3.2 Chromosomes Flashcards
What are bacterial chromosomes like?
have one chromosome consisting of a circular DNA molecule.
What are plasmids? Where are they found?
small extra DNA molecules that are commonly found in prokaryotes but are very unusual in eukaryotes - small, circular and naked, containing a few genes that may be useful to the cell
but not those needed for its basic life processes.
What are prokaryote chromosomes like?
referred to as ‘naked’ because they are not associated with any proteins.
What are eukaryote chromosomes like?
linear DNA molecules associated with histone proteins.
How are chromosomes in eukaryotes different?
They differ both in length and in the position of the centromere where the two chromatids are held together; the centromere can be positioned anywhere from close to an end to the centre of the chromosome
What are homologous chromosomes?
Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.
- are not usually identical to each other because the alleles are different
- If two eukaryotes are members of the same species, we can expect each of the chromosomes in one eukaryote to be homologous with at least one chromosome in the other eukaryote, which allows members of a species to interbreed
Name the genome sizes for the following: T2 phage; Escherichia coli (E.coli;) Drosophila melanogaster; Homo sapiens; Paris japonica
0.18 5 140 3,000 150,000
Distinguish between haploid and diploid nuclei.
Haploid nuclei have one chromosome of each pair.
- Has one full set of chromosomes that are found in its species; haploid nuclei in humans contain 23 chromosomes
- Gametes have haploid nuclei, so in humans both egg and sperm cells contain 23 chromosomes
Diploid nuclei have pairs of homologous chromosomes.
- Have two chromosomes of each type an two copies of every gene apart from genes on the sex chromosomes
- Has two sets of chromosomes; diploid nuclei in humans contain 46 chromosomes
- When haploid gametes fuse together during sexual reproduction, a zygote with a diploid nucleus is produced
What is the importance of chromosome numbers in characterising species?
The number of chromosomes is a characteristic feature of members of a species.
- Organisms with a different number of chromosomes are unlikely to be able to interbreed so all the interbreeding members of a species need to have the same number of chromosomes
- The number of chromosomes can change during the evolution of a species; can decrease if chromosomes become fused together or increase if splits occur
Name the number of chromosomes for the following: Parascaris equorum; Oryza sativa; Homo sapiens; Pan troglodytes; Canis familiaris
4 24 46 48 78
How is sex determined in an organism?
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.
Two chromosomes in humans that determine sex (sex chromosomes): the X chromosome which is relatively large and has its centromere near the middle, and the Y chromosome which is much smaller and has its centromere near the end
Other chromosomes are autosomes and do not affect the sex
The X chromosome has many genes that are essential in both males and females; all humans must have at least one X chromosome
The Y chromosome only has 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 remaining genes of the Y chromosome are not found on the X chromosome, and are not needed for female development → male: XY, female: XX
All offspring inherit an X chromosome from their mother; daughters inherit their father’s X chromosome and sons inherit his Y chromosome
What is a karyogram? What is it used for?
Karyogram: image of the chromosomes of an organism arranged in homologous pairs of decreasing length
- shows the chromosomes of an organism in homologous pairs of decreasing length.
- The chromosomes are arranged according to their size and structure
- The position of the centromere and the pattern of banding allow chromosomes that are of a different type but similar size to be distinguished
How are karyotypes used to determine Down syndrome?
Karyotype: property of a cell - the number and type of chromosomes present in the nucleus
Used to deduce whether an individual is male or female: if two XX chromosomes are present the individual is female, whereas XY indicates a male
Used to diagnose Down syndrome and other chromosome abnormalities: normal karyotypes have a pair of each chromosome type including a pair of sex chromosomes. If there are more or less than two of each pair, the person has a chromosome abnormality. The most common type is Down syndrome which is due to having 3 copies of chromosome 21 (trisomy 21)
