DNA, genes and protein synthesis

Question 1

What is a gene?

Answer 1

a gene is a section of DNA that contains the coded info for making polypeptides and functional RNA
- Polypeptides make up proteins and so genes determine the proteins of an organism. Enzymes are proteins
The gene is a base sequence of DNA that codes for:
- amino acid sequence of a polypeptide
-or a functional RNA including ribosomal RNA and transfer RNA

Question 2

Genetic code

Answer 2

• degenerate code: most amino acids are coded for more than one triplet
• non overlapping: sequence is only read once
• universal: each triplet codes for the same amino acid in all organims
• Exons: only certain sequences code for amino acids
• Introns: within the gene these exons are separated by further non-coding sequences

Question 3

Prokaryotic and Eukaryotic DNA

Answer 3

• in prokaryotic cells, in bacteria the DNA molecules are shorter and they form a circle and are not associated with protein molecules. Prokaryotic don’t obviously have chromosomes
• in Eukaryotic cells, the DNA is longer and form a line rather than a circle and occur in association with proteins called histones to form structures called chromosomes.
  The mitochondria and chloroplasts of eukaryotic cells also contain DNA which is short and circular and not associated with proteins

Question 4

Chromosomes structure

Answer 4

• they’re really visible when a cell is dividing for the rest of the time they’re dispersed throughout the nucleus
• Each thread is called a chromatid because DNA has already replicated to give two identical DNA molecules
• The DNA in chromosomes is held by histone

Question 5

DNA and histones

Answer 5

• a helix is wound around histones to fix it in position

- This DNA histone complex is then coiled and is further coiled before being packed into the chromosome

Question 6

Homologous chromosomes

Answer 6

-one of each pair is derived from the chromosomes provided by the mother and father and the total number is referred to the diploid. in humans this is 46

Question 7

What is an allele

Answer 7

an allele is one of a number of alternatives forms a gene

• each allele has a different base sequence and different amino acid sequence and produces a different polypeptide
• any change base sequence of a gene produces a new allele of that gene (mutation) and results in a different sequence of amino acids being coded for, this different sequence results in production of different polypeptide

Question 8

Transferring the coded info

Answer 8

• sections of the DNA code are transcribed onto single stranded RNA so the DNA can be translated into proteins

Question 9

proteins are synthesised where?

Answer 9

mRNA (messenger RNA)

Question 10

what’s a codon

Answer 10

three bases on a mRNA that codes for a single amino acid

Question 11

Genome

Answer 11

the complete set of genes in a cell including those in mitochondria and chloroplasts

Question 12

Proteome/ complete proteome

Answer 12

the full range of proteins produced by the genome,

Question 13

DNA structure

Answer 13

DNA is composed of two nucleotide chains wound around each other

Question 14

RNA structure

Answer 14

RNA is a polymer made up of repeating mononucleotide subunits.
Each nucleotide is made up of:
- a pentose sugar ribose
- organic bases, adenine, guanine, cytosine and uracil
- a phosphate group

Question 15

two types of RNA in protein synthesis

Answer 15

• messenger RNA

- transfer RNA

Question 16

Messenger RNA (mRNA)

Answer 16

• mRNA is a long strand that is arranged in a single helix
• the base sequence of mRNA is determined by the sequence of bases on a length of DNA
• mRNA leaves the nucleus via pores in nuclear envelope and enters the cytoplasm where it associates with ribosomes
• There it acts as a template for protein synthesis
  > It’s structure is suited to this function because it possesses info in the form of codons
• The sequence of codons determines the amino acid sequence of a specific polypeptide

Question 17

Transfer RNA (tRNA)

Answer 17

• small molecule made up of around 80 nucleotides
• it’s a single-stranded chain folded into a clover leaf shape
• this is the part of tRNA molecule to which an amino acid can easily attach
• At the opposite end of the tRNA molecule is a sequence of three other organic bases, known as the anticodon
• there’s coding triplets and tRNA is specific to one amino acid and has an anticodon that is specific to that amino acid

Question 18

Complementary base pairings that RNA forms are therefore

Answer 18

• guanine with cytosine

- adenine with uracil (in RNA) or thymine (in DNA)

Question 19

During protein synthesis:

Answer 19

an anticodon pairs with the three complementary organic bases that make up the codon on mRNA
-The tRNA structure with its end chain for attaching amino acids and its anticodon for complementary base pairing with the codon of the mRNA and is structurally suited to its role of lining up amino acids on the mRNA template during protein synthesis

Question 20

Instructions by the DNA in cell’s nucleus

Answer 20

• DNA provides the instructions in the form of a long sequence of bases
• transcription take place when a complementary section of part of this sequence is made up in the form of a molecule called pre-mRNA
• the pre mRNA is spliced to form mRNA
• translation takes place where mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry are linked to form a polypeptide

Question 21

Transcription

Answer 21

the process of making pre-mRNA using part of the DNA as a template:

• An enzyme acts on a specific region of the DNA causing the two strands to separate and expose the nucleotide bases in that region
• nucleotide bases on one of the two DNA strands known as the template strand, pair with their complementary nuceltoides. The enzyme RNA polymerase the moves along the strand and joins the nucelotides together to form a pre mRNA molecule
• exposed guanine base on the DNA binds to cytosine base
• As the RNA polymerase adds the nucleotides one at a time to build a strand of pre-mRNA, the DNA strands rejoin behind it. As a result, only 12 bases on the DNA are exposed at any one time
• When the RNA polymerase reaches a particular sequence of bases on the DNA that it recognises as a ‘stop’ triplet code, it detaches and the production of pre-mRNA is then complete

Question 22

Splicing of pre-mRNA

Answer 22

• in prokaryotic cells, transcription results directly in the production of mRNA from DNA
• in eukaryotic cells transcription resukts in the production of pre-mRNA which is spliced to form mRNA
• DNA of a gene eukaryotic cells is made up of sections called exons that code for proteins and introns that don’t
• splicing is the base sequences corresponding to the introns are removed and the functional exons are joined together during a process called splicing
• mRNA are too large to diffuse out of the nucleus so once they have been spliced they can leave via a nuclear pore

Question 23

Synthesising a polypeptide

Answer 23

The first part is transcription, which produces a molecule of messenger RNA (mRNA) from the gene. Transcription takes place in the nucleus of the cell. First, the gene to be transcribed dips into the nucleolus and the DNA is unwound, so the bases of the transcribing DNA strand are exposed. Complementary nucleotide base pairs which are in the nucleus then line up next to the exposed bases. C pairs with G, and A pairs with U, because U replaces T in RNA molecules. Phosphodiester bonds form between the RNA bases to form the mRNA molecule, catalysed by the enzyme RNA polymerase. This molecule of mRNA can then leave the nucleus through a nuclear pore, and the DNA rewinds.

The second part of protein synthesis is translation. It takes place outside the nucleus of the cell, on a ribosome. The mRNA produced in transcription attaches to the groove in the ribosome. Three bases on the mRNA make up a codon, and each group of three codes for a specific amino acid found in the cell. A tRNA molecule-another type of RNA-has the anticodon which is specific to each codon on the mRNA, and a specific amino acid is bound to the opposite end of the tRNA. tRNAs line up alongside the complementary codons on the mRNA, and then peptide bonds form between the adjacent amino acids. The mRNA moves through the ribosome, adding amino acids until the entire length of the mRNA has been translated. The completed polypeptide then leaves the ribosome and may go to another organelle for completion before it can perform its function.

Question 24

Assembling a protein

Answer 24

sometimes a single polypeptide chain is a functional protein:

• polypeptide is coiled or folded producing its secondary structure
• the secondary structure is folded, producing the tertiary structure
• Different polypeptide chains along with non protein groups are linked to form the quarternary structure