MS Science

1
Biology Unit 8 Cell Division Notes 2
(Chapter 5)
2
Lesson 1
What is sexual reproduction?

• Sexual reproduction is a type of reproduction in
  which the genetic materials from two different
  cells combine, producing a genetically distinct
  offspring.
• Vs.
• Asexual reproduction IS genetically identical to
  the the parent

3
Lesson 1
Diploid Cells

• Organisms that reproduce sexually form body cells
  (skin, nerve, etc) and sex cells (sperm and
  egg).
• In body cells of most organisms, similar
  chromosomes occur in pairs.
• Ex. Human body cells have 46 chromosomes (23
  pairs)
• Diploid cells are cells that have pairs of
  chromosomes.
• Referred to as 2n

4
Lesson 1
Diploid Cells (cont.)

• Pairs of chromosomes that have genes for the same
  traits arranged in the same order are called
  homologous pairs
• The pairs are alike,
• even having
• centromeres in the
• same location

5
Diploid Cells (cont.)

• The only one that can be different
• The chromosome for
• Female is XX, while
• The chromosome for
• Male is XY

6
Lesson 1
Haploid Cells

• Haploid cells are cells that have only one
  chromosome from each pair of chromosomes.

Ex Human haploid cells (n) Total of 23
chromosomes called gametes (sex cells) Fusion
creates (n n) 2n Are produced by meiosis
7
Lesson 1
Haploid Cells (cont.)

• In meiosis, one diploid cell divides and makes
  four haploid sex cells.
• Meiosis occurs only during the formation of sex
  cells.
• Meiosis involves two divisions of the nucleus,
  meiosis I and meiosis II.
• A reproductive cell goes through interphase
  (replicating chromosomes) before beginning
  meiosis I.

8
Lesson 1
The Phases of Meiosis (cont.)

• There are four phases of meiosis I.

9
Prophase 1
Mom 1
Dad 1

• Similar as mitosis, except
• 1) Synapsis Pairing of homologous chromosomes
• Dads 1 chromosome pairs with Moms 1, etc
• The four paired chromatids equals a TETRAD
• 2) Crossing Over Exchange of genes

10
Lesson 1
The Phases of Meiosis (cont.)

• There are four phases of meiosis II.

11
The Phases of Meiosis (cont.)
12
Lesson 1
13
Lesson 1
Why is meiosis important?

• Meiosis forms sex cells with the correct haploid
  number of chromosomes.
• Meiosis also creates genetic variation by
  producing haploid cells.
• When haploid sex cells join together during
  fertilization, they make a diploid zygote, or
  fertilized egg.

14
Lesson 1
Why is meiosis important? (cont.)

• The fertilized egg, formed when sex cells join
  together, divides by mitosis to create a diploid
  organism.

15
Lesson 1
How are mitosis and meiosis the same?

• During mitosis and meiosis, cells and nuclei
  divide and produce new cells.
• Both have similar steps
• Replication of chromosomes
• Disappearance of nuclei
• Movement of
• chromosomes to
• opposite ends of the cell

16
Lesson 1
How do mitosis and meiosis differ? (cont.)

• During meiosis, a reproductive cell and its
  nucleus divide twice and produce four cellstwo
  pairs of identical haploid cells.
• Not genetically identical offspring
• Mitosis makes 2 diploid
• genetically identical offspring
• Meiosis forms sex cells used for sexual
  reproduction.
• Mitosis body cells

17
Lesson 1
Advantages of Sexual Reproduction

• Genetic variation occurs in all organisms that
  reproduce sexually.
• Due to genetic variation, individuals within a
  population have slight differences.
• Genetic variation may enable one plant to be more
  disease-resistant than another within the same
  species.

18
Lesson 1
Disadvantages of Sexual Reproduction

• Organisms have to grow and develop until they are
  mature enough to produce sex cells.
• Organisms must form sex cellseither eggs or
  sperm.