3.2 Chromosomes

Question 1

what do prokaryotes possess instead of a nucleus?

Answer 1

nucleoid (genetic material found free in the cytoplasm)

Question 2

what does the genetic material of a prokaryote consist of? (2)

Answer 2

• single chromosome consisting of a circular DNA molecule (genophore)
• the DNA of prokaryotic cells is naked (not associated with proteins for additional packaging)

Question 3

what do prokaryotic cells possess in addition to the genophere?

Answer 3

plasmids (circular DNA molecules)

Question 4

what are plasmids?

Answer 4

• small circular DNA molecules that contain only a few genes and are capable of self-replication

Question 5

what do bacterial cells do with plasmids?

Answer 5

• exchange plasmids via sex pili in a process called bacterial conjunction

Question 6

what does the exchange of genetic material allow for bacteria?

Answer 6

allows bacteria to evolve new features within a generation (horizontal gene transfer)

Question 7

why are plasmids ideal vectors for gene manipulation in labs? (2)

Answer 7

they can self-replicate and autonomously synthesise proteins

Question 8

what does the genetic material of eukaryotic cells consist of? (2)

Answer 8

• multiple linear molecules of DNA that are associated with histone proteins
• the packaging of DNA with histone proteins result in a compact structure for ore efficient storage

Question 9

what is the organisation of eukaryotic chromosomes? (5)

Answer 9

• DNA is complexed with 8 histone proteins (octamer) to form a nucleosome
• nucleosomes are linked by an additional histone protein (H1 histone)
• these coil to form a solenoid structure which is condensed to form a 30nm fibre
• these fibres then form loops which are compressed and folded around a protein scaffold to form chromatin
• chromatin will supercoil during cell division to form chromosomes that re visible (stained) under microscope

Question 10

what is the constriction point of a chromosome called?

Answer 10

centromere

Question 11

why is it significant that sexually reproducing organisms inherit their genetic sequences from both parents?

Answer 11

these organism will possess 2 copies of each chromosome

Question 12

what are maternal and paternal chromosome pairs called?

Answer 12

homologous chromosomes

Question 13

what do homologous chromosomes share?

Answer 13

• the same structural features (eg. same size, banding patterns, centromere positions)
• same genes at the same loci positions

Question 14

why must homologous chromosomes be separated in gametes (via meiosis)?

Answer 14

to prevent chromosome numbers continually doubling with each generation

Question 15

why do sexually reproducing organisms have 2 sets of chromosomes (diploid)?

Answer 15

receive genetic material from both parents

Question 16

what must organisms must do to create gametes from the diploid chromosomes?

Answer 16

• they must create gamete with half the number of chromosomes (haploid)

Question 17

what occurs when 2 haploid gametes fuse?

Answer 17

it results in a diploid cell (zygote) which grows into new organism

Question 18

what are the characteristics of diploid cells? (4)

Answer 18

• possess pairs of homologous chromosomes(in nuclei)
• nuclei will possess 2 gene copies (allele) for each trait
• all somatic (body) cell in the organism will be diploid with new diploid cells created via mitosis
• diploid cells are present in most animals and many plants

Question 19

what are the characteristics of haploid cells? (4)

Answer 19

• nuclei possessing only 1 set of chromosomes are haploid
• these nuclei will possess a single gene copy(allele) for each trait
• all sex cells (gametes) in the organism will be haploid and are derived from diploid cells via meoisis
• haploid cells are also present in bacteria (asexual) and funji (except when reproducing)

Question 20

how is sex determined in humans?

Answer 20

sex chromosomes (pairs of chromosomes)

Question 21

what sex chromosomes do females possess?

Answer 21

2 copies of X chromosome (XX)

Question 22

what sex chromosomes do males possess?

Answer 22

1 copy of X and 1 copy of Y (shorter)

Question 23

what will the absence of a Y chromosome lead to?

Answer 23

female sex organs developing

Question 24

which parent is always responsible for determining the sex of offspring?

Answer 24

• father
• is male sperm contain X chromosome then girl
• if sperm contain Y chromosome then boy

Question 25

what are the remaining chromosomes in the body called (the ones which do not determine sex)?

Answer 25

autosomes

Question 26

what are karyotypes?

Answer 26

the number and types of chromosomes in a eukaryotic cells

Question 27

what is the process which determines karyotypes? (3)

Answer 27

• harvesting cells (from foetus/white blood cells of adults)
• chemically inducing cell division, then arresting mitosis while chromosomes are condensed
• stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids or not

Question 28

how do you produce a visual profile?

Answer 28

chromosomes are stained and photographed to generate a karyogram

Question 29

What is karyotyping used for? (2)

Answer 29

• to determine the gender of the unborn child (via identification of sex chromosomes)
• test for chromosomal abnormalities

Question 30

what is down syndrome?

Answer 30

• a condition whereby the individual has three copes of chromosome

Question 31

what is down syndrome caused y? (2)

Answer 31

• by a non-disjunction event in one of the parental gametes
• the extra genetic material causes mental and physical delays in child development

Question 32

What is the process of autoradiographie? (7)

Answer 32

• cells grown in a solution containing radioactive thymidine
• triaged thymidine is incorporated into the chromosomal DNA of the cell
• the chromosomes are isolated by gently losing the cells and fixing the chromosomes to photographic surface
• the surface is then immersed in a radtioactively-sensitive emulsion containing silver bromide
• the radiation released from the triaged thymidine converts AG+ ions in silver bro moi de into insoluble metal grains
• following a period of exposure,excess bromide is washed away, leaving the silver grins to appear as small black dots
• when the photographic film is developed, the chromosomal DNA can be visualised with an electon microscope

Question 33

How did John Cairns measure autoradiography?

Answer 33

• Used autoradiography to visualise the chromosome whilst uncoiled for more accurate indications of length
• by using triaged uracil region of active transcription can be identifies within uncoiled chromosome

Question 34

What other discoveries did John Cairns find out by autoradiography? (2)

Answer 34

• DNA replication involves formation of a replication bubble (and prokaryotic replication involves a single origin of replication)
• DNA replication is bi-directional (it occurs independently at both ends of the replication bubble)

Question 35

What are the general rule for genome size?(3)

Answer 35

• viruses and bacteria tend to have very small genomes
• prokaryotes typically have smaller genome than eukaryotes
• sizes of plant gem ones can vary dramatically due to the capacity for plant species to self-fertilise and become polyploid