Inheritance

Question 1

What is the genome

Answer 1

• The entire set of the genetic material of an organism is known as its genome
• Each gene within the genome is made up of a small section of DNA that codes for a particular sequence of amino acids
• These sequences of amino acids form different types of proteins

Question 2

What is an allele

Answer 2

different versions of a gene
o e.g. there are different alleles that code for brown or blue eye colour
o Having 2 alleles of the same type is called homozygous
o Having 2 alleles that are different to one another is called heterozygous

Question 3

What is a chromosomes

Answer 3

• In the nucleus of a cell, the DNA double helix supercoils to form structures called chromosomes
• They are only visible during cell division
• Ordinary human body cells contain 23 pairs of chromosomes
  o This is a diploid number (often shown as 2n)
  o One chromosome from a pair is inherited from each parent
  o Each chromosome pair is called a homologous pair
• The 23rd pair of chromosomes are the sex chromosomes
  o In females, the sex chromosomes are the same, a homozygous genotype (XX)
  o In males, the sex chromosomes are different, a heterozygous genotype (XY)
• Gametes contain just 23 individual chromosomes
  o This is half the full number of a body cell and is called the haploid number (or n)

Question 4

The Structure of DNA

Answer 4

• DNA, or deoxyribonucleic acid, is the genetic material found in the nucleus of a cell
• DNA, is a polymer made up of two strands coiled around to make a double helix
• The strands are formed from a sugar phosphate backbone with bases joined together by complementary base pairing
  o Adenine pairs with thymine
  o Guanine pairs with cytosine
• Cell division and protein synthesis both rely on these base-pairing rules because each half of the DNA double helix acts as a template to be copied in order to create a complete new double helix

Question 5

RNA structure

Answer 5

• Like DNA, the nucleic acid RNA (ribonucleic acid) is a polynucleotide – it is made up of many nucleotides linked together in a long chain
• Like DNA, RNA nucleotides contain the nitrogenous bases adenine (A), guanine (G) and cytosine (C)
• Unlike DNA, RNA nucleotides never contain the nitrogenous base thymine (T) – in place of this they contain the nitrogenous base uracil (U)
• Unlike DNA, RNA molecules are only made up of one polynucleotide strand (they are single-stranded)
• Each RNA polynucleotide strand is made up of alternating ribose sugars and phosphate groups linked together, with the nitrogenous bases of each nucleotide projecting out sideways from the single-stranded RNA molecule
• An example of an RNA molecule is messenger RNA (mRNA), which is the transcript copy of a gene that encodes a specific polypeptide. Two other examples are transfer RNA (tRNA) and ribosomal RNA (rRNA)

Question 6

Transcription & Translation

Answer 6

• A gene is a sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids, that in turn make up a specific polypeptide (protein)
• This process of protein synthesis occurs in two stages:
  o Transcription – DNA is transcribed and an mRNA molecule is produced
  o Translation – mRNA (messenger RNA) is translated and an amino acid sequence (protein) is produced

Question 7

Transcription

Answer 7

• This stage of protein synthesis occurs in the nucleus of the cell
• Part of a DNA molecule unwinds (the hydrogen bonds between the complementary base pairs break)
• This exposes the gene to be transcribed (the gene from which a particular polypeptide will be produced)
• A complementary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA (messenger RNA)
• The mRNA molecule leaves the nucleus via a pore in the nuclear envelope

Question 8

Translation

Answer 8

• This stage of protein synthesis occurs in the cytoplasm of the cell
• After leaving the nucleus, the mRNA molecule attaches to a ribosome
• In the cytoplasm, there are free molecules of tRNA (transfer RNA)
• These tRNA molecules have a triplet of unpaired bases at one end (known as the anticodon) and a region where a specific amino acid can attach at the other
• There are at least 20 different tRNA molecules, each with a specific anticodon and specific amino acid binding site
• The tRNA molecules bind with their specific amino acids (also in the cytoplasm) and bring them to the mRNA molecule on the ribosome
• The triplet of bases (anticodon) on each tRNA molecule pairs with a complementary triplet (codon) on the mRNA molecule
• Two tRNA molecules fit onto the ribosome at any one time, bringing the amino acid they are each carrying side by side
• A peptide bond is then formed between the two amino acids
• This process continues until a ‘stop’ codon on the mRNA molecule is reached – this acts as a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete
• This amino acid chain then forms the final polypeptide (protein)