3.2 Chromosomes
Understanding:
Prokaryotes have one chromosome consisting of a circular DNA molecule.
Bacterial Chromosomes:
Some prokaryotes also have plasmids but eukaryotes do not.
Plasmids:
Eukaryote chromosomes are linear DNA molecules associated with histone proteins.
Eukaryote Chromosomes:
In a eukaryote species there are different chromosomes that carry different genes.
Difference between chromosomes:
Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.
Homologous Chromosomes:
Haploid nuclei have on chromosome of each pair.
Haploid Nuclei:
Diploid nuclei have pairs of homologous chromosomes.
Diploid Nuclei:
The number of chromosomes is a characteristic feature of members of a species.
Chromosome Numbers:
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.
Sex Determination:
A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
Karyograms:
Bacterial Chromosomes:
- most prokaryotes have only one chromosome
- consisted of a circular DNA molecule containing all the genes needed for the basic life processes
- DNA in bacteria is not associated with proteins, so is described as naked
- since prokaryotes only have one chromosome, there is usually only a single copy of each gene (unless the cell is preparing for cell division, then it would have two copies)
Some prokaryotes also have plasmids but eukaryotes do not.
Plasmids:
- small extra DNA molecules that are commonly found in prokaryotes but very unusual in eukaryotes
- usually small, circular and naked, containing a few genes that maybe be useful to the cell, but not those needed for its basic life processes (ex: genes are beneficial when an antibiotic is present in the environment but are not at other times)
- plasmids are not replicated at the same time as the chromosome of a prokaryotic cell or at the same rate
- there can be multiple copies of plasmids in a cell, but may not be passed on to cells formed by cell division
- when a prokaryotic cell dies, it's plasmid can be absorbed by a cell of a different species
- plasmids are also used by biologist to transfer genes between species artificially
Eukaryote chromosomes are linear DNA molecules associated with histone proteins.
Eukaryote Chromosomes:
- chromosomes in eukaryotes are composed of DNA and protein
- the DNA is a single immensely long linear DNA molecule
- histones are globular in shape and are wider than the DNA
- histone molecules in a chromosomes, with the DNA molecule wound around them
- adjacent histones in the chromosome are separated by short stretches of the DNA molecule that are not in contact with histones (give the eukaryotic chromosome the appearance of a string of beads during interphase)
In a eukaryote species there are different chromosomes that carry different genes.
Difference between chromosomes:
- at least two different types of chromosome in eukaryotes
- there are 23 types of chromosomes in humans
- every gene in eukaryotes occupy a specific position on one type of chromosome (each chromosome therefore carries a sequence of genes arranged along the linear DNA molecule)
Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.
Homologous Chromosomes:
- if two chromosomes have the same sequence of genes they are homologous (homologous chromosomes are not usually identical to each other because, for at least some of the genes on them, the alleles are different)
- if two eukaryotes are members of the same species, each of the chromosomes in one of them are homologous with at least one chromosome in the other (this allows for interbreeding)
Haploid nuclei have on chromosome of each pair.
Haploid Nuclei:
- has one chromosome of each type
- has one full set of the chromosomes found in its species (haploid nuclei in humans contain 23 chromosomes)
- Gametes (sex cells) have haploid nuclei, so egg and sperm cells contain 23 chromosomes
Diploid nuclei have pairs of homologous chromosomes.
Diploid Nuclei:
- has two chromosomes of each type
- two full sets of the chromosomes that are found in its species (diploid nuclei in humans contain 46 chromosomes)
- when haploid gametes fuse together during sexual reproduction, a zygote with a diploid nucleus is produced, when this divides, more diploid nuclei are produced (many plants and animals consist entirely of diploid cells, apart from the cells (haploid) that they are using to produce gametes for sexual reproduction)
- diploid nuclei have two copies of every gene, apart from the genes on the sex chromosomes
- so harmful recessive mutations can be avoided if a dominant allele is also present
- organisms are more vigorous if they have two different alleles of genes instead of just one (hybrid vigour, reason for strong growth in F1 hybrid crop plants)
The number of chromosomes is a characteristic feature of members of a species.
Chromosome Numbers:
- organisms with a different number of chromosomes are unlikely to be able to interbreed
- number of chromosomes can change after the evolution of a species
- # of chromosomes can decrease if chromosomes become fused together
- # of chromosomes can increase if chromosomes split
- and also mechanisms that may double the # of chromosomes
- but all of the above are very unlikely (numbers tend to remain unchanged over million of years of evolution)
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.
Sex Determination:
- there are two chromosomes in humans that determine sex
- the X chromosome is relatively large and has its centromere near the middle
- the Y chromosome is much smaller and has its centromere near the end
- all the other chromosomes are autosomes and do not affect whether a fetus develops as a male or female
- the X chromosomes has many genes that are essential in males and females (all humans must have one X chromosome)
- the Y chromosome only has a small number of genes (a small part of the Y chromosome has the same sequence of genes as a small part of the X chromosome, but the remainder of the genes on the Y chromosome are not found on the X chromosome and are not needed for female development)
- one Y chromosome gene, called the SRY or TDF, causes the fetus to develop as a male because it initiates the development of testes and testosterone production (male features) because of this gene, a fetus with XY chromosomes will develop as a male
- a fetus with XX chromosomes does not have the TDF gene so ovaries develop instead of testes and female sex hormones are produced, not testosterone
- females have two X chromosomes (females pass on one of their two X chromosomes in each egg cell, so all offspring inherit an X chromosome from their mother)
- gender is determined at the moment of fertilization by one chromosome carried in the sperm (which can either be an X or Y chromosome)
- when sperm are formed, half contain the X and the other half contain the Y chromosome (daughters inherit their father's X chromosome and sons inherit his Y chromosome)
A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
Karyograms:
- an image of the chromosomes of an organism
- Size is arranged from longest to shortest (decreasing length)
- karyotype is a property of an organism - the number and type of chromosomes that the organisms has in its nuclei
- if there are three copies of chromosome 21 in the karyotype, then the child has Down's Syndrome (known as trisomy 21) features of Down's: hearing loss, heart and vision disorders, mental and growth retradation are also common