What is the cell cycle?
The series of events in which a cell grows, prepares for
division, and divides to form two daughter cells.
What are the 3 steps of the cell cycle?
Interphase, Mitosis, Cytokinesis
What are the 4 phases of Mitosis?
Prophase, Metaphase, Anaphase, Telophase
Why do cells go through the cell cycle?
Growth, Repair, or Asexual reproduction
The process in sexual reproduction in which an egg
cell and a sperm cell join to form a new cell (a new
organism)
Following fertilization, cell division begins and a new
organism grows from one cell to many cells.
Mendel discovered that “factors”
(genes) can control traits in offspring
 Scientists did not know where in the cell
genes were located

American geneticist (scientist who
studies genetics)
 1900s- studied chromosomes in
grasshopper cells
 Hypothesis: chromosomes are the key to
learning how offspring have traits similar
to those of their parents

Look at Page 94, Figure 1
How many chromosomes are there in the
body cells of a grasshopper?
24
How many chromosomes are there in a
grasshopper egg cell?
12
How many chromosomes are there in a
grasshopper sperm cell?
12
How is the chromosome number returned to
normal?
When a sperm and egg join in fertilization,
their individual 12 chromosomes add up to 24
chromosomes.
In your notebook, answer questions 1 & 2
1. How does the number of chromosomes
in grasshopper sex cells compare to the
number in body cells?
Sex cells have half the chromosomes
of body cells.
2. How is the inheritance of chromosomes
similar to what you know about alleles?
Just like alleles, one chromosome of
each pair is inherited from each parent.
When the egg and sperm combined, the
new cell had 24 chromosomes (the
same number as each parent’s body
cells)
 The 24 chromosomes existed as 12 pairs
 One chromosome in each pair comes
from male parent, the other from female
parent.


Remember: Mendel knew that alleles
exist in pairs in an organism
› one allele came from the male parent, one
allele from the female parent

Sutton realized that paired alleles are
carried on paired chromosomes
Genes pass from parents to their
offspring on chromosomes
Human body cells- 46 chromosomes (23
pairs)
 Chromosomes are made up of many
genes joined together like beads on a
string.
 Your body cells contain between 20,000
to 25,000 genes
 Genes control traits

Page 95,
Figure 2
Meiosis!
•
•
A cell divides into
two cells.
Then each of
these cells divides
again, forming a
total of four sex
cells.
•
•
DNA replication
only occurs
before the first
cell division
Each of the four
sex cells has half
the number of
chromosomes

https://www.youtube.com/watch?v=to
WK0fIyFlY

Sex cells

Two
Pages 96-97, Figure 3
How would you describe the shape of a
chromosome?
An X
What are the two sides of the X?
The chromatids
What holds the chromatids together?
The centromere
How do the chromatids compare?
They are identical
When do the chromatids separate?
During the second division
4 chromosomes
 DNA replication- 4
pairs of
chromosomes
 Divide into 2
daughter cells with 4
chromosomes (2
pairs)
 Divide into 4
daughter cells with 2
chromosomes

Chromosome
pairs line up
together and then move
apart

Chromosomes split into identical halves,
which move to new cells

http://www.cellsalive.com/meiosis_js.htm