Download Document

B2 Cell division: gametes, body, mitosis, once,
repair, asexual, copied, growth, two, testes,
twice, differentiate, four, fertilisation, gamete,
genetic, random, ovaries, identical
Mitosis
Body cells divide by mitosis to
form new cells needed for growth
and repair.
First each chromosome is copied
(the DNA molecules replicate). The
cell then divides once to make two
genetically identical cells.
Meiosis
Gametes (sex cells) are made by
meiosis. In humans sperm are
made in the testes and eggs are
made in ovaries.
First each chromosome is copied
(the DNA molecules replicate).
The cell then divides twice to
make four gametes, each with one set of chromosomes
(23 in human gametes).
Fertilisation
Gametes join at fertilisation forming a single body cell. This cell
then divides by mitosis repeatedly.
Most types of animal cells differentiate at an early stage and
become specialised cells (eg nerve cells, muscle cells)
Asexual vs sexual reproduction
Some organisms can reproduce using a single parent and no
gametes. Cells of offspring made by asexual reproduction are
produced by mitosis, so they are genetically identical to their
parents (no variation in alleles).
Sexual reproduction results in genetic variation because…
a) Gametes fuse at random.
b) Every gamete is unique, with a different combination of
alleles.
B2 Chromosomes and stem cells: DNA, cancer, nucleus,
46, helix, 23, gene, alleles, embryos, animals, medical,
long, plant
Chromosomes
Most body cells have a nucleus containing two
sets of chromosomes. Human cells have 23
pairs, making a total of 46.
Chromosomes contain genetic
information and are made
from DNA. Each DNA
molecule has a double helix structure.
Stem cells
Stem cells from human embryos and adult bone
marrow can differentiate into many different
types of cells.
Treatment with stem cells
may be able to help conditions
such as paralysis.
A short section of chromosome DNA which
codes for a characteristic is called a gene.
Genes can exist as different forms called
alleles, eg. the eye colour gene has a blue (b)
allele and brown (B) allele.
Differentiation.
When cells become specialised, they are said to
have differentiated. In animals, most cells
differentiate at an early stage (in the embryo).
Many plant cells can differentiate throughout life.
Say NO
to stem cells:
1. Destroys embryos
which have a right to
life.
2. Unknown long term
effects – stem cells
may cause cancer.
3. Money could be
spent on other
medical research
Say YES
to stem cells:
1. May cure
paralysis.
2. Makes use of
spare IVF
embryos.