2.6 - Cell Cycle Flashcards
Name of asexual reproduction in different organisms
Animals - mitosis
Plants - mitosis
Bacteria - binary fission
Fungi - budding
What is meant by a homologous pair of chromosomes?
- One chromosome is maternal and the other is paternal
- Both have the same genes
- But have different alleles at the same locus
- Are usually the same or similar in length
- The centromere is usually in the same position
- They from bivalents when they pair up during meiosis
Differences between mitosis in plant and animal cells
- Plants don’t have centrioles - can’t form spindle fibres
- In plants, cell plate forms between new cells
- Mitosis only occurs in meristem cells
- Nuclear envelope does not reform in plant cells in telophase
- Cytokinesis starts from the middle of the cell in plants
Outline the process of budding in yeast.
• Nucleus divides by mitosis • Cell swells on one side /bulges ° Nucleus moves into the bulge • Nucleus pinches off → new cell wall forms • New yeast cell forms.
Name one adaptation of the guard cell and explains how this aids its function
- Guard cell has a large vacuole
- Can take up water and become turgid - Cell wall thicker on one side
- Causes the cell to open the stomata on one side - Lots of mitochondria
- Generates a lot of ATP for active transport of molecules into the plant
Homologous chromosomes def
Groups of chromosomes that have different alleles of the same gene at the same loci/position
Random assortment mechanism
- takes place during anaphase 2
- In diploid organisms chromosomes come in homologous pairs
- They are homologous because they have the same genes but different alleles
- In gamete formation the chromosome number reduces by half
- The gamete needs one copy of each chromosome
- That copy can come from the paternal or maternal chromosome - random (random assortment)
- which of each pair goes into the gamete is random and generates variants
- Number of possible combinations is 2^n - where n is the haploid number (no. Of chromosomes in a sperm or egg cell)
Describe what is meant by independent assortment (3 Marks)
- Occurs during prophase/metaphase 1 of meiosis
- In prophase 1, homologous chromosomes pair up
- In metaphase 1, homologous chromosomes are pulled towards the equator of the spindle
- Each pair is arranged randomly
- Orientation of one homologous chromosome is independent to orientation of the other pair
- This produces a random different combination of alleles in daughter cells
- in humans, 2^23 defines the number of different combinations of chromosomes possible
Describe how crossing over occurs in meiosis (3 Marks)
- Crossing over of non-sister chromatids occurs in prophase 1
- Non-sister chromatids can cross over and get entangled
- These crossing points are called chiasmata
- Entanglement places stress on the DNA molecules
- Section of chromatin May break off chromosome and rejoin with the chromatids from the other chromosome
- This produces a new combination of alleles in the two chromosomes
- Called Bivalents
Three methods to produce variation in meiosis
- Crossing over of non-sister chromatids
- Independent Assortment of homologous chromosomes
- Random fertilisation/fusion of gametes
Random fusion of gametes mechanism
- Meiosis creates genetic variation between the gametes produced by crossing over and independent assortment
- This means each gamete carries substantially different alleles
- During fertilisation, any male gamete can fuse with a female egg to form a zygote
- Each zygote will have a unique combination of alleles from random fusion of gametes
Significance of meiosis
- Having genetically different offspring can be advantageous for natural selection
- Crossing over and independent assortment result in different combinations of alleles in gametes
Meiosis is used in many organisms for the production of gametes.
Explain why meiosis needs to have twice as many stages as mitosis.
(2 Marks)
- To halve chromosome number
- To separate homologous pairs (of chromosomes) and sister chromatids ;
- because, DNA (previously) replicated /
- chromosomes are two chromatids at start ;
Discuss the possible effects that mutation can have on the structure and function of a
protein.
( 3 Marks)
- Changes DNA base sequence
- Can change primary structure of proteins produced
- Protein can be unchanged due to silent (substitution) mutation
- Protein can be shorter due to deletion mutation
- Changes shape of protein
- Function can be worse/better
3 functions of mitosis
- Growth of tissues
- Repair/replacement of tissues
- Asexual reproduction - E.g. bacteria
