Genetics and Cellular Reproduction

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Genetics and Cellular Reproduction
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Cell Reproduction

• All living things must reproduce to keep species
  alive.
• This is a major driving force for zoology
  -Evolution
• These two facts, reproduction and passing traits
  necessary for success on to offspring, are
  essentially interelated.

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Cell Reproduction

• Cell reproduction serves different functions for
  different organisms.
• Unicellular organisms - producing a copy of
  itself.
• Multicellular organisms - growth, repair of
  damaged structures, development of new
  structures, and most importantly
• Formation of gametes!

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Cell Reproduction

• For reproduction to occur, there must be a set of
  instructions.
• These instructions are carried in the code of
  DNA.
• DNA codes for specific proteins
• Proteins make EVERYTHING!

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Eukaryotic Chromosomes

• DNA that is responsible for the code in
  organisms, is contained in the form of
  chromosomes.
• When in the chromatin state, the units of
  inheritance called genes can actively participate
  in protein production.
• Chromatin consists of DNA and histone proteins.
• The proteins serve as a coiling point for the DNA
  and when combined becomes the nucleosome.

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Types of Chromosomes

• All of the cells in the body contain chromosomes
  and the same genetic information.
• Somatic cells (body cells) have the code for male
  and female.
• However, these cells are not responsible for the
  reproduction of the organism.
• Sex cells are the cells that actually determine
  sex in the organism.

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Chromosome Numbers

• The number of chromosomes in a species is
  constant.
• However, the number of chromosomes between
  species varies greatly.
• As seen in the picture, the chromosomes are
  present in pairs.
• If N represents the number of different
  chromosomes, most animals have two sets and are
  2N.

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Chromosome Numbers

• This 2N condition is what we call diploid in
  animals.
• Some animals may only have one set of
  chromosomes, which we refer to as haploid.
• Even more bizarre and rare, some animals have
  more than two sets of chromosomes, which we refer
  to as polyploidy.

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Mitosis

• There are two types of cellular division
• Mitosis
• Meiosis
• Mitosis - the type of division necessary for
  growth and repair processes.
• Involves two stages, mitosis (dividing of the
  nucleus) and cytokinesis (dividing of the
  cytoplasm).

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Mitosis

• The normal cell cycle consists of distinct
  phases.
• We are mainly concerned with everything but
  interphase.
• Mitosis is divided into four specific stages.
• Prophase
• Metaphase
• Anaphase
• Telophase

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Mitosis

• Prophase - first stage of mitosis begins when
  the cells become visible.
• Nuclear envelope begins to dissolve and the
  centrioles begin to move apart.
• The centrioles, asters, and microtubules once
  formed make up the mitotic spindle.

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Mitosis

• The next phase for the dividing cell is
  metaphase.
• Chromatids begin to move towards the center of
  the cell.
• Near the end of this phase the centromeres will
  detach the two chromatids.

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Mitosis

• Once the cell has formed chromosomes, the cell
  moves into anaphase.
• The microtubules of the mitotic spindle begin to
  shorten.
• This phase ends when each chromosome reaches its
  respective pole.

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Mitosis

• Once the chromosomes are at opposite poles, the
  cell begins telophase.
• The mitotic spindle begins to disassembles, and
  the nuclear envelope reforms.
• The chromosomes begin to uncoil which allows for
  gene expression and the nucleolus is reformed.

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Mitosis

• The final step for this cell to complete its
  division is cytokinesis.
• The cytoplasm will begin to divide either late in
  anaphase or early telophase.
• The contractile ring will pinch the cell in half
  eventually forming two new cells.

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Meiosis

• We have already said that reproduction involves
  the combination of two different cells called
  gametes.
• Uniting of these gametes forms a zygote - the
  first new cell of the animal.
• To have the number of chromosomes remain constant
  in future reproductions, the reproducing animals
  must produce gametes with half the number of
  chromosomes as somatic cells.

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Meiosis

• Meiosis involves two nuclear divisions - Meiosis
  I and II.
• These divisions result in four daughter cells,
  each with half the number of chromosomes as
  parent cell.
• It is important to note that these daughter cells
  are NOT genetically identical!

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Meiosis

• Just as prophase was the first step in Mitosis,
  it is called prophase I in Meiosis.
• The chromosomes become visible.
• The cells contain a diploid number of chromosome
  at this point.

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Meiosis

• Crossing over - a series of events where
  non-sister chromatids of two homologous
  chromosomes exchange DNA.
• This allows for the exchange of genetic
  information which is a major source of genetic
  variation in populations.

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Meiosis

• Again, the second phase of this division is
  referred to as Metaphase I.
• The microtubules form spindles just as in
  mitosis.
• The difference in this division is that the
  chromosomes do not pair up like in mitosis.
• The chromosomes remain in the center of the cell
  unpaired.

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Meiosis

• Anaphase is when the homologous chromosomes move
  toward each pole.
• Each pole randomly receives a pair of homologues.
• This results in independent assortment of the
  homologues.

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Meiosis

• Just as in mitosis, there is a disassembly of the
  spindle.
• This is called Telophase I in meiosis.
• The transition to the second nuclear division is
  called interkinesis.

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Meiosis

• Meiosis differs from mitosis in that there is a
  second nuclear division.
• This division resembles the first division,
  except the number of chromosomes will be reduced
  by half.
• Repectively, the phases are Prophase II,
  Metaphase II, Anaphase II, and Telophase II.
• The final step is cytokinesis as in mitosis.

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Meiosis
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Meiosis

• The end result of this 2 phase nuclear division
  is the formation of two gametes (sperm and egg
  cells).
• Spermatogenesis results in the production of the
  sperm cell.
• Oogenesis produces a mature ovum or egg.
• Oogenesis differs slightly in that only one of
  the four cells produced in meiosis is functional.

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Modes of Reproduction

• There are many modes of reproduction in the
  animal kingdom
• Viviparous giving birth to live young (ex.
  Mammals)
• Oviparous animals lay their eggs (ex.
  Butterflies)
• Ovoviviparous eggs are held within the mother
  and hatched giving birth to young (ex. Sharks)

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Genetic Material

• For cells to divide, there has to be a molecule
  that can perform certain functions
• The substance must serve as a code for the amino
  acids and proteins.
• It must be able to copy itself.
• It must be able to be contained in the nucleus.
• It must be able to undergo changes over time to
  account for adaptations.

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Genetic Material

• This substance can be none other than DNA.
• DNA (deoxyribonucleic acid).
• Another genetic molecule just as important is RNA
  and it is responsible for protein synthesis.

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Genetic Material

• DNA and RNA are made up of molecules called
  nucleotides.
• A nucleotide consists of
• A nitrogen base (either a purine or pyrimidine).
• A pentose sugar
• A phosphate group
• There are some differences between DNA and RNA in
  terms of the nitrogen base and the sugar group.

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Genetic Material

• The DNA structure is ladder like with the rails
  consisting of alternating sugar and phosphate
  groups.
• The pairing of the nitrogenous bases is
  complimentary and is held by a hydrogen bond.
• Each strand of DNA is a template for a new
  strand. The complimentary structure of DNA
  allows for the coding of a new strand.

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Genetic Material

• For a trait to be expressed, there has to be a
  gene to code for that trait.
• A gene is a sequence in the bases of DNA that
  codes for the synthesis of a polypeptide.
• When RNA is synthesized from DNA it is called
  transciption.
• When RNA then forms a protein in the ribosome,
  the process is called translation.

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Genetic Material

• There are three types of RNA, each with a
  specific role in protein synthesis.
• mRNA is the strand that carries the instructions
  for proteins to the cytoplasm.
• tRNA pick up the amino acid in the cytoplasm and
  carries them to ribosome for formation of the
  polypeptide.
• rRNA and proteins make up the ribosome.

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Genetic Material

• There are 20 different amino acids found in
  living organisms.
• However, there are only 4 different nitrogen
  bases.
• How can we possibly code for these different
  amino acids?

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Genetic Information

• The genetic code is a triplet code or a three
  base code called a codon.
• There are a total of 64 possible codons coding
  for 20 amino acids.
• Some of the codons do not code for amino acids,
  but rather for stop and start signals.

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Transcription

• Information from DNA is transcribed into mRNA.
• This involves the unwinding of a portion of DNA
  and copying the code to mRNA.
• Unlike direct DNA transcription where an exact
  copy is made, with RNA only one or a few genes
  are exposed and only one of the two DNA strands
  is copied.

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Translation

• Translation is the process of protein synthesis
  in the ribosome in the cytoplasm.
• This occurs because of the information
  transcribed to mRNA.
• mRNA codes for different amino acids which are
  then aligned by tRNA so that a polypeptide can be
  made.

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Mutations

• DNA, while it has the ability to undergo
  replication, also is at risk for mistakes to be
  made during replication.
• These mistakes are another means of driving the
  evolutionary process.
• Mutations are changes in the sequencing of bases
  in DNA.

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Mutations

• Point mutations change result from changes in the
  base sequence of DNA.
• This may be due to replacement, addition, or
  deletion of a nucleotide base.

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Changes in Chromosome Number

• Sometimes the number of chromosomes can be
  changed.
• This may happen to an entire set of chromosomes
  or may happen to one particular set.
• Aneuploidy is the addition or deletion of one or
  more chromosomes.
• Another problem is when chromatids fail to
  separate during meiosis.
• This condition is referred to as nondisjunction.

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Changes in Chromosome Structure

• A final problem with chromosomes can occur when
  the chromosome breaks.
• This can result in pieces of the chromosome being
  lost or the piece may re-attach, but in the wrong
  direction.
• Either of these situations can result in the
  wrong protein being synthesized.

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Inheritance Patterns

• One of the main principles in genetics is
  segregation.
• Segregation is the idea that pairs of genes are
  distributed between gametes during gamete
  formation.
• This ultimately means that genes from each parent
  are mixed and passed onto offspring.

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Inheritance Patterns

• The genes that exhibit specific traits exist in
  alternative forms called alleles.
• Alleles can be present in dominant and recessive
  forms
• Dominant always shows up when present
• AA vs Aa