hydras

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Homologous Chromosomes

• In humans a typical body cell, somatic cell, has
  46 chromosomes
• 23 matched pairs (4 chromosomes all together),
  each set of chromosomes has a twin nearly
  identical in length and centromere position
• These matched pairs are called homolgous
  chromosomes

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Homologous Chromosomes

• Both carry the genes controlling the same
  inherited characteristics
• Both have the gene controlling the characteristic
  but they may have a different version of that
  gene
• One has the blue eye version, the other the brown
  eye version

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Sex Chromosomes

• Of the 23 pairs
• 22 pairs are autosomes-found in both males and
  females
• The other pair are sex chromosomes that determine
  gender
• Females have a pair of X chromosomes
• Males have an X chromosome and a Y chromosome
• X and Y chromosomes differ in size and shape

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Gametes

• Cells with two sets of chromosomes are said to be
  diploid
• Gametes are the sex cells sperm and eggs
• Gametes are haploid, with only one set of
  chromosomes
• Gametes are formed by a process called meiosis

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Meiosis

• Meiosis, like mitosis, is preceded by chromosome
  duplication
• However, in meiosis the cell divides twice to
  form four daughter cells
• In the first division, meiosis I, homologous
  chromosomes are paired
• While they are paired, they cross over and
  exchange genetic information
• The homologous pairs are then separated, and two
  daughter cells are produced

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Meiosis reduces the chromosome number from
diploid to haploid

• Events in the nucleus during meiosis I
• Prophase I
• Chromosomes coil and become compact
• Homologous chromosomes come together as pairs by
  synapsis
• Each pair, with four chromatids, is called a
  tetrad
• Nonsister chromatids exchange genetic material by
  crossing over

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Meiosis reduces the chromosome number from
diploid to haploid

• Metaphase I
• Tetrads align at the cell equator
• Anaphase I
• Homologous pairs separate and move toward
  opposite
• poles of the cell
• Telophase I
• Duplicated chromosomes have reached the poles
• A nuclear envelope forms around chromosomes in
  some species
• Each nucleus has the haploid number of chromosomes

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Meiosis reduces the chromosome number from
diploid to haploid

• Meiosis II follows meiosis I without chromosome
  duplication
• Each of the two haploid products enters meiosis
  II
• Events in the nucleus during meiosis II
• Prophase II
• Chromosomes coil and become compact
• Metaphase II
• Duplicated chromosomes align at the cell equator

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Meiosis reduces the chromosome number from
diploid to haploid

• Anaphase II
• Sister chromatids separate and chromosomes move
  toward opposite poles
• Telophase II
• Chromosomes have reached the poles of the cell
• A nuclear envelope forms around each set of
  chromosomes
• With cytokinesis, four haploid cells are produced

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Meiosis I
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Meiosis II

• Meiosis II is essentially the same as mitosis
• The sister chromatids of each chromosome separate
• The result is four haploid daughter cells

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Mitosis vs Meiosis
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Causes of Genetic Variation

• 1. Different homologous chromosomes
• Each chromosome of a homologous pair comes from a
  different parent
• Each chromosome thus differs at many points from
  the other member of the pair
• The large number of possible arrangements of
  chromosome pairs at metaphase I of meiosis leads
  to many different combinations of chromosomes in
  gametes
• Random fertilization also increases variation in
  offspring

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Causes of Genetic Variation
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Causes of Genetic Variation

• 2. Different versions of the same gene
• The differences between homologous chromosomes
  are based on the fact that they can carry
  different versions of a gene at corresponding
  loci
• One chromosome carries one version of a gene, the
  other carries another

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Causes of Genetic Variation
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3. Crossing Over

• Crossing over- the exchange of corresponding
  segments between two homologous chromosomes
• Chiasma- sites of crossing over
• During synapsis (when the homologous chromosomes
  are lined up together) the chromosomes may
  overlap
• When these segments overlap, the overlapping
  segments may be detached and re-attached to the
  opposite chromosome

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Crossing Over
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Alterations of Chromosome Number

• Errors in meiosis can lead to gametes containing
  abnormal chromosome numbers
• This results in offspring with abnormal
  chromosome numbers and abnormalities
• An extra copy of chromosome number 21 causes Down
  syndrome

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Errors in Meiosis

• Nondisjunction- the members of a chromosome pair
  fail to separate
• Can be in meiosis I or II

Either homologous pairs fail to separate
during meiosis I
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Errors in Meiosis

• Sister chromatids fail to separate during meiosis
  II
• Fertilization after nondisjunction in the mother
  results in a zygote with an extra chromosome

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Alterations of chromosome structure can cause
birth defects and cancer

• Structure changes result from breakage and
  rejoining of chromosome segments
• Deletion is the loss of a chromosome segment
• Duplication is the repeat of a chromosome segment
  
• Inversion is the reversal of a chromosome segment
• Translocation is the attachment of a segment to a
  nonhomologous chromosome can be reciprocal
• Altered chromosomes carried by gametes cause
  birth defects
• Chromosomal alterations in somatic cells can
  cause cancer

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Chromosome Breakage

• Chromosome breakage can lead to rearrangements
  that can produce genetic disorders or cancer
• Chromosomal changes in a somatic cell can cause
  cancer

• A chromosomal translocation in the bone marrow is
  associated with chronic myelogenous leukemia