1. Biology
Unit 8 – Cell Division
Notes #2 (Chapter 5)

2. 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
Lesson 1

3. 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
Lesson 1

4. 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
Lesson 1

5. Diploid Cells (cont.) The only one that can be different
The chromosome for
Female is “XX”, while
Male is XY

6. 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
Lesson 1

7. 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.
Lesson 1

8. The Phases of Meiosis (cont.)
There are four phases of meiosis I.
Lesson 1

9. Prophase 1 Similar as mitosis, except…
Mom 1
Dad 1
Similar as mitosis, except…
1) Synapsis: Pairing of homologous chromosomes
Dad’s #1 chromosome pairs with Mom’s #1, etc…
The four paired chromatids equals a TETRAD
2) Crossing Over: Exchange of genes

10. The Phases of Meiosis (cont.)
There are four phases of meiosis II.
Lesson 1

11. The Phases of Meiosis (cont.)

12. Lesson 1

13. 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.
Lesson 1

14. Why is meiosis important? (cont.)
The fertilized egg, formed when sex cells join together, divides by mitosis to create a diploid organism.
Lesson 1

15. 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
Lesson 1

16. How do mitosis and meiosis differ? (cont.)
During meiosis, a reproductive cell and its nucleus divide twice and produce four cells—two 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
Lesson 1

17. 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.
Lesson 1

18. Disadvantages of Sexual Reproduction
Organisms have to grow and develop until they are mature enough to produce sex cells.
Organisms must form sex cells—either eggs or sperm.
Lesson 1