monohybrid cross for dominance

Aa x Aa

incomplete or co-dominance for monohybrid cross

Aa x Aa

test cross for monohybrid cross

Aa x aa

lethal gene (AA dies)

Aa x Aa

monohybrid

sex determination for monohybrid cross

XX x XY

Sex linked with monohybrid cross

X^CX^c x X^cY

dominance for dihybrid cross

AaBb x AaBb

Test cross for dominance

AaBb x aabb

polygene (dihybrid cross)

AaBb x AaBb

linked genes in dihybrid cross

AaBb x AaBb

linked genes test cross

dihybrid cross

AaBb x aabb

DNA Replication Process

(11 points)

1. The coiled DNA is allowed to uncoil and the double helix unwinds

2. The enzyme helicase breaks the weak hydrogen bonds between the bases

3. New pieces of DNA are formed from free nucleotide units joined together by the enzyme polymerase

4. The free nucleotides are matched up to the complementary nucleotides in the original strand

5. DNA polymerase enzyme can build DNA strands only in the 5' to 3' direction

6. The leading strand can be synthesised as a continuous strand

7. The other complementary strand (lagging strang) must be constructed in fragments (Okasaki fragments) initiated by RNA primers

8. The Okasaki fragments are joined by DNA ligase

9. The 2 new DNA strands coil up in a helix (each strand will go into forming a chromatid)

10. In cell division these 2 strands appear as sister chromatids joined at the centromere

11. DNA replication is initiated at many points along the chromosome in eukaryotes and occurs in interphase in mitosis

Sickle cell anaemia

(9 points)

1. Hemoglobin molecules are made up of 2a chains and 2B chains linked together

2. Sickle cell anaemia is caused by autosomal recessive mutation on chromosome 11

3. This mutation results in the substitution of a single nucleotide base in the gene coding for the beta chain of haemoglobin in which GAG mutates to GTG, changing the amino acid from glutamic acid to valine

4. The mutation changes the structure of the haemoglobin molecule

5. Sickle cells containing mutant haemoglobin (less soluble) are less efficient at carrying oxygen

6. The haemoglobin clusters together to form fiberm which deforms the red blood cell into a sickle shape

7. Sickle cell anaemia is carried by a single gene with two alleles Hb^A and Hb^S

8. Hb^A Hb^A = normal blood

Hb^A Hb^S = carrier of trait

Hb^S Hb^S = sickle cell anaemia

9. Heterozygous advantage has some resistance to malaria (selective advantage, environmental selective criteria means the individuals are more viable)

Translation

(7 points)

1. Initiation: brings together mRNA with the start cdon (AUG), a tRNA bearing the start amino acid of a polypeptide and ribosomes

2. Elongation: amino acids are added one by one by tRNA as the ribosome moves along the mRNA

- the tRNA brings the appropriate amino acid up to the ribosome. The anticodon on the tRNA matches with the codon on the mRNA

- the new amino acid is then joined to the polypeptide chain. As each new amino acid is joined, the ribosome nudges the mRNA strand along 3 notches and the next tRNA falls into place

- A peptide bond is formed between adjacent amino acids

- the tRNA is released

3. Termination: the final stage of protein syntehsis occurs when the ribosome reaches a stop codon. The polypeptide chain disconnects from the ribosome

Packaging of DNA

(6 points)

1. Prokaryote DNA is circular and can only be seen under an electron microscope

2. Eukaryote DNA is wound around histones (proteins)

3. The DNA and histones are grouped into bead-like structures of 8 molecules. The group is called a nucleosome

4. The beaded coils form tight chromatin coils called chromatin fibres

5. These form looped domains which are attached to a non-histone protein scaffold

6. The looped domains coil and fold even more to form the characteristic chromosome during cell division

Gene expression

(6 points)

1. A section of DNA (called a transcription unit) that codes for a polypeptide unzips

2. A copy of the transcription unit is made called mRNA (transcription)

3. mRNA moves out of the nucleus to the ribosomes

4. Three bases code for an amino acid

5. Amino acids are assembled in the correct order by carrier tRNA to make the polypeptide chain (translation)

6. Several polypeptides join together to make a protein

mRNA

(however, mRNA has U and DNA has T)

tRNA

(tRNA has U and DNA has T)

the 3 bases that code for an amino acid are called:

1. triplets:

2. codons:

3. anticodons:

1. DNA

2. mRNA

3. tRNA

1. A section of DNA unwinds inside the nucleus

2. One side of the DNA acts as a template on which the complementary molecule of mRNA is synthesized. This is catalysed by RNA polymerase

3. In the mRNA molecule, the thymine is replaced by uracil

4. The mRNA molecule moves through to take ribosomes where the protein synthesis takes place

5. Only the exons (coding regions) of the DNA are copied. The introns (non-coding regions) are copied out

total of all areas where living things are found; including the deep ocean and the lower part of the atmosphere

1. A DNA sample is obtained and is denatured

2. The sample is cool and primers are annealed (bonded) to each DNA strand

3. Free nucleotides and the enzyme DNA polymerase are added. DNA polymerase binds to the primers and synthesises complementray strands of DNA

4. After one cycle there are 2 copies of the original DNA

5. The cycle is repeated

repeated sequences that vary in leangth

some are duplicated thousands of time, forming long stretches of DNA called VNTR regions

1. Adding a foreign gene (organisms are then referred to 

as transgenic)

2. altering an existing gene

3. deleting or 'turning off' a gene