Transcript Cell Structure & Function

Inheritance and variation
• Introduction:
•
There are two kinds of
reproduction, sexual reproduction and
asexual reproduction.
• Sexual reproduction necessitates the
presence of two individuals male and
female for production of sexual gametes
.Sexual reproduction is a major source of
genetic variation.
-Asexual
reproduction
comprises isolation of a body
part either a spore cell or
tissues and their growth into a
new organism that fully
resembles the original from
which the above part has been
isolated.
functions of sexual reproduction.
•
Sexual reproduction allows genetic
material to be passed from one
generation to the next.
•
Sexual reproduction allows genetic
material from two individuals to be used
in generating unique combinations of
genes in the reproduction of new
offspring.
The offspring may possess new
characteristics that enhance their
successful survival in a changing
environment.
to be used in generating unique
combinations of genes e reproduction
of new offspring.
characteristics of gametes in humans,
as in all mammals•
Sperm and eggs are highly
specialised haploid cells.
• Sperm are relatively small.
They survive for a matter of only
hours or a few days in the
vagina.
•
Sperm are produced constantly
and in enormous numbers (400 million
in one ejaculation in man). There is a
very high mortality rate so they need to
be released in such high numbers.
•
The sperm head contains the
haploid nucleus. The head is tipped by
a special body, the acrosome, which
contains hydrolytic enzymes that help
the sperm penetrate the egg.
•
Sperm contain large numbers of
mitochondria behind the head. These
provide ATP for movement of the tail, a
flagellum.
•
Sperm have to swim to the egg
through the female reproductive tract.
•
Eggs are relatively large. They
have a large food reserve to nourish the
fertilised egg, the zygote, in its first days
•
•
Eggs are produced in relatively
low numbers in mammals. Primates
normally produce single eggs because
the survival rate of both the egg and the
baby is very high. In addition, a single
baby’s head (and brain) can achieve an
optimum size with little competition in
utero and the long period of
dependency of the baby on the parent
can more readily be satisfied with a
single child
•
•Eggs are non-motile as the sperm
swim to meet them and the eggs are
moved passively into position in the
uterus by the ciliated cells of the
oviduct.
•
-Asexual reproduction depends on
mitotic division of cells , where cells
resulting from this division receive a
complete copy (2N) of original
chromosomes.
•
--The nucleus of the plant or animal
cell contains chromosomes. Each
chromosome is formed of one molecule
of the nucleic acid DNA which extends
along the length of the chromosome in
the form of a thread coiled around some
basic proteins. The nucleic acid DNA
carries the genetic information of the
cell.
•
Chromosome is a double structure
made of two identical chromatides
,each chromatid contains one DNA
molecule ,appears in nucleus just
before cell division, it carries genes
..
•
-When the cell is about to divide,
each chromosome condenses and
become visible. Each chromosome
appears as two threads. Each of the
threads is called a chromatid and they
are joined together at a point called
centromere
..
•
-The gene is a part of DNA, a
sequence of nucleotide at a specific
locus ,which codes for just one
polypeptide.
..
•
-The number of chromosomes
remains constant within a certain
species , yet the number varies from
one species to another. For example in
Man number of chromosomes in
somatic cell is 46 chromosomes ,in pea
plant, the number of chromosomes is
14 ,in maize number of chromosomes
is 20.The cells of the maize plant that
have twenty(20)chromosomes are
called somatic cells
•and referred to as 2N because it
contains diploid number of
chromosomes and these cells are
produced from mitotic division. On the
other hand , the gametes (pollen grains,
ovules or sperm)are produced from the
gonads of higher plants or animals
contain half the number of
chromosomes and referred to as N
because it is haploid. It contains one
copy of the chromosomes.
• These cells are produced from meiotic
division and referred to as 2N because
it contains diploid number of
chromosomes and these cells are
produced from mitotic division. On the
other hand , the gametes (pollen grains,
ovules or sperm)are produced from the
gonads of higher plants or animals
contain half the number of
chromosomes and referred to as N
because it is haploid. It contains one
• These cells are produced from meiotic
division
Variation is the difference between
individual animals and plants and
others of the same species even when
they are offspring of the same parents.
Variation can be due to environmental
factors, or inherited factors. The
following is a basic overview of variation
and inheritance:
Environmental variation can be caused by
diet, physical accidents, culture, climate,
or lifestyle. Both environmental factors
and genetic factors can influence variation
as our genes decide what characteristics
we inherit and our environment affects
how these inherited characteristics
develop. For example, an individual might
inherit a predisposition to thinness, but a
poor diet can result in the individual
becoming overweight :
Although identical twins are genetically
the same there will be differences in
tastes, aptitude or personality according
to the differences in their experience or
environment. A good example of this
would be identical twins adopted into two
separate families. The difference in their
experience and environment will lead to
many variations between the two.
Continuous and Discontinuous Variation:
Variation can be either continuous or
discontinuous. Continuous variation is
gradual or not so clear cut variation.
Height is an example of continuous
variation - individuals can have a
complete range of heights, for example,
1.98, 1.99 or 1.995 metres high. Other
examples include weight, hand span,
colour of skin, size of feet etc. Genes,
environmental variation or both can cause
continuous variation.
Discontinuous variation is where
individuals have to be within a distinct
class or categories. These are cut and
dried characteristics. You either have the
characteristic or you don't. Genes cause
discontinuous variation. For example, you
are either one blood group or another you cannot be in between. Another
example is natural hair colour. The
environment has little to no effect on this
type of variation.
discontinous variation
-Not affected by environment
-Shows distinct phenotypes, no
intermediates
-controlled by one or few genes
e.g, blood groups ,gender, rolling of tongue
continuous variation
- affected by environment
-no distinct phenotypes, many
intermediates
-controlled by one or many genes
e.G human height , intelligence
:
1- environmental variation (not inhrtited)
Environment can affect the expression of a
gene and induce it, so phenotypes
:
2-genteic variation (inherited) due to;
a- Meiosis due to
i-Random independent assortment of
homologous chromosomes
ii-Crossing over between chromatids of
homologues chromosome
:
b- Random mating
c-Mutation
a sudden ,spontaneous
change in the sequence of bases in a DNS
molecule (a gene mutation )or in the
structure or number of chromosomes in a
cell (chromosome mutation
:
Kinds of mutation
1- Gene mutation: chemical change in
genes, can’t be detected by microscope
2- Chromosome mutation: changes in
structure or number of chromosomes, can
be detected by microscope.
:
Mutagens : ‘’ factors that increase the
frequency of mutations’’
1- Chemicals ; such as nitrous acid,
chemicals in tobacco smoke tar
2-Radiations;such as x- rays gamma rays,
ultraviolet light ,atomic radiation
3- Virus infection such as cancer cervix
.
:
What Causes Genetic Variation?
The mixing of genetic material during
sexual reproduction causes genetic
variation. Inherited differences are due to
genes we get from our parents. Genes
control the characteristics we develop. The
human body and other plant and animal
species contain chromosomes, which carry
the genes that control different
characteristics
:
On fertilisation a unique set of
chromosomes is created – half from the
mother and half from the father. The new
individual is genetically unique.