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Summary 1.5 - Changing Man
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Problem 1What is DNA and how is DNA copied during cell division/can it change?
Terminology:
Genotype: genes that one inherits
Phenotype: person’s observable characteristics
Autosome: the 22 chromosomes that aren’t sex chromosomes
Genotype - The symbolic representation of the pair of alleles possessed
by an organism, typically represented by two letters. Examples: Bb, GG,
Phenotype - The characteristics or traits of an organism. Examples: five
fingers on each hand, colour blindness
Dominant allele - An allele that has the same effect on the phenotype
whether it is paired with the same allele or a different one – they are
always expressed in the phenotype. Example: the genotype Aa gives the
dominant A trait because the a allele is masked; the a allele is not
transcribed or translated during protein synthesis.
Recessive allele - An allele that has an effect on the phenotype only when
present in the homozygous state. Example: aa gives rise to the recessive
trait because no dominant allele is there to mask it.
Co-dominant alleles - Pairs of alleles that both affect the phenotype
when present in a heterozygote. Example: a parent with curly hair and a
parent with straight hair can have children with different degrees of hair
curliness, because both alleles influence hair condition when both are
present in the genotype.
Locus -The particular position on homologous chromosomes of a gene –
each gene is found at a specific place on a specific pair of chromosomes.
Homozygous - Having two identical alleles of a gene Example: AA is a
genotype that is homozygous dominant, whereas aa is the genotype of
someone who is homozygous recessive for that trait
Heterozygous - Having two different alleles of a gene – results from the
fact that the paternal allele is different from the maternal one. Example:
Aa is a heterozygous genotype.
Carrier - An individual who has a recessive allele of a gene that does not
have an effect on the phenotype. Example: Aa carries the gene for
albinism, but has pigmented skin, which means an ancestor must have
been albino and some offspring might be albino; if both parents are
unaffected by a recessive condition yet both are carriers, some of their
progeny could be affected (because they would be aa)
Test cross - Testing a suspected heterozygote plant or animal by crossing
it with a known homozygous recessive (aa). Because a recessive allele can
be masked, it is often impossible to tell whether an organism is AA or Aa
unless they produce offspring that have the recessive trait.
,Genetic Transmission #1
After fertilization once the sperm penetrates an ovum, both the sperm &
the ovum start disintegrating and start releasing genetic material
- Zygote: the new cell nucleus is formed which contains some
hereditary information from both the father and the mother
- This initially tiny zygote will develop into a human
46 chromosomes, each of which containing thousands of chemical
segments, which are the genes
Genes are actually (deoxyribonucleic add) DNA – a double helix
molecule (looks like a twisted ladder) that provides the chemical basis for
development
- DNA can actually duplicate itself by splitting in half (where the
“ladder like” shape) is connected
- This split in two is what actually allows the one cell zygote to
duplicate enough times in order to become a complex human
Prokaryotes (Bacteria) & Plasmids
Prokaryotes have only one chromosome, not 2
Escherichia coli, like many prokaryotes (bacteria), have small loops of
DNA that are extra copies of some of the genetic material of the
organism.
- These loops are called plasmids – small, circular, DNA molecules that
aren’t connected to the main bacterial chromosome.
- Plasmids replicate independently of the chromosomal DNA.
- Plasmid DNA is not required by the cell under normal conditions, but
it may help the cell adapt to unusual circumstances.
- They can also be found in Archaea as well as in bacteria
Eukaryotic Cells – DNA -2
DNA of eukaryotic cells most often comes in the form of chromosomes –
which are vital for the cells to survive
DNA: the genetic material of cells & what allows genes to be passed
down
When the cell is not dividing, the chromosomes are not visible structures.
During this phase, the cell's DNA is in the form of chromatin
- Chromatin is formed of strands of DNA and proteins called histones.
Unfolded DNA looks like beads on a string - each of the beads is a
nucleosome
- A nucleosome consists of two molecules of each of four different
histones
Chromosomes are the DNA molecules which contain half of the genetic
material from the mom & half from the dad
The DNA wraps twice around these eight protein molecules
, The DNA is attracted to the histones because DNA is negatively charged
and the histones are positively charged
There is often a fifth type of histone attached to the linking string of DNA
near each nucleosome
- This fifth histone leads to further wrapping (packaging) of the DNA
molecule and eventually to the highly condensed or supercoiled
chromosomes
- When DNA is wrapped around the histones and then further wrapped
in even more elaborate structures, it is inaccessible to transcription
enzymes
- Therefore, the wrapping or packaging of DNA regulates the
transcription process
- This allows only certain areas of the DNA molecule to be involved in
protein synthesis.
Which types of cell-division exist and how do they differ? You do need to
know the major differences between meiosis & mitosis
Growth of Zygote & Production of Cells – Mitosis #2
When the zygote moves from the fallopian tube to its prenatal
environment, the uterus starts replicating itself (zygote) through mitosis
- The cell starts by dividing into 2 cells, then the 2 cells divide into 4
and so on
- Before each division, the cell duplicates the 46 chromosomes, moving
these duplicates into opposite directions
- The division of the cell continues, resulting in two new cells which
both have 23 identical chromosomes, meaning the same genetic
material that was found in the first zygote
When child is born, they are made up of billions of cells (making up
muscles, etc.) all through the process of mitosis
Mitosis continues throughout life, creating new needed cells & replacing
the damaged ones
- Every new (duplicate) cell will always contain the original 46
chromosomes
Mitosis
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