Genetics: Transcription and Translation

TUTORIAL 9 - Student
Genetics: Transcription and Translation
AIMS:
1. To understand keywords and use them as an aid to study facts 2. To get a better understanding of transcription and translation
LEARNING OBJECTIVES: 1. Comprehensive reading 2. To associate certain facts with each other, therefore creating a bigger picture

Exercise 1a: Firstly, fill in the structure names for each picture. Secondly, describe in a short paragraph what u looking at, where u find it and the function of the structure.

The DNA strand is read in the 3' to 5' direction and the mRNA is transcribed in the 5' to 3' direction by the RNA polymerase. Transcription occurs in the cell nucleus, where the DNA is held. The DNA structure of the cell is made up of two helixes made up of sugar and phosphate held together by the bases. The sugar and the phosphate are joined together by a hydrogen bond. The DNA is "unzipped" by the enzyme helicase, leaving the single nucleotide chain open to be copied. RNA polymerase reads the DNA strand from 3-prime (3') end to the 5-prime (5') end, while it synthesizes a single strand of messenger RNA in the 5'-to-3' direction. The general RNA structure is very similar to the DNA structure, but in RNA the nucleotide uracil takes the place that thymine occupies in DNA. The single strand of mRNA leaves the nucleus through nuclear pores, and migrates into the cytoplasm.

Exercise 1b:

Translation occurs in the cytoplasm, where the ribosomes are located. Ribosomes are made of a small and large subunit that surround the mRNA. In translation, messenger RNA (mRNA) is decoded to produce a specific polypeptide according to the rules specified by the trinucleotide genetic code. This uses an mRNA sequence as a template to guide the synthesis of a chain of amino acids that form a protein. In activation, the correct amino acid (AA) is joined to the correct transfer RNA (tRNA). While this is not, in the technical sense, a step in translation, it is required for translation to proceed. When the tRNA has an amino acid linked to it, it is termed "charged". Initiation involves the small subunit of the ribosome binding to 5' end of mRNA with the help of initiation factors (IF), other proteins that assist the process. Elongation occurs when the next aminoacyl-tRNA (charged tRNA) in line binds to the ribosome along with GTP and an elongation factor. Termination of the polypeptide happens when the A site of the ribosome faces a stop codon (UAA, UAG, or UGA). When this happens, no tRNA can recognize it, but releasing factor can recognize nonsense codons and causes the release of the polypeptide chain.

Exercise 1c:
The bigger picture of transcription and translation in the cell itself.

Exercise 2: Transcription of the lac gene is said to be “induced” by lactose. Provide a written summary and use sketches to describe the scenario where required. Glucose | Lactose | | Summary | - | + | | When glucose is absent there are high levels of cAMP (cyclic AMP) in the cell. cAMP binds to the CAP (catabolite activator protein) molecule which activates it, making it able to bind to the CAP-binding site bending the DNA. This bending of the DNA allows RNA polymerase to associate with the promoter (Plac). | + | + | | When glucose is present cAMP levels are low therefore CAP is inactive and does not bind to the binding site. Due to this the RNA polymerase does not associate with the lac operon | + | - | | There is also a regulatory protein called the lac repressor protein. This protein is capable of binding to the operator site and preventing transcription when lactose is absent, glucose present. The repressor covers part of the promoter resulting in RNA polymerase being unable to associate with the Lac operon, preventing the making of mRNA. | - | + | | When lactose is present, a lactose isomer binds to the repressor and inactivated it. This prevents the repressor from binding to the operator site. RNA polymerase can associate with the promoter and carry out transcription |

Practice multiple questions

1. Prior to cell division, every chromosome is composed of 2 identical parts called
a. sister chromatids d. centromeres
b. homologous pairs e. twins

2. Which of the following pairs is mismatched?
a. prophase - separation of chromatids
b. anaphase - movement of chromatids to poles
c. telophase - formation of nuclear membrane
d. metaphase - chromosomes line up at equator of cell
e. interphase - chromosomes are duplicated

3. DNA replication occurs during
a. prophase d. G2 period
b. G1 period e. M period
c. S period

4. Which of the following statements is NOT true?
a. meiosis involves two nuclear divisions
b. there is no DNA replication between meiosis I and II
c. meiosis produces gametes
d. meiosis allows for genetic variation
e. meiosis produces two identical daughter cells

5. A nine month old human baby grows up to two inches in height in one month. Her cells are undergoing rapid
a. meiosis d. mitosis
b. fertilization e. synapsis
c. gamete production

6. If a tiger (2N = 20) and a lion (2N = 22) could mate and produce offspring, how many chromosomes would a “tiglon” have? (this is hypothetical only)
a. 21 b. 20 c. 22 d. 42 e. 84

7. Exchange of chromosomal fragments occurs in which of the following stages?
a. metaphase II of meiosis
b. metaphase of mitosis
c. prophase I of meiosis
d. prophase II of meiosis
e. prophase of mitosis

8. Replication of the chromosomes occurs
a. prior to mitosis
b. prior to meiosis I
c. prior to meiosis II
d. during prophase of mitosis or meiosis I
e. a and b only

9. Independent assortment of chromosomes
a. allows for genetic variation in the gametes
b. occurs in metaphase I
c. is also called synapsis
d. means that any sperm can fertilize any ovum
e. a and b only

10. If a mouse ovum has 10 chromosomes, how many chromosomes will be found in the liver cells? a. 5 b. 10 c. 20 d. 15

11. Homologous pairs are lined up at the equator of a cell. The cell could be in
a. metaphase d. metaphase I
b. prophase I e. metaphase II
c. anaphase II

12. Sister chromatids separate and move to opposite ends of the cell. The cell could be in
a. anaphase d. metaphase I
b. anaphase II e. a and b only
c. prophase II

13. Which of the following does NOT provide genetic variation to a species?
a. mutation
b. independent assortment of chromosomes
c. mitosis
d. crossing over
e. random fertilization

14. If a cell has 20 chromosomes, then, after meiosis, there will be ______ number of chromosomes in each of _____ cells
a. 20; 2 b. 40; 2 c. 20; 4 d. 10; 4 e. 10; 2

15. Billy has a widow's peak, and Bob has a continuous hairline. This is a description of their
a. genotype d. gametes
b. phenotype e. chromosomes
c. alleles