Chromosomes and Cell Reproduction by HC120727041531


									Chromosomes and Cell
Chromosome structure and role in
development and reproduction
What allows a few cells to grow into a
human being?
During cell division, what allows each
cell to receive the proper genes?
• Identify examples of cell division in prokaryotes
  and eukaryotes
• Distinguish between a gene, a DNA molecule, a
  chromosome, and a chromatid
• Differentiate between homologous
  chromosomes, autosomes, and sex
• Compare haploid and diploid cells
• Predict how changes in chromosomes can affect
Why cell division?
•   Growth
•   Tissue repair
•   Replace worn out cells
•   Asexual reproduction
Why are we interested in
• Vitally important that each
  new cell receives the proper set
  of chromosomes to function

• Each organism has a
  characteristic number of

• We need to understand the
  structure of chromosomes and
  the role of chromosomes in the
Different types of cell division – all
involve passing on DNA
• Bacterial cell division – Binary Fission

• Division of body cells in eukaryotes - Mitosis

• Formation of gametes (reproductive cells) in
  eukaryotes - Meiosis
Prokaryotic Cell Division
• Called Binary Fission

• Form of asexual reproduction

• Single parent passes on exact
  copies of all DNA to offspring

• First stage = single circular
  DNA molecule is copied

• Second stage = Cell divides
Eukaryotic Cell Division
• Much more complicated
  process due to larger amount
  of genetic information (genes)
• Genes = segment of DNA that
  codes for a protein or RNA
• When genes being used, DNA
  (chromatin – DNA and its
  associated proteins) is
  stretched out
• Before cell division, DNA must
  divide and then coil up into
Chromatids and Centromere
                • Chromatids will become
                  separated during cell division

                • 1 Chromatid will be placed into
                  each new cell

                • This ensures that each new cell
                  will have the same genetic
                  material as the original cell
Chromosome Number and Structure
• Each human body cell
  (excluding sperm and egg
  cells) has 2 copies of 23
  different chromosomes

• Total chromosomes = 46

• Each chromosome contains
  thousands of genes

• Genes play important role in
  how person’s body develops
  and function
Homologous Chromosomes
• Each pair of chromosomes =
  homologous chromosomes

• Chromosomes similar in size,
  shape and genetic content

• Each homologue comes from
  one of the two parents

• One set = Ma
• One set = Pa
Somatic Vs Sex Cells =
Diploid Vs Haploid
• Body cells (non-sex cells) = somatic cells
• A cell that contains 2 sets of chromosomes (i.e. a somatic cell) =
  diploid or 2n
• A cell that contains only one set of chromosomes (i.e. a sex cell or
  gamete) = haploid or n
• For humans n = 23. That is how many chromosomes our gametes
  have. 2n = 46. That is how many chromosomes our somatic cells
• Fusion of two haploid
  gametes = fertilization

• Diploid zygote formed by
  fertilization = first cell of
  a new individual
Different Types of Chromosomes
Autosomes                        Sex Chromosomes
• Chromosomes that are not       • One of the 23 pairs of
  directly involved in             chromosomes in humans
  determining the sex (gender)   • Determine the sex of the
  of an individual                 individual
                                 • Humans and many other
                                   animals - X and Y
                                 • Anyone with a Y chromosome
                                   is male.
                                 • Female = XX; Male = XY
                                 • Female can only give an X, so
                                   sex of offspring determined by
Change in chromosome number
• Genes help determine how body
  develops and functions

• All 46 chromosomes must be
  present for normal

• Too many or too few
  chromosomes leads to problems

• Abnormalities in chromosome
  number detected by a karyotype
  - a photo of chromosomes in
  dividing cell - shows
  chromosomes arranged by size
Down Syndrome – Trisomy 21
• Extra copy of chromosome 21

• Short stature, round face,
  mental retardation

• Occurs more frequently in
  older mothers

• Mothers younger than 30 – 1
  in 1500 births

• Mothers over 45 – 1 in 46
How does someone end up with an
extra chromosome?
• When sperm and egg cells
  form (during meiosis) each
  homologous pair separates

• If chromosomes fail to
  separate (non-disjunction) one
  gamete ends up with both
  chromosomes and the other
  gamete gets none

• Trisomy – gamete with both
  chromosomes fuses with
  normal gamete
Changes in chromosome structure
• Mutations = changes in    • Duplication = chromosome
  chromosome structure        fragment attaches to
                              homologous chromosome
                              (which then carries 2 copies of
                              set of genes)
                            • Inversion = chromosome
                              fragment reattaches to original
• Chromosome breakage  4     chromosome but in reverse
  types of mutations          order
                            • Translocation =
                              chromosome fragment
                              reattaches to non-homologous
                            • Deletion = a piece of a
                              chromosome breaks off
Types of mutations
Inversion            Translocation
Types of Mutations
Deletion             Duplication

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