The gentic code and cell function

Question 1

What was the experiment to test DNA replication?

Answer 1

Cultured the bacterium, Escherichia Coli, for several generation on a medium containing amino acids made with the heavy isotope nitrogen 15N. The bacteria incorporated the 15N into their nucleotides and then into their DNA so that all the DNA contained 15N. They extracted the bacterial DNA and centrifuged it. The DNA settled at a low point in the tube. The 15N bacteria were washed, then transferred to a medium containing the normal lighter form of Nitrogen, 14N and were allowed to divide once more. When extracts of DNA from this first generation culture were centrifuged, it was shown to have a mid-point density, since half the strand was made up of the original strand of 15N DNA and the other half was made up of the new strand containing 14N. When extracts were taken from the second generation grown in 14N the DNA settled at midpoints and high points in the tube. This was conclusive evidence for the semi-conservative hypothesis.

Question 2

How does replication occur? (semi-conservative hypothesis)

Answer 2

1) Hydrogen bonds holding the base pairs together break and the two halves of the molecule separate
2) DNA unwinds and as the strands separate, DNA polymerase catalyses the addition of free nucleotides to the exposed bases
3) Each chain acts as a template so that free nucleotides can be joined to their complementary bases by the enzyme, DNA polymerase.
4) The result is two DNA molecules, each made up of one newly synthesised chain and one chain conserved from the original nucleotide.

Question 3

What do the codes carried by DNA determine?

Answer 3

What reactions take place in an organism

Question 4

If one base coded for one amino acid how many amino acids could be made?

Answer 4

4- A,T,G,C for each code

Question 5

If two bases coded for one amino acid how many amino acids could be made?

Answer 5

16- 4 squared

Question 6

If three bases coded for one amino acid how many amino acids could be made?

Answer 6

64 - 4 cubed which is more than enough to make 2o amino acids

Question 7

Why are codons exactly the same for all living organisms?

Answer 7

Codons are universal

Question 8

What’s the basic process of protein synthesis?

Answer 8

1) DNA acts as a template providing the instructions in the form of a long sequence of nucleotides
2) A complementary section of part of this sequence is made into mRNA by a process called transcription
3) The mRNA acts as a template to which complementary tRNA molecules attach and the amino acids they carry are linked to form a polypeptide by a process called translation.

Question 9

What’s Transcription?

Answer 9

Process whereby part of the DNA, the gene, acts as a template for the production of mRNA

Question 10

What’s the process in Transcription?

Answer 10

1) The enzyme, DNA helicase, acts on a specific region of the DNA molecule, called the cistron, to break the hydrogen bonds between the bases. This causes the two strands to separate and expose the nucleotide bases in that region
2) The enzyme RNA polymerase links to the template strand of DNA ant the beginning of the sequence to be copied. The double-stranded DNA first unwinds and then unzips in the relevant region
3) Transcription occurs when free RNA nucleotides align themselves opposite the template strand
4) Because of the complementary relationship between the bases in DNA and the free nucleotides, cytosine in the DNA attracts a guanine, guanine a cytosine, thymine and adenine and adenine a uracil
5) RNA polymerase moves along the DNA forming bonds that add nucleotides one at a time to the RNA. This results in the synthesis of a molecule of mRNA alongside the unzipped portion of DNA. Behind the RNA polymerase the DNA strands re-join to reform the double helix
6) The RNA polymerase separates when it reaches a “stop” code. Then the production of mRNA is complete.
7) The mRNA carries the DNA code out of the nucleus through a nuclear pore to the cytoplasm and attaches itself to a ribosome consisting of ribosomal RNA and protein.

Question 11

What’s the process of Translation?

Answer 11

1) A ribosome attached to the starting codon at one end of the mRNA molecule
2) The first tRNA with the anticodon complementary to the first codon on the mRNA attaches itself to the ribosome. Then a second tRNA with an anticodon complementary to the second codon on the mRNA attaches to the other attachment site. the two amino acids are sufficiently close for a peptide bond to form between them., The first tRNA leaves the ribosome. leaving an attachmeny site vacant. The ribosome moves one codon along the mRNA strand.
3) One site binds with tRNA with the growing polypeptide; the other site is for RNA carrying the next amino acid in the sequence.
4) Translation by ribosomes allows the assembly of amino acids into polypeptides according to the original DNA code. A ribosomal enzyme catalyses peptide bond formation between an amino acid on one tRNA and the growing polypeptide on the other tRNA.
5) A ribosome passes along mRNA, one codon at a time, the tRNA with the appropriate anticodon fills the vacant slot and the amino acid forms a peptide bond with the last member of the chain until a stop codon is reached.
6) Each time one ribosome moves along the mRNA a molecule of polypeptide is produced
7) Usually a number of ribosomes can be found on a single mRNA, each reading from the coded information at the same time. This is called a polysome system and so many molecules of a polypeptide are formed.

Question 12

What’s referred to as activation?

Answer 12

Once the tRNA is released from its specific amino acid, it is free to collect another amino acid from the amino acid to attach itself to he tRNA.

Question 13

Where does Meiosis take place?

Answer 13

Reproductive organs of both plants and animals forming haploid gametes as to maintain a constant number of chromosomes they must be halved.

Question 14

How does meiosis result in genetic variation?

Answer 14

1) Independent assortment of homologous chromosomes

2) Crossing over between the chromatids of homologous chromosomes

Question 15

What happens in meiosis I?

Answer 15

Homologous chromosomes pair up and their chromatids wrap around each other and equivalent portions of theses chromatids may be exchanged in a process caked crossing over. By the end of his stage the homologous pair separate with one chromosome of each pair going into one of he two daughter cells. This results in two daughter nuclei containing half the number of chromosomes of the aren’t of the parent nucleus.

Question 16

What happens in meiosis II?

Answer 16

Chromatids move apart and he two new haploid nuclei divide again in a division identical to that of mitosis. The net result is that 4 haploid nuclei are formed from he parent nucleus each containing half the number of chromosomes.

Question 17

What happens in Prophase I?

Answer 17

The chromosomes become shorter and thicker and can be seen as two chromatids. In cells where centrioles are present, the centrioles, move to the poles of the cells and microtubules begin to radiate from them, forming asters resulting in formation of spindle. This stage differs from that of mitosis as homologous chromosomes associate in pairs and each pair is called a bivalent. Each bivalent consist of four strands, made up of two chromosomes, each comprising two chromatids. These chromatids wrap around each other and then partially repel each other but remain joined at certain points called chiasmata. At these points chromatids may break and recombine with a different but equivalent chromatid. This exchange of pieces of chromosomes is called crossing over. By the end of this stage the nuclear envelope has disintegrated and the nucleolus has disappeared.

Question 18

What happens in metaphase I?

Answer 18

At this stage, when the pairs of homologous chromosomes align themselves on the equator of the spindle, the maternal and paternal chromosomes are arranged randomly. This random distribution and consequent independent assortment of chromosomes produces new genetic combinations.

Question 19

What happens in Anaphase I?

Answer 19

The chromosomes in each bivalent separate and one of each pair is pulled to one pole, its sister chromosome to the opposite pole. This each pole receives only one of each homologous pair of chromosomes and, because of their random arrangements at metaphase, these will be a random mixture of maternal and paternal chromosomes. The chromosomes reach the opposite poles and the nuclear envelope re-forms around each group of haploid chromosomes.

Question 20

What happens in Telophase I?

Answer 20

Usually the chromosomes stay in their condensed form and meiosis II follows on immediately. In animal cells cytokinesis occurs, that is, the division of the cytoplasm to give two haploid cells. many plant cells go straight into meiosis II with no re-formation of the spindle.

Question 21

What happens in meiosis II?

Answer 21

1) Prophase II- new spindle develops at right angles to the old spindle
2) Metaphase II- The chromosomes line up separately on the equator of the spindle with each chromosome attached to a spindle fibre by its centromere.
3) Anaphase II- Centromeres divide and the chromatids are pulled to opposite poles
4) Telophase II- On reaching the poles the chromatids lengthen and are indistinct. the spindle disappears and the nuclear membrane reforms. Cytokinesis takes place.

Question 22

What is the significance of meiosis?

Answer 22

Meiosis is the reduction division that occurs during gamete formation in sexually reproducing organisms. In his division the diploid number of chromosomes (2n) is reduced to the haploid (n). Thus when two gametes join together at fertilisation, the zygote that is formed has two complete sets of chromosomes returning to the diploid condition. However, meiosis does more than halve the number of chromosomes into a cell; it also introduces genetic variation into the gametes and therefore the zygotes that are produced. When these genetically different gametes fuse, randomly, at fertilisation, more variation produced amongst the offspring.

Question 23

How is variety created?

Answer 23

1) Each of the chromosomes making up homologous pair carries different genetic material. During sexual reproduction the genotype of one parent is mixed with that of the other when haploid gametes fuse.
2) The different pairs of homologous chromosomes arrange themselves on the spindle during metaphase I of meiosis. When, they subsequently separate, they do so entirely independently separate, they do so entirely independently of each other so that the daughter cells contain different combinations of maternal and paternal chromosomes.
3) Crossing over during chiasmata formation during prophase I of meiosis.

Question 24

What recombination?

Answer 24

Equivalent parts of homologous chromosomes may be exchanged, thus producing new combinations and the separation of linked genes. This single cross-over occurs during meiosis I results in four haploid gametes having a different genetic composition.