OCR 21st Century Science - BIOLOGY B1

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BIOLOGY
B1 - You and Your Genes
Variation
Differences between individuals of the same species are called variation. Variation may be due to
genetic factors or environmental factors.
Genetic Information
Genes carry the information needed for you to develop. Different genes control different
characteristics, e.g. the colour of your eyes.
Genes...occur in long strings called chromosomes and are located inside the nucleus of every cell.
Chromosomes are made of DNA (deoxyribonucleic acid) molecules. DNA molecules are...made up of
two strands and coiled to form a double helix.
DNA molecules form a complete set of instructions for...how the organism should be constructed
and how each cell should function.
Genes are section of DNA. Genes control the development of different characteristics by issuing
instructions to the cell. The cell carries out these instructions by producing proteins.
The proteins formed inside a cell can be...structural proteins (for cell growth and repair) or
enzymes (to speed up chemical reactions).
Genetic Modification
All organisms have DNA. This means it's possible to introduce genetic information from one organism
into another. This will produce a new combination of genes and characteristics. It is called genetic
modification.
Chromosomes
Chromosomes normally come in pairs: Both chromosomes in a pair have the same sequence of
genes, i.e. the same genes in the same place. Different species have different numbers of pairs.
Human cells contain 23 pairs of chromosomes (46 in total).
Sperm + Egg = Fertilized Egg Cell IS THE SAME AS 23 chromosomes + 23 chromosomes = 46
chromosomes (23 pairs)-half from the mother (egg) and half from the father (sperm).
Sex cells contain single chromosomes. In humans they have a total of 23 chromosomes; half the
number of a normal body cell.

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Alleles
A gene can have different versions, called alleles. For example, the gene for eye colour has two
alleles: brown and blue. For each gene, you inherit one allele from your father and one from your
mother. This is why you can have similarities to both of you parents.
You can inherit two alleles that are the same or two that are different. Brothers and sisters can
randomly inherit different combinations for all the different genes, which is why they can be very
different.…read more

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If the gene isn't present, i.e. if there are two X chromosomes present, the embryo will develop into a
female.
If the gene is present, i.e. if there is an X chromosome and a Y chromosome, testes begin to develop.
After six weeks the testes start producing a hormone called androgen. Specialised receptors in the
developing embryo detect the androgen and male reproductive organs begin to grow.
Sometimes the Y chromosome is present but androgen isn't detected. When this happens...…read more

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Genetic testing
It's possible to test a person for a faulty allele. If the tests are positive, couples must choose
whether to have children and risk passing on the disorder, to adopt a child instead, or to use embryo
selection.
Fetuses can also be tested. If the faulty allele is present in a developing fetus, parents may decide
whether to terminate the pregnancy.
Testing the fetus
There are two ways if removing cells so that a genetic test can be carried out on a fetus.…read more

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The Implications of Genetic Testing
Before genetic testing can become a common practice, the following questions should be
addressed:
How can mistakes be prevented? Is it right to interfere with nature? Who has the right to decide if a
disorder is worth living with?
There is always a difference between what can be done and what should be done.…read more

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Gene Therapy
Gene therapy is a potential treatment for certain genetic disorders. It involves inserting `healthy'
genes into cells in order to treat a disease.
The most common method uses genes from healthy people.
The genes are inserted into a modified virus, which infects the patient. The genes become a part of
the patient's cells, correcting the faulty allele.
Gene Therapy and Ethics
New procedures raise questions. Some can be answered by further scientific research, e.g.…read more

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Asexual Reproduction
Bacteria and other single-cell organisms can reproduce by dividing to form two `new' individuals. The
new individuals are clones (genetically identical to the parent).
This is asexual reproduction. Most plants and some animals can reproduce in this way.
Variation in organisms that reproduce asexually is normally only caused by environmental factors.
Clones can occur naturally: The cells or an embryo sometimes separate. The two new embryos
develop into identical twins.…read more

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