Unit 3.1
• A gene is a heritable factor that consists of a length of DNA and influences a specific
characteristic.
• The entire base sequence of human genes was sequenced in the Human Genome Project. The
genome is the whole of the genetic information of an organism
• A gene occupies a specific position on a chromosome known as the gene locus.
• Alleles are different forms of a specific gene such
as having brown or blonde hair. They differ from
each other by only 1 or 2 bases. New alleles are
formed by a mutation.
• Sickle cell anaemia is caused by base substitution
mutation where a thymine is replaced with alanine.
• This causes the haemoglobin to be unable to carry
oxygen properly so the cell develops a sickle
shape.
• Rice has more genes than humans, E. Coli has less. Humans have around 21,000.
Unit 3.2
• Prokaryotes have one chromosome consisting of a circular DNA molecule. They may also have
extra DNA in the form of plasmids.
• Bacterial cells may exchange plasmids via their sex pili, in a process known as bacterial
conjugation.
• Eukaryotic chromosomes are linear DNA molecules associated with histone proteins.
• In humans there are 22 pairs of autosomal chromosomes and one pair of sex chromosomes.
• On a eukaryotic chromosome the p arm is short and the q arm is long. Gene loci are named
based on their chromosome number, arm and band location e.g 7p20.
• Homologous chromosomes (one from each parent) carry the same sequence of genes but not
necessarily the same alleles of those genes.
• Diploid nuclei (2n) have pairs of homologous chromosomes whereas haploid nuclei (n) have one
chromosome of each pair.
• A karyogram shows the chromosomes of an organism in homologous pairs of decreasing
length. They can be used to determine the sex or to identify trisomy (Down’s Syndrome is
trisomy 21).
• The chromosome number does not give an indication of how complex a species is:
Unit 3.3
• During meiosis one diploid nucleus divides to produce four haploid nuclei.
• DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
• See Unit 10.1 for stages of meiosis.
• A gene is a heritable factor that consists of a length of DNA and influences a specific
characteristic.
• The entire base sequence of human genes was sequenced in the Human Genome Project. The
genome is the whole of the genetic information of an organism
• A gene occupies a specific position on a chromosome known as the gene locus.
• Alleles are different forms of a specific gene such
as having brown or blonde hair. They differ from
each other by only 1 or 2 bases. New alleles are
formed by a mutation.
• Sickle cell anaemia is caused by base substitution
mutation where a thymine is replaced with alanine.
• This causes the haemoglobin to be unable to carry
oxygen properly so the cell develops a sickle
shape.
• Rice has more genes than humans, E. Coli has less. Humans have around 21,000.
Unit 3.2
• Prokaryotes have one chromosome consisting of a circular DNA molecule. They may also have
extra DNA in the form of plasmids.
• Bacterial cells may exchange plasmids via their sex pili, in a process known as bacterial
conjugation.
• Eukaryotic chromosomes are linear DNA molecules associated with histone proteins.
• In humans there are 22 pairs of autosomal chromosomes and one pair of sex chromosomes.
• On a eukaryotic chromosome the p arm is short and the q arm is long. Gene loci are named
based on their chromosome number, arm and band location e.g 7p20.
• Homologous chromosomes (one from each parent) carry the same sequence of genes but not
necessarily the same alleles of those genes.
• Diploid nuclei (2n) have pairs of homologous chromosomes whereas haploid nuclei (n) have one
chromosome of each pair.
• A karyogram shows the chromosomes of an organism in homologous pairs of decreasing
length. They can be used to determine the sex or to identify trisomy (Down’s Syndrome is
trisomy 21).
• The chromosome number does not give an indication of how complex a species is:
Unit 3.3
• During meiosis one diploid nucleus divides to produce four haploid nuclei.
• DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
• See Unit 10.1 for stages of meiosis.
