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Biology Paper 1 > Topic 4: Genes and Protein Synthesis > Flashcards

Topic 4: Genes and Protein Synthesis Flashcards

1
Q

Describe the differences between prokaryotic and eukaryotic DNA

A
  • Located in nucleus in eukaryotes, nucleoid in prokaryotes
  • Is a chromosome molecule in eukaryotes, is a nucleoid molecule in prokaryotes
  • Linear in eukaryotes, circular in prokaryotes
  • Associated with histone proteins in eukaryotes, not in prokaryotes
  • Longer in eukaryotes
  • Eukaryotes have introns, prokaryotes do not
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2
Q

What is a gene?

A

A section of DNA at a fixed position (locus) that contains the coded information for making polypeptides and functional RNA (e.g ribosomal RNA, transfer RNA)

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3
Q

What is a locus?

A

The fixed position of a gene

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4
Q

What gives the phenotype of an organism?

A

A combination of proteins (coded for by DNA) interacting with their environment

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5
Q

What is an intron?
What is an exon?

A

Intron - non-coding sections of DNA that occur between and within genes

Exon - sections of DNA that code for polypeptides

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6
Q

How is each amino acid coded for?

How do we know?

A

1 amino acid is coded for by a DNA triplet (3 bases)

There are 20 amino acids with their own code and only 4 bases. If 1 base = 1 amino acid, there could only be 4 amino acids. If 2 bases = 1 amino acid, there could only be 6 codes.
3 bases gives 64 codes, which is enough

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7
Q

Explain the properties of the genetic code

A
  • Degenerate: most amino acids are coded for by multiple triplets
  • Non-overlapping: every base belongs to 1 triplet and is read only once
  • Universal: with minor exceptions, each triplet codes for the same amino acid in all organisms (indirect evidence for evolution)
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8
Q

What is a chromatid?
How many chromatids are in a chromosome at any time?

A

One thread of a chromosome after DNA replication.

After DNA replication, chromosomes have 2 sister chromatids joined at the centromere, but usually it only has 1

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9
Q

Describe the structure of a chromosome

A
  • DNA double helix wound around histone proteins to fix its position.
  • DNA-histone complex is coiled, looped and coiled further before being packed into chromosome
  • One chromosome has a single DNA molecule
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10
Q

What are homologous chromosomes?

A

A pair of chromosomes, one maternal and one paternal set. Contain the same genes at the same loci but are not genetically identical (have different alleles)

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11
Q

Describe the number of chromosomes in a cell

A

The number of chromosomes is the same for normal individuals of a species, but varies between species. Humans have 46

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12
Q

What is the diploid number?

A

The total number of homologous pairs of chromosomes in a cell

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13
Q

What is a haploid cell?
How are they produced?

A

Cells with half the chromosome number.

During meiosis, the halving of chromosome number is done so each gamete receives one chromosome from each homologous pair - each receives one gene for each characteristic

When the gametes combine, the diploid state with homologous chromosomes is restored

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14
Q

What is an allele?

A

An alternative form of a gene.

Each individual inherits one allele from each parent. When they’re different, alleles have different base sequences so produce different polypeptides.

Any mutation forms an allele

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15
Q

What is the genome?

A

The complete set of genes in a cell, including mitochondria and/or chloroplasts

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16
Q

What is the proteome?

A

The full range of proteins produced by the genome.

Sometimes called the complete proteome, where the proteome is the proteins produced by a given type of cell under a certain set of conditions

17
Q

Describe the structure of messenger RNA (mRNA)

A
  • A strand of bases in a single helix, complementary to DNA
  • Small enough to leave the nucleus via nuclear pores where DNA is not
  • 3 mRNA nucleotides are a codon, codes for 1 amino acid, complementary to a DNA triplet
  • Easily hydrolysed so only exists while manufacture of a particular protein is needed
18
Q

Describe the function of mRNA

A

Used to transfer information on DNA in the nucleus to the cytoplasm where, in conjunction with a ribosome, it is translated and acts as a scaffold to form a protein on

19
Q

Describe the structure of transfer RNA (tRNA)

A
  • A single-stranded chain folded into a clover-leaf shape, made from around 80 nucleotides
  • One end of the chain extends beyond the other (amino acid binding site)
  • At the opposite end is the anticodon: 3 bases complementary to an mRNA codon
  • Binds to a specific amino acid so have many types (must be as many molecules as there are coding triplets)
20
Q

Describe the function of tRNA

A

To bring amino acids coded for by the mRNA molecule to the ribosome in the correct order, in order to form a polypeptide

21
Q

Describe the differences between DNA, mRNA and tRNA

A
  • DNA is a double polynucleotide chain, mRNA and tRNA are single
  • DNA is the largest, then mRNA, then tRNA
  • DNA is a double helix, mRNA is a single helix, tRNA is clover shaped
  • DNA has deoxyribose sugar, mRNA and tRNA have ribose
  • DNA has A,T,G,C, mRNA and tRNA have A,U,G,C
  • DNA is only found in nucleus, mRNA and tRNA are manufactured in nucleus but found throughout cell
  • DNA quantity is constant in all cells but gametes, mRNA and tRNA quantities vary between levels of metabolic activity
  • DNA is chemically very stable, then tRNA, then mRNA (hydrolysed within a few days)
22
Q

What are the stages of protein synthesis?

A
  • Transcription: gene is transcribed onto an mRNA molecule from DNA template
  • Translation: production of polypeptides by sequence of mRNA codons in a ribosome
23
Q

Describe the steps of transcription in protein synthesis

A
  • DNA helicase breaks hydrogen bonds between DNA bases to unwind + expose required gene
  • Free RNA nucleotides in nucleus pair with DNA bases
  • RNA polymerase attaches to start of a polynucleotide strand + moves along, joining RNA nucleotides, forming a pre-mRNA molecule
  • As RNA polymerase adds nucleotides, DNA strands rejoin behind it
  • pre-mRNA is spliced to form an mRNA molecule that is small enough to leave nucleus via nuclear pore
  • Outside nucleus, mRNA is attracted to ribosomes + becomes attached ready for translation
24
Q

What is splicing?

A

Eukaryotes produce pre-mRNA, which must be spliced before translation to remove introns, as they would prevent polypeptide synthesis

Not necessary in prokaryotes as they don’t have introns

25
Q

Describe the process of translation in protein sythesis

A
  • mRNA associates with a ribosome in cytoplasm - 1st 2 codons are on ribosome + attract complementary tRNA molecules, carrying specific amino acids
  • Anticodons on tRNA form hydrogen bonds with mRNA codons
  • Peptide bond formed between 2 adjacent amino acids (catalysed by peptidyl transferase), also requires ATP hydrolysis
  • Ribosome moves forward 1 codon, new tRNA enters + first leaves ribosome
  • Another peptide bond is formed = polypeptide
  • Ribosome continues along mRNA until it reaches a stop codon, then detaches due to release factor
  • Polypeptide chain is released + folded into a protein

Biology Paper 1 (9 decks)

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