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DNA, cell division and genetics - Coggle Diagram
DNA, cell division and genetics
the genome, chromosomes, genes and DNA
chromosome occur as functional pairs, except in gametes
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chromosomes
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In most cells, chromosomes are located in functional pairs in the nucleus.
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In contrast, gametes do not have pairs of chromosomes in their nuclei and bacterial cells do not have a nucleus.
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gene
- A gene is a short length of DNA found on a chromosome that codes for a particular characteristic or protein.
- Alleles are variations of the same gene. For example, eye colour is the gene but blue, green, brown etc. are alleles.
the structure of DNA
structure
dna consists of two phosphate and sugar held together by bases linked by hydrogen bonds. this unit it is repeated along the length of the dna molecule
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the four can combine only in the order
- adenine-thymine
- guanine-cytosine
theses combinations are referred to as base pairing
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terms
chromosomes
- genetic structures that occur in functional pairs in the nucleus of cells ( expect gametes, where there is only one chromosomes from each pair, and bacteria, which don't have a nucleus and only have a single chromosome
gene
- a short section of dna that codes a particular characteristic
genome
- the entire genetic material found in an organisim
cell division
mitosis
a type of cell division that produces cells genetically identical to the parent cell and to each other
is important for growth, replacing worn out cells and repairing damage tissue
ensures that new daughter cells have exactly the same chromosome arrangement as each other and as parent cell
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genetics
definitions
allele
- a particular form of a gene
gene
- a short section of chromosomes that codes for a particular characteristic e.g. gene for eye colours
heterozygous
- describes the situation when both alleles of a gene are the same
homozygous
- describes the situation when both alleles of a gene are the same
genotype
- paired symbols showing the allele arrangement in an individual
phenotype
- outward appearance of an individual
dominant
- in the heterozygous condition the dominant allele over rides the non- dominant allele
recessive
- the recessive allele is dominated by the dominant allele - it shows itself in phenotytpe if there are two recessive alleles
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sex linkage
The sex chromosomes, although mainly responsible for determining gender, carry some genes that code for a number of body functions.
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The difference in size between the X chromosome and the shorter Y chromosome means some genes appear only on the X chromosome.
The effect of sex-linked genes is more obvious in males because the X chromosome can contain genes the Y does not.
A recessive allele on the X chromosome will show in the phenotype as there is no dominant allele present to override it.
A female will have another allele on her other X chromosome which, if dominant, would mask the effect of a recessive allele.
genetic conditions
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examples
cystic fibrosis
Mainly affects the lungs and digestive system, which become clogged with mucus leading to frequent infections.
It is caused by a recessive allele, which means only homozygous recessive individuals (cc) will be affected.
Huntington’s disease
Affects nerve cells in the brain, leading to brain damage.
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down syndrome
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It is caused by the presence of an extra chromosome – a sperm cell with 23 chromosomes fertilises an egg cell with 24 chromosomes.
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ethical and moral issues
Parents can be faced with difficult decisions if genetic screening shows their foetus has a genetic condition.
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Many parents will argue ‘yes’, as it prevents having a child who could have a poor quality of life; a lot of time may need to be spent caring for the child at the possible expense of time with their other children.
Other parents will argue ‘no’, as the unborn child doesn’t have a say, or they will argue that it is not morally right to ‘kill’ a foetus. Additionally, abortion is banned in some religions
other issues arising
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It is possible to screen people in childhood and adulthood, providing us with an individual genetic profile.
There are ethical issues associated with making this information more public such as making it available to life insurance companies and employers.
genetic screening
Genetic screening involves testing people or groups of people for the presence of a particular allele or other genetic abnormality.
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Foetal cells in the fluid are then examined for the presence of genetic abnormalities such as Down syndrome and cystic fibrosis.
As amniocentesis carries a 1% risk of miscarriage, it is usually only offered to pregnant women who are at a higher risk of having a child with a genetic abnormality.
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