Download Mitosis and Meiosis MA

Mitosis and Meiosis
• Chromosomes
One linear strand of DNA is coiled tightly around
proteins. This is how 3m of DNA material can fit
into a nucleus
• In body cells (somatic cells, not gametes) each
cell has 2 copies of every chromosome=diploid
number
• Humans have 46 in total
-22 pairs are the same in males and females and
the 23rd pair determines the sex
(XX for females and XY for males)
The cell cycle
• Series of events in
which cells grow and
divide to produce two
identical daughter cells
-Interphase, Mitosis,
Cytokinesis
• Includes growth and
division
First up there is interphase!!!
Sister chromatids
• Before S phase
chromosomes are
composed of one DNA
molecule
• During S phase
chromosomes replicate –
two DNA molecules
• These molecules are called
chromatids
• Sister chromatids are held
together by a centromere
• Sister chromatids end up in
two genetically identical
daughter cells
Mitosis-division of the nucleus
• 4 phases
• Produces 2 identical daughter cells from the
parent cell
• Each daughter cell has the exact same genetic
information as parent
• Same number and type of chromosomes as
parent
Prophase
• Chromatin coils tightly
in chromosomesbecomes visible
• Nuclear membrane
disintegrates
• Centrosomes
(organelle) move to
opposite poles of the
cell-produce
microtubules that link
poles of the cell
Metaphase
Microtubules
attach to sister
chromatids at
centromere
Sister chromatids
lie in the equator
of the cell pulled
in line by the
microtubules
Anaphase
Two sister
chromatids split at
the centromere
Chromosomes
move to opposite
poles (ends) of the
cell pulled by
microtubules that
contract
Telophase
• Chromosomes elongate
to form chromatin and
re not visible
• Chromosomes are at
each pole
• Nuclear membrane
begins to reform
around each set of
chromosomes
Cytokinesis-division of cytoplasm and cell
Animal Cells
• Inward pinching of cell
membrane to form
cleavage furrows
Plant Cells
• Plant cells have a firm
cell wall and form a
cell plate
Meiosis
• Sexual reproduction is the fusion of male and
female gametes (sex cells) to form a zygote
(fertilised egg)
• A fertilised egg contains a combination of genetic
material from both parents-50% of inherited
characteristics from the maternal (mother’s side)
and 50% from the paternal (father’s side)
• Our cells contain genetic information from both
parents through a process called meiosis
• For humans to pass on genes there needs to be a
method to divide a parents 46 chromosomes
(diploid) to 23 (haploid)=meiosis
• Meiosis-produces gametes with half the number
of chromosomes-they only have one copy of each
chromosome in each cell (haploid)
• Gamete- sex cells (sperm and ovum-egg)
• This is so when gametes (sperm and egg cells)
come together at fertilisation the total number of
chromosomes, 46, will be restored
• Meiosis occurs in reproductive organs called
gonads (ovaries and testes)
• Meiosis=2 stages
- Meiosis 1 is one cell divides into 2
– Crossing over occurs which swaps regions of DNA
between matching chromosomes (homologous
chromosomes) to make 4 unique chromatidsproduces variation in offspring
– Homologous chromosomes (matching) are separated
- Meiosis 2 is 2 cells into 4 (haploid)
- sister chromatids are separated
• Results in a change of the DNA code (base
sequence) or chromosome number
• Can cause a change in hair colour, skin colour =
harmless
• Others can impede or stop cell function, others
can affect cell division (cause tumours and
cancer) = harmful
• If a change during meiosis, can be passed on to
offspring- variation in species
• Mutagens (environmental factors) can cause
mutation during DNA replication-UV light,
cigarette smoke, X rays
Inherited characteristics
• Gregor Mendel in the 19th Century paved the way
for the understanding of how certain conditions
and genes are passed on to the next generation
• We have homologous pairs of chromosomes (one
from our mother and one from our father)
• For every gene on one chromosome there is a
corresponding gene on the other.ie. For hair
colour or eye colour
• These different forms of the same gene are called
alleles
• X and Y chromosomes carry different genes
• Genes carried on either the X or Y
chromosome are ‘ sex-linked’
• If a recessive gene is carried on either the X or
Y chromosome of males it will be expressed in
the phenotype (the appearance of the
organism)
• Why?
• Because the gene is the only allele present
• Females have two X’s (XX) so they have two
alleles of the same gene so the gene can be
recessive or the female can be a ‘ carrier’ of
the X-linked recessive disease
• These alleles can be dominant or recessive
• Dominant -only needs to occur on one allele
for the trait to be expressed eg. Brown hair (B)
• Recessive -needs to occur on both alleles to
be expressed. eg. Blonde hair (b)
• Homozygous-an individual has two identical
alleles. Eg. BB (B for brown hair) or bb (for
blonde hair)
• Heterozygous-an individual has one of each
type of allele. Eg. Bb
• Will the above person have brown or blonde
hair?