Most humans have 46 chromosomes (23 pairs) in their body cells. They have only 23 chromsomes in their gametes. Why do you think gametes only have half the number of chromsomes?
Jan 02, 2016
Most humans have 46 chromosomes (23 pairs) in their body cells.
They have only 23 chromsomes in their gametes.
Why do you think gametes only have half the number of chromsomes?
Talk to the people around you about what a Punnett square is
used for.
Talk to the people around you about the difference between
heterozygous and homozygous.
Talk to the people around you about the Law of Segregation.
Talk to the people around you about the Law of Independent
Assortment.
• Genes do not exist free in the nucleus of a cell; they are lined up on chromosomes.
• Typically, a chromosome can contain a thousand or more genes along its length.
• Your body cells are diploid, and contain 46 chromosomes.
• A cell with two of each kind of chromosome is called a diploid cell.
Diploid Cells
• The two chromosomes of each pair in a diploid cell are called homologous chromosomes.
• Each pair of homologous chromosomes has genes for the same traits.
Homologous Chromosomes
• Organisms produce gametes (sex cells) that contain one of each kind of chromosome.
• A cell containing one of each kind of chromosome is called a haploid cell.
Haploid Cells
Chromosome Numbers of Common Organisms
Organism Body Cell (2n)
Fruit fly 8Garden pea 14Corn 20Tomato 24Leopard Frog 26 13Apple 34Human 46Chimpanzee 48Dog 78Adder’s tongue fern 1260
Gamete (n)47
1012
17232439
630
Diploid and Haploid Cells
In Humans, how many chromosomes would each of these cells have…..
CHEEK CELL
46
SPERM CELL
23
EGG CELL
23
NERVE CELL
46
Since they have different amounts of chromosomes, the way they are made differ.
● Body cells are made by the division process MITOSIS.
● Sex cells are made by the division process MEIOSIS.
Before a cell can divide by either method, it must first prepare for cell division.
The cell cycle (or entire life cycle of the cell) consists of:
INTERPHASE: The longest/busiest stage in a cell’s life
1. Chromosomes (DNA) replicates
2. Cell grows bigger
CELL DIVISION: Mitosis or Meiosis
CYTOKINESIS: When the cytoplasm divides and the cell completely separates.
Mitosis: cell division that makes BODY CELLS.
It starts with a DIPLOID cell 46
Chromosomes Replicate
92
4646
It ends with TWO IDENTICAL DIPLOID cells
PHASES OF MITOSIS (Paul Makes A Taco)
PROPHASE
1. Chromosomes coil up
2. Nuclear envelope disappears
3. Spindle fibers form
METAPHASE
1. Chromosomes Line up in the middle
2. Spindle attaches to chromosomes
PHASES OF MITOSIS (Paul Makes A Taco)
ANAPHASE
1. Chromosomes split and are pulled apart
TELOPHASE
1. Chromosomes uncoil
2. Nuclear envelopes reappear
3. Cell is splitting into two
Meiosis: cell division that makes SEX CELLS.
It starts with a DIPLOID cell 46
Chromosomes Replicate
92
46 46
23 23 23 23
It ends with FOUR HAPLOID cells (not identical)
• These haploid cells are called sex cells— gametes.
• Male gametes are called sperm.
• Female gametes are called eggs.
Meiosis
• When a sperm fertilizes an egg, the resulting zygote once again has the diploid number of chromosomes.
Meiosis
Crossing-Over Crossing-Over
In Meiosis:
When homologous chromosomes actually break and exchange genetic material- is a process known as crossing over.
Crossing-OverCrossing-Over
Crossing over causes variations in the alleles on each chromosome.
• The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction.
•This would occur when homologous chromosomes are suppose to separate from each other.
Nondisjunction
• This chart of chromosome pairs is called a karyotype, and it is valuable in identifying unusual chromosome
numbers in cells.
• Down syndrome is the only autosomal trisomy in which affected individuals
survive to adulthood.
Down syndrome: Trisomy 21
Down syndrome: Trisomy 21
• The incidence of Down syndrome births is higher in older mothers, especially those over 40.
• Extra chromosome on 21
• 1 in every 1100 live births
• Range from mild to severe physical and mental disabilities
• Many abnormalities in the number of sex chromosomes are known to exist.
Abnormal numbers of sex chromosomes
• Any individual with at least one Y chromosome is a male, and any individual without a Y chromosome is a
female.
Turners Syndrome
• Absence of one X chromosome
• Occurs in 1 out of every 2,500 live female births.
• Physical characteristics
• No mental disability
Abnormal numbers of sex chromosomes
• Most of these individuals lead normal lives, but they cannot have children and some have varying degrees of mental retardation.
XXX
“Trisomy X”
XXY
“Klinefelter’s”
XYY
“Jacob’s Syndrome”
A cell with two of each kind of chromosome is __________.
Question 1
D. polyploid
C. biploid
B. haploid
A. diploid
The answer is A. The two chromosomes of each pair in a diploid cell are called homologous chromosomes. Each has genes for the same traits.
Homologous Chromosome 4
a A
Terminal Axial
InflatedD
Constrictedd
Tall
T
Short
t
What is the importance of meiosis in sexual reproduction?
Question 2Meiosis
Meiosis
Sperm Cell
Egg Cell
Haploid gametes
(n=23)
Fertilization
Diploid zygote
(2n=46)
Mitosis and Development
Multicellular
diploid adults
(2n=46)
Meiosis is cell division that produces haploid gametes. If meiosis did not occur, each generation would have twice as many chromosomes as the preceding generation.
Meiosis
Meiosis
Sperm Cell
Egg Cell
Haploid gametes
(n=23)
Fertilization
Diploid zygote
(2n=46)
Mitosis and Development
Multicellular
diploid adults
(2n=46)
Trisomy usually results from _______.(TX Obj 2; 6C)
Question 2
D. twenty-two pairs of chromosomes
C. nondisjunction
B. incomplete dominance
A. polygenic inheritance
The answer is C.