Term 2 Lecture 11: chromosomes and the cell cycle Flashcards
Genetics can be divided into 3 main subgenres
Transmission (‘classical’) genetics - covered in upcoming lectures
Molecular genetics - covered in previous lectures
Population genetics - used in ecology
Genetic variation can be inherited by
Meiosis
Polyploidy (having more than one genome)
Mobile DNA e.g. transposons and retrotransposons
The mechanisms involved include
Mendelian inheritance
Linkage
Sex-linked genes
Extensions and modifications to Mendelian rules
Humans have 23 pairs of chromosomes
Humans are diploid organisms meaning they have two sets of chromosomes organised into homologous pairs
diploid = cells carry two sets of genetic information
haploid = cells carry one set of genetic information
Chromosome structure
At times a chromosome consists of a single chromatid, other times it consists of two sister (identical) chromatids.
At the ends of a chromosome are telomere regions that protect the ends and keep them stable - telomeres get shorter as we age.
The centromere is a constricted region of the chromosome (often near the middle) this is where the kinetochores form and spindle fibres attach to them.
You only see chromosomes condensed during cell division, normally the DNA is fully unravelled to be actively accessible.
The structure of a chromosome can be described by the location of its centromere
Submetacentric - just above halfway
Metacentric - in the middle (halfway down typical X shape)
Telocentric - at the top so chromosome resembles inverted U
Acrocentric - very close to the top
Chromosomes retain the same structure every time they condense making their shape a factor in identification. Fluorescent markers may also be used to ID chromosomes by banding patterns
Why you may want to perform a foetal karyotype screening
you may screen the chromosomes of a foetus if you are concerned about the genetic outcomes of a pregnancy e.g. if the mother is older (mid 30’s) there is a higher risk of chromosomal disorders e.g. Downs Syndrome is 50% more likely in children conceived after 34
Nowadays PCR may be used instead
The Cell cycle
G1 - cell grows
G0 - cell may then enter this non-dividing phase
G1/S after this point the cell is committed to dividing
S DNA duplicates
G2 cell prepares for mitosis (supercoiling)
G2/M after this checkpoint the cell can divide
M mitosis and cytokinesis - nuclear and cellular division
2 phases
interphase - cell growth (all steps before M)
M phase - nuclear and cell division
In the M phase
Mitosis - separation of sister chromatids
Cytokinesis - separation of cytoplasm and formation of new cell membranes
Mitosis
(Follows interphase)
prophase - condensing
prometaphase - spindles form
metaphase - chromosomes line up along ‘metaphase plate’ with spindles attached to centromeres either end of cell
Anaphase - spindles contract and sister chromatids pulled apart
Telophase - 2 new nuclear membranes form
All autosomal cells (non-gametes) carry out mitosis to produce 2 genetically identical daughter cells each with a full compliment of chromosomes and half of the cytoplasm/ organelles of the original cell. Over the G1 growth period these aspects are increased.
Spindle fibres - microtubules made of tubulin
Positive end attaches to the centromeres and the negative end connects them to the centrosomes. In anaphase they contract pulling the sister chromatids apart. The spindles then disintegrate (reforming in the next cell cycle)
Difference between homologous chromosomes and sister chromatids
Homologous chromosomes: are alike in size and structure and carry information for the same characteristics but are not identical
Sister chromatids: arise from DNA replication during the S phase of the cell cycle, therefore, they are identical in DNA content and separated in anaphase
No. of chromosomes per cell/ No. of DNA molecules per cell
G1 4/4
S 4/4-8
G2 4/8
prophase and prometaphase: 4/8
metaphase 4/8
anaphase 8/8
telophase and cytokinesis 4/4 (2 cells formed)
Meiosis
the production of haploid gametes for sexual reproduction
fertilisation is the fusion of compatible haploid gametes to form a diploid zygote.
genetic variation is a consequence of meiosis
Meiosis steps
Interphase - DNA synthesis and chromosome replication phase
Meiosis l - separation of homologous chromosome pairs and reduction of chromosome number by half
Meiosis ll - separation of sister chromatids also known as equational division (similar to mitosis)
outcome: haploid cells with just one chromosome for each of the 23 pairs - must fuse to a complimentary gamete to form a diploid zygote
Meiosis l
Prophase 1
-synapsis - close pairing of homologous chromosomes
-tetrad- closely associated 4 sister chromatids of two homologous chromosomes
crossing over: crossing over of chromosome segments from the sister chromatid of one chromosome to the sister chromatid of the other synapsed chromosome
- exchange of genetic material - the first mechanism in generation of genetic variation in newly formed gametes
Metaphase 1
random alignment of homologous chromosome pairs along the metaphase plate (midline of the cell)
Anaphase 1
Separation of homologous chromosome pairs and the random distribution of chromosomes into two newly divided cells - the second mechanism in generation of genetic variation in newly formed gametes
Telophase 1
Interkinesis
