Topic 8 - DNA, RNA and Protein Synthesis

Question 1

Gene

Answer 1

Sequence of DNA bases which code for a polypeptide or functional RNA

Question 2

Primary Structure of protein

Answer 2

Sequence (no. + order) of amino acids

Question 3

Why are polypeptide chains different?

Answer 3

Different primary structures.
Different no. + order of amino acids
Order of amino acids is determined by the order of DNA bases in the gene

Question 4

DNA Triplet

Answer 4

Sequence of 3 DNA bases in a gene which code for an amino acid

Question 5

Functional RNA

Answer 5

RNA molecules other than mRNA which have a specifc role in protein synthesis e.g. tRNA, rRNA.

Question 6

Genome

Answer 6

Full set of genes in cell

Question 7

Protome

Answer 7

Full range of proteins cell is able to produce

Question 8

Alleles

Answer 8

1. Different versions of the same gene.
2. Alleles of the same gene have a slighly different order of bases in the gene
3. Therefore, code for a slightly different version of the polypeptide
4. For example, gene for blood type has 3 alleles. One allele codes for blood type A, one for type B, one for type O

Question 9

Homologous Pairs of chromosomes

Answer 9

1. Humans have 23 matching pairs of chromosomes - homologous pairs
2. In each homologous pair, the chromosomes are the same size, contain the same genes however, could have different alleles. Alleles which code for same characteristic, in a homologous pair, found on same locus.

Question 10

Locus

Answer 10

Fixed position of gene on a chromosome

Question 11

mRNA - Messenger RNA

Answer 11

1. Produced during transcrption
2. Carries genetic code from nucleus to ribosome where protein is made during translation
3. One single polynucleotide strand
4. Sequence of 3 bases on mRNA is a CODON

Question 12

tRNA - Transfer RNA

Answer 12

1. Carries amino acids to the ribosome to make protein during translation
2. Single polynucleotide strand folded into clover shape due to hydrogen bonds between specific base pairs
3. Each tRNA molecule has a specific sequence of three bases on one end called ANTI-CODON
4. Each tRNA molecule has an amino acid binding site on one end

Question 13

Products of trannscription are different in prokaryotic cells and eukaryotic cells. Explain why.

Answer 13

1. In prokaryotic cells, mRNA is produced directly from DNA. No need for splicing. Prokaryotic DNA does not contain introns
2. In eukaryotic cells, pre-mRNA is produced from DNA. This is then spliced to form mRNA
3. This is because eukaryotic DNA contains introns and exons. During transcription, introns and exons are both copied into mRNA strand, so mRNA strand contains introns and exons, so its now called pre-mRNA
4. Splicing then occurs to pre-mRNA. This is where the introns are removed and the exons are joined together to form mRNA. This happens in the nucleus

Question 14

DNA in eukaryotic and prokaryotic cells

Answer 14

The structure of DNA is the same in eukaroytic cells and prokaryotic cells. The difference is the way it is stored

In eukaryotic cells:

1. DNA is linear in the form of chromosomes which are in the nucleus
2. DNA is long and is “wound” up so it fits in the nucleus
3. DNA is associated histone proteins
4. Mitochondria and chloroplasts contain DNA however, this DNA is more similar to prokayotic DNA, circular, shorter, not associated with histone proteins

In prokaryotic cells:

1. DNA is circular and in form of chromosomes however, no nucleus, so free floating in the cytoplasm
2. DNA is shorter and is condensed by supercoiling so it can fit in cell
3. Not associated with histone proteins

Question 15

Protein Synthesis

Answer 15

Split into 2 stages:

1. Transcription
2. Translation

Question 16

Transcription

Answer 16

mRNA is produced from DNA. It carries the genetic code from nuclues to ribosome where translation occurs
In eukaryotic cells, occurs in the nucleus
In prokaryotic cells, occurs in cytoplasm

Process:

1. RNA polymerase attaches to DNA double helix at start of gene
2. DNA helicase breaks hydrogen bonds between complementary base pairs between the 2 DNA polyncleotide strands, causing them to uncoil and unwind. We now have to seperate DNA strands with bases exposed
3. One strand is used as a template strand to form the mRNA copy
4. RNA polymerase lines up free floating RNA nucleotides alongside DNA template strand. Bases on RNA nucleotides join with exposed bases on DNA template strand via complementary base pairing
5. Due to complementary base pairing, mRNA produced is a complimentary copy of the DNA template strand. Except, in RNA base thymine is replaced with uracil
6. RNA polymerase moves along DNA template strand and seperates the DNA template strand from the mRNA strand, assembling mRNA strand
7. RNA polymerase reaches a stop signal on DNA and detaches from DNA
8. When RNA polymerase leaves, hydrogen bonds reform between the two uncoiled DNA polynucleotide strands and the DNA double helix reforms
9. mRNA is produced and in eukaryotic cells, it moves out of the nuclues via nuclear pore, goes into cytoplasm and attaches to ribosome. The next stage of protein synthesis occurs which is translation

Question 17

Translation

Answer 17

Second stage of protein synthesis. It is where tRNA molecules bring amino acids to the ribosome, where they are joined together by peptide bonds to form a polypeptide, following the sequennce of codons in mRNA

In both eukarkaryotic cells and prokaryotic cells, translation occurs in the cytoplasm at the ribosome

Process

1. mRNA attaches to ribosome. tRNA molecyles bring amino acids to ribosome. ATP provides energy for bond to form between tRNA molecules and amino acids
2. First tRNA molecule (carrying an amino acid) has an anti-codon complimentary to the first codon on mRNA. tRNA molecule then joins to mRNA by complimentary base pairing between mRNA codon and tRNA anti-codon
3. Second tRNA molecule (carrying another amino acid) has an anti-codon complimentary to the second codon on mRNA. This tRNA molecule then joins to mRNA via complimentary base pairing between codon and anti-codon
4. The first two amino acids (still attached to tRNA molecules) join together via peptide bond. Once peptide bond formed, the first tRNA molecule leaves, leaving its amino acid behind
5. Third tRNA molecule (carrying another amino acid) has an anti-codon complimentary to the third codon on mRNA. This tRNA molecule then joins to mRNA via complimentary base pairing between codon and anti-codon
6. This amino acid then joins to the second amino acid (both still attached to tRNA molecules) via peptide bond. Once peptide bond formed, the second tRNA molecule leaves, leaving its amino acid behind
7. This process continues till a stop signal on mRNA is reached
8. Polypeptide formed. Polypeptide moves away from ribosome and trabslation is complete.

Question 18

Gametes

Answer 18

1. Gametes are sex cells
2. Male gametes are sperm cells and female gametes are egg cells
3. Gametes have a haploid number of chromosomes (n). They only have one of each chromosome

Question 19

Meiosis

Answer 19

Cell division which produces gametes.
It occurs in the reproductive organs
It is important for sexual reproduction as it produces gametes with half the no. of chromosomes as parent cell

Question 20

Meiosis Process (7 steps)

Answer 20

1. DNA unravels + replicates so that there are 2 copies of each chromosome called chromatids
2. DNA then condenses and forms double armed chromosomes where each chromosome consists of two sister chromatids joined together by a centromere
3. Meiosis 1 (the first division now happens) where homologous chromosomes are arranged in pairs
4. Crossing over now occurs. This is where chromatids twist around each other and bits of chromatids are swapped over. Chromatids still have the same genes but different alleles
5. Homologous chromosomes seperate. This halves chromosome no.
6. Meiosis 2 (the second division) now happens. This is where the centromeres divide and the sister chromatids which make up the chromosomes seperate
7. This forms four haploid daughter cells (gametes) which are genetically different from each other

Question 21

How does meiosis lead to genetic variation

Answer 21

Two events occur in meiosis which lead to genetic variation:

1. Crossing over
2. Independent segregation of chromosomes

Question 22

Crossing Over

Answer 22

1. Occurs during meiosis 1 - first division
2. Chromosomes arrange themselves into homologous pairs
3. Chromatids twist around each other, and bits of chromnatids swap over
4. Chromatids still contain same genes but different alleles
5. So, the daughter cells produced will each have chromatids with different alleles
6. This increases genetic diversity

Question 23

Independent segregation of chromosomes

Answer 23

1. Homologous pairs of chromosomes consist of one chromosome from mother and one from father
2. During meiosis 1, first division, chromosomes arrange themselves into homologous pairs
3. The homologous pairs of chromosomes then seperate (this halves the chromosome no.)
4. When the homologous pairs of chromosomes eperate, which chromosomes from each pair that end up in which daughter cell is completewly random
5. Each daughter cell produced by meiosis will have different combinations of these paternal and maternal chromosomes
6. This increases genetic diversity

Question 24

Describe and explain the different outcomes of mitosis and meiosis

Answer 24

1. Mitosis produces 2 genetically identical daughter cells. Both cells are genetically identical to each other and the parent cell. Also, both daughter cells contain the same no. of chromosomes as each other and the parent cell
2. Meiosis produces 4 genetically different daughter cells. Each cell is genetically different from each other and the parent cell and each cell contains half the no. of chromosomes as the parent cell

Mitosis produces genetically identical cells and meiosis produces genetically different cells. This is because, duiring mitosis, there is no pairing or seperation of homologous chromosomes, so there is no crossing over of chromatids and no independent segration of chromosomes.

Meiosis produces 4 daughter cells and mitosis produces 2 daughter cells. This is because mitosis has one division (which is the seperation of sister chromatids) whereas meiosis has 2 divisions (first: seperation of homologous chromosomes, then seperation of sister chromatids)

Mitosis produces daughter cells with the same no. of chromosomes as parent cell. Meiosis produces daughter cells with half no. of chromosomes has parent cells. Ths is because during meiosis 1, division 1, chromosomes arrange themselves into homologous pairs and then homologous pairs are seperated halving the chromosome no. 2 divisions in meiosis. 1 division in mitosis - seperation of sister chromatids.

Question 25

Differntial Reproductive Sucess

Answer 25

1. Some organisms in a population of a specific species are more likely to reproduce than others

Question 26

Evolution

Answer 26

Gradual change in a species ober time

Question 27

Natural Selection

Answer 27

1. Random mutation occurs to gene. This creates new allele of the gene. The allele is beneficial. It increases the organism’s chance of survival. Therefore, organisms with this beneficial allele more likely to survive, reproduce and pass on their genes (including this beneficial allele)
2. This means that in the next generation, a higher propotion of individuals in the population will have the beneficial allele
3. In this generation, those who have the beneficial allele will have a higher chance of survival, and will be more likely to reproduce and pass on their genes (including beneficial allele)
4. So, from generation to generation, allele frequency of beneficial allele increases in population
5. So, over generations, this beneficial allele becomes more common, this leads to evolution