3.2 chromosomes Flashcards
what are autosomes?
chromosomes that do not determine sex
what is the difference between genome size and genome number?
genome number: number of genes
genome size: total length of dna in organism
what is the difference between karyotype and karyogram?
karyotype: property of a cell; number and type of chromosomes present in nucleus of eukaryotic cell; determined by a process that involves:
- harvesting cells (usually from foetus / white blood cells of adults)
- chemically inducing cell division, then arresting mitosis while the chromosomes are condensed
- stage during which mitosis is halted (metaphase: chromosomes most aligned and visible) will determine whether chromosomes appear with sister chromatids or not
- chromosomes stained and photographed to generate a visual profile known as a karyogram
karyogram: photograph / diagram of chromosomes
- chromosomes of an organism are arranged into homologous pairs according to size (with sex chromosomes shown last)
what kinds of dna do prokaryotes have?
2 kinds of dna used as genetic material:
- single circular chromosomal dna
- multiple optional and accessorial dna (plasmids)
- single circular chromosomal dna coiled up and concentrated within nucleoid region in prokaryote
- no concept of being diploid; each gene exists as single copy
- chromsomal dna replicated prior to binary fission
what are the features of dna plasmids?
- extra chromosomal
- naked dna; not associated with proteins
- small and circular dna
- not responsible for essential and normal life processes of the cells
- commonly contain additional survival characteristics like antibiotic resistance / resistance to certain conditions
- can be passed between prokaryotes
- plasmids are not naturally occurring in eukaryotic cels but some are capable of carrying and expressing genes on plasmid dna
how did scientists measure the length of dna molecules?
- cairns’ technique for measuring length of dna molecules by autoradiography
- e. coli cultured on a medium containing radioactively labelled thymidine (made using isotope of hydrogen)
- as bacterial cells grew in numbers, dna was actively synthesised utilising labelled thymine ( “t” nucleotide)
- bacterial cells were lysed using enzymes and dna fixed onto a photographic surface and immersed into radioactively sensitive photographic emulsion
- radiation released by labelled thymine caused darkening on photographic emulsion, thus allowing scientists to view dna chromosomes
- resulting images: dna in e. coli had an average length of 1, 100 μm while e. coli cells were about 2 μm long
ACTUAL PROCESS:
PREPARATION
1. incubate cells in radioactive H^3 – T (thymidine)
2. gently lyse cells and fix contents
3. embed contents onto slide
4. immerse slide in radioactively-sensitive emulsion
PROCESS
- slide dipped in silver bromide (AgBr) emulsion (AgBr crystals present)
- radiation activates nearby Ag+ ions
- activated Ag+ ions reduced to Ag metal
- soluble ions washed away to leave metal
DEVELOP
- expose slide to photographic film
- develop to transfer image to film
- autoradiograph produced
what are eukaryote [NOT BACTERIA] chromosomes?
- linear dna molecules associated w histone proteins
- dna highly packaged to fit into nucleus of cell
- dna: 2 strands in double helix structure
- coiled around globular pieces of dna known as histones (wound around the histones in a “beads on a string” structure)
- nucleosomes: structures formed when dna wraps around single histone
bacteria has no histones
dna –> nucleosome –> chromatine –> chromatine loops –> condensed chromatine loops –> chromosome
how are chromosomes formed?
- at simplest level, chromatin is a double-stranded helical structure of dna
- dna is complexed with 8 histone proteins (octamer) to form nucleosomes
- linked by an additional histone protein (h1 histone) to form a string of chromatosomes
- coil to form a solenoid structure (~6 chromosomes per turn) which is condensed to form 30 nm fibre
- fibres form loops, which are compressed and folded around a protein scaffold to form chromatin
- chromatin then supercoils during cell division to form chromosomes that are visible (when stained) under microscope
dna –> nucleosome –> chromatosome –> solenoid –> 30 nm fibre –> chromatin –> chromosomes
what is a centromere?
constriction point on a chromosome that divides the chromosome into 2 sections
- shorter section: p arm
- longer section: q arm
how do eukaryotic chromosomes differ from one another and how can this be identified?
- chromosomes may differ in size and position of centromere
- chromosomes can be stained with particular dyes e.g. giemsa stain to generate unique banding patterns
what is the locus? (plural: loci)
each chromosome carries specific genes; position of particular gene on chromosome is the locus
- consistent throughout the species
how do you identify the locus using codes like 7q31?
- first number denotes chromosome (in 7q31, chromosome 7)
- letter denotes which arm the locus is positioned on (in 7q31, the longer arm q)
- number corresponding to G (gene?) band location
why are there homologous chromosomes?
- sexually reproducing organisms inherit genetic sequences from both parents
- fertilisation event: 2 haploid cells fuse together to form 1 zygote; each haploid gamete contributes 1 chromosome
- possess 2 copies of each chromosome; 1 maternal 1 paternal
- maternal and paternal pairs: homologous chromosomes
what are homologous chromosomes?
chromosomes that:
- same structural features (e.g. same size, same banding patterns, same centromere positions
- same genes at same loci positions (genes same, alleles may be different)
- must be separated in gametes (via meiosis) prior to reproduction to prevent chromosome numbers continually doubling w each generation
what are diploid nuclei?
- nuclei possessing pairs of homologous chromosomes (2 copies of each chromosome)
- 2n
- possess 2 gene copies (alleles) for each trait but the genes do not have to be identical
- all somatic cells are diploid, with new diploid cells created by mitosis
- present in most animals and many plants
- diploid organism more likely to survive chromosomal damage and mutations as there is still chance of having 1 functional chromosome / gene
