Genome, Chromosomes, Genes, DNA and Genetics

Question 1

Where are genomes in the body?

Answer 1

All the DNA in an individual is referred to as their genome.

Question 2

What are chromosomes?

Answer 2

Chromosomes are genetic structures occurring in functional pairs in the nucleus of cells, except gametes and bacteria.

Question 3

What are genes and alleles?

Answer 3

Genes and alleles are sections of chromosomes made up of short lengths of DNA that operate as functional units to control characteristics e.g. eye or hair colour.

Alleles are different forms of the same gene.

Question 4

What are DNA molecules made up of?

Answer 4

A DNA molecule is made up of thousands of sub-units called nucleotides.

Question 5

What is the structure of a nucleotide?

Answer 5

Nucleotides consist of 3 sub units:

phosphate

Sugar (deoxyribose)

base

The sugar and phosphate always join up and form the backbone of the DNA molecule and the base is always attached to the sugar.

Question 6

What is DNA made up of?

Answer 6

DNA is made up of two strands of nucleotides (like a ladder). The sugar and phosphates make up the uprights of the ladder and the bases make up the rungs. The whole molecule is twisted to form a double helix (like a spiral staircase).

Question 7

What are the 4 forms of bases and what do they pair with?

Answer 7

The 4 forms are:

1. Adenine
2. Thymine
3. Cytosine

4.Guanine

Adenine pairs with Thymine.

Cytosine pairs with Guanine.

(AT CG)

Question 8

Does anyone have the same DNA?

Answer 8

No, No two people have the same sequence of bases along the entire length of the DNA of all of their chromosomes (except identical twins).

Question 9

How does DNA work?

Answer 9

The DNA codes for proteins, which include enzymes and some hormones. Each set of 3 bases is called a base triplet, they code for 1 amino acid and the different amino acids combine to form proteins. In this way the sequence on the DNA determines the order of the amino acids in a particular protein. The DNA that codes for a particular protein is referred to as a gene.

Question 10

What is mitosis?

Answer 10

It is the exact duplication of
chromosomes producing daughter cells that are genetically identical to parent cells and clones.

• It takes place through the body in animals and in meristem regions in plants.
• It is important in growth and replacing worn out/ damaged cells.
• It ensures that all new cells have exactly the same chromosomes as each other and as the parent cell (they are clones of the parent cell).

-It produces two new cells in one division.

Question 11

What are the 6 steps of mitosis?

Answer 11

1. The chromosomes become visible in the nucleus.
2. Each chromosome replicates, forms an identical copy, into 2 identical chromatids held together by a centromere.
3. The chromosomes line up along the centre (equator) of the cell.
4. The chromatids in each chromosome separate and are pulled to opposite ends of the cell.
5. The cell splits in 2 forming 2 identical daughter cells.
6. The 2 new daughter cells contain the same chromosomes as each other and the parent cell.

Question 12

What is meiosis?

Answer 12

-It occurs in sex organs (testes and ovaries).

-It produces gametes (eggs/ova, sperm)

• It is a reduction division as it produces gametes with half the number of chromosomes (haploid number) of the parent cell (diploid).
• produced 4 cells, in 2 divisions.
• All cells are different from each other and the parent cell.

Question 13

What is a homozygous allele?

Answer 13

Both alleles are the same, either in the dominant or recessive form.

E.g. both brown eye colour (BB) or both blue eyes (bb).

Question 14

What is a heterozygous allele?

Answer 14

The 2 alleles are different; usually one is dominant over the other.

E.g. one for brown eye colour and one for blue eye colour (Bb).

Question 15

What is a genotype?

Answer 15

Paired letters showing the alleles present in an individual.

E.g. BB or Bb or bb.

Question 16

What is a phenotype?

Answer 16

It is the physical appearance of the individual.

E.g. brown eyes.

A way to remember it:

Ph Phenotype

Ph Physical appearance

Question 17

What is a dominant allele?

Answer 17

This is an allele that will be expressed when it is heterozygous. E.g. in Bb Brown (B) is dominant.

Question 18

What is a recessive allele?

Answer 18

This is an allele that will only be expressed when it is homozygous. E.g. bb

Question 19

What is a Punnett square?

Answer 19

You can find out the percentage of offspring’s phenotype.

E.g.

Tall. X. Dwarf

TT. X. tt

Gametes T T
t Tt Tt
t Tt Tt

Conclusion: 100% of offspring will be tall as all offspring have the dominant T gene.

All offspring will be heterozygous.

Question 20

Solve: black dog could be (BB) or (Bb) crossed with yellow dog (bb). What are the outcomes of both test crosses?

Answer 20

1.

    B       B   b   Bb    Bb

b Bb Bb

100% black dogs.

2.
B b

b  Bb     bb

b  Bb     bb

1 Black : 1 yellow

1Bb : 1bb

50% chance of either black or yellow dog.

Question 21

What is a pedigree diagram?

Answer 21

They show the way in which a genetic condition is inherited in a family or group of biologically related people.

You need to be able to determine a person’s phenotype, genotype and chance of a condition from a pedigree diagram.

Question 22

How is sex determined in Humans?

Answer 22

Humans have 23 pairs of chromosomes. 22 pairs of autosomes that look the same and 1 pair of sex chromosomes, which in females are the same (XX) and in males are different (XY).

Question 23

Explain what sex-linked conditions are?

Answer 23

As well as determining sex, the sex chromosomes can carry genes and alleles that control other characteristics. As the Y chromosome doesn’t have any alleles, any recessive alleles that are on the X chromosome in males will not be masked by a dominant one and so the recessive trait will be expressed in the phenotype.

However, in females there are 2 X chromosomes and so the recessive allele can be masked by a dominant one.

Question 24

What are some examples of sex-linked conditions?

Answer 24

Red-green colour blindness - you can’t distinguish between red and green colours.

Haemophilia- people with haemophilia can’t make blood products to let their blood clot to seal a cut. Haemophilia is inherited in a recessive allele.

Question 25

What is cystic fibrosis and what allele is it?

Answer 25

It 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.

Question 26

What is Huntington's disease?

Answer 26

It affects nerve cells in the brain, leading to brain damage and starts to take effect in middle age. It is fatal, there is no cure and is caused by a **dominant** allele.

Question 27

What is Down's syndrome and what causes it?

Answer 27

Individuals have reduced muscle tone and cognitive development. This is where a baby is born with an extra chromosome. They have 47 chromosomes instead of 46. It is usually caused by the mum passing on an extra chromosome of number 21.

Question 28

What is genetic screening?

Answer 28

It is taking a sample of cells and looking to see what genetic conditions the person is likely to suffer from.

Question 29

What are the ethical issues associated with genetic screening?

Answer 29

The parents may consider having an abortion if the child will have a genetic condition e.g. Down's syndrome The process of having an amniocentesis increases the risk of miscarriage.

Question 30

What is amniocentesis?

Answer 30

A needle is inserted into the amniotic fluid that surrounds the foetus and fluid is withdrawn. Foetal cells in the fluid are then examined for the presence of genetic abnormalities such as Down syndrome and cystic fibrosis.

Question 31

What does a blood test taken before amniocentesis determine?

Answer 31

When screening for Down syndrome, pregnant mothers are offered a blood test between 10–14 weeks of pregnancy. The blood test determines if the possibility of having a Down syndrome child is raised. Amniocentesis is normally then offered to those mothers who appear to have a higher risk.

Question 32

What are the benefits and risks of amniocentesis compared to blood tests?

Answer 32

Benefit: Amniocentesis is more accurate than blood tests. Risk: Amniocentesis has a 1% risk of miscarriage, whereas blood tests serve no danger to the mother or baby.

Question 33

What do life insurance companies think they should have the right to and why could that be a problem?

Answer 33

They believe that they should have access to genetic information, if they had access to genetic information they may not offer life insurance to some people or it may be too expensive.

Question 34

What could be a problem if employers have access to an individual's genetic information?

Answer 34

They may discriminate between potential employees.

Question 35

What is genetic engineering?

Answer 35

It is a process that modifies the genome of an organism to introduce desirable characteristics.

Question 36

What are the main steps in genetic engineering for insulin production?

Answer 36

1. The healthy human gene for insulin is removed from a chromosome (using restriction enzymes). A plasmid is removed from a bacterial cell. 2. The plasmid is cut open (using the same restriction enzymes). 3. The gene is placed back into the plasmid and the plasmid is put back into the bacterium. (The bacterium has now been genetically engineered). 4. The genetically engineered bacterium is placed into a vat with amino acids, glucose, oxygen etc so that they can grow and multiply. 5. The insulin is extracted and purified. 6. The insulin is tested (safety checks), bottled and sold as Humulin.

Question 37

What are restriction enzymes?

Answer 37

They are enzymes that cut and isolate the human insulin gene and the plasmid.

Question 38

What do "sticky ends" allow?

Answer 38

Sticky ends allow pairing to take place and link human insulin DNA into the bacterial plasmid.

Question 39

What needs to happen following the production of insulin and what is it called?

Answer 39

Following the production of insulin, it needs to be extracted, purified and packaged before it can be used for medical purposes. These processes are called downstreaming.

Question 40

What are the advantages of producing human insulin by genetic engineering?

Answer 40

Before genetically engineered insulin, the amount of insulin available was limited by the number of animals brought to the abattoirs for slaughter. Using human insulin will not create any ethical issues for people. The extraction process is much quicker and there isn't a risk of transferring infections. Human insulin is more effective than animal insulin.