MGD S4 - Inheritance of Genes Flashcards
Outline the cell cycle
See diagram
Give a brief overview of DNA replication
Double helix separates and each strand is a template for the new copy. Catalysed by DNA polymerase. Extend 3’ end of existing DNA. (dNMP)n primer strand + dNTP nucleotide ➡️ (dNMP)n+1 + PPi - Stepwise reaction driven by pyrophosphate hydrolysis - Chain growth is directional, from 5’ to 3’
Briefly outline the stages of DNA replication in prokaryotes
Circular “naked” chromosome: - Initiation - Elongation - Termination
Describe the process of initiation in prokaryotic DNA replication
- Recognition of origin of replication - Requires recruitment of DNA polymerase plus other specific proteins - Requires a “kick-start” by primase (2/3 RNA nucleotides) as DNA polymerase can only extend a 3’ end - Template 3’ to 5’ to extend 5’ to 3’. DNA polymerase reads in opposite direction
Describe the process of elongation in prokaryotic DNA replication
- Moving replication forks - Helicase unwinds double helix bit by bit - DNA polymerase extends 3’ ends only: leading to leading strand (continuous), lagging strand (discontinuous) and Okazaki fragments - DNA ligament joins fragments
What is the difference between the leading and lagging strands?
- Lagging strand: made in small parts (discontinuous) - Leading strand: made continuously
Describe the process of termination in prokaryotic DNA replication
- When 2 facing replication forks meet and DNA ligase joins final fragments - Chromosome number stays the same - One replicated chromosome consists of two sister chromatids (maternal copy - non-sister chromatids from paternal copy)
SECOND DIAGRAM - see slide 18
What is the product of DNA replication in prokaryotes?
2 new double stranded antiparallel DNA molecules
Describe some features of DNA replication in eukaryotes
- Double stranded so 5’ and 3’ on both ends - Can start at different positions - DNA replication forks in both directions - One chromosome = one DNA molecule - After replication: one replicated chromosome = 2 identical DNA molecules (2 chromatids) - Takes human cell ~8 hours to replicate all DNA
Describe chromosome structure
The classical X-shape is a replicated chromosome containing two identical DNA molecules
Give an overview of mitosis
- Cell division for somatic cells - Production of two identical daughter cells (with same chromosome content as parental cell) - ~50 mitotic rounds during development - Mitotic growth necessary for some tissue (epidermis, mucosae, bone marrow, spermatogonia)
Describe prophase
- Spindle fibres appear - Chromosomes condense - Nuclear membrane disintegrates
Describe prometaphase
- Spindle fibres attach to chromosome - Chromosomes condense - Spindle fibres connected to centromere by kinetochore
Describe metaphase
- Chromosomes align - Chromosomes line up randomly in middle of cell (at metaphase plate)
Describe anaphase
- Centromeres divide - Chromosomes split - Sister chromatids move to opposite poles
Describe telophase
- Nuclear membrane reforms - Chromosomes decondense - Spindle fibres disappear
Describe cytokinesis
- Cytoplasm divides - Parent cell becomes two daughter cells with identical genetic information
Give an overview of meiosis
- Special division for germ line cells - Production of four non-identical cells (gametes) with half chromosome content of parental cell - One round of replication followed by two rounds of division: meiosis I and II - Diploid (2n: 46 chromosomes) reduced to haploid (n: 23 chromosomes) - Production of eggs and sperm
Image=meiosis I
Describe prophase I
Disintegration of nuclear membrane
Describe metaphase I
- Chromosomes line up in pairs on metaphase plate - Crossing over occurs
Describe crossing over
- If it doesn’t occur meiosis goes wrong - Occurs between sister chromatids too but has no effect as they’re identical - 1 complete form of each maternal and paternal chromosome
Describe anaphase I
Each replicated chromosome goes to either end of the cell. Still random lining up: random assortment of chromosomes. Whole replicated chromosome of each goes into daughter cell
Describe telophase I
Nuclear membrane reforms
Describe cytokinesis I
Cell divides into two daughter cells
Which stage of meiosis is similar to mitosis?
Meiosis II
Describe prophase II
Nuclear membrane disintegrates
Describe metaphase II
(Still crossed over) random line up
Describe anaphase II
Chromosomes split
Describe telophase II
Nuclear membrane reforms
Describe cytokinesis II
Cells divide into 4 haploid daughter cells
What is the difference between meiosis I and meiosis II?
- Meiosis I: homologous pairs line up and chromosomes separate - Meiosis II: chromosomes line up and chromatids separate. As soon as chromatids separate, they’re called chromosomes again
What are the consequences of meiosis?
- Maintaining constant chromosome number from generation to generation - Generation of genetic diversity: 1. Random assortment of chromosome 2. Crossing over of genetic material
What is spermatogenesis?
- Cell division to produce male gametes - 1 spermatocyte (2n) ➡️ 4 sperm (n) - Length of spermatogenesis is ~48 days
What is oogenesis?
- Cell division to produce female gametes - 1 oocyte (2n) ➡️ 1 egg (n) + 3 polar bodies (discarded) - Length of oogenesis is 12-50 years - All oocytes already in foetus, arrested in meiosis I - Period - 1 oocyte goes through meiosis to metaphase II - Old mothers: DNA can be damaged - Meiosis only complete at fertilisation
What happens if crossing over does not occur?
See image
Describe mistakes in meiosis and their consequences
- Humans are not very good at meiosis. Missegregation: 30 in 100 - Consequences: a third of all identified (they are many more) miscarriages, infertility, leading cause of mental retardation
Contrast mitosis and meiosis
See image
What are the environmental factors that affect genotype and phenotype?
- Radiation - Mutagens - Chemicals that affect cell growth - Diet - Lifestyle - etc
What creates genetic variation?
Mutation Meiosis: - Independent assortment of chromosomes (2^n possibilities, n is the number of chromosome pairs) - Crossing-over of genetic material
Give a brief overview of inheritance
- Each human has 25,000 genes - Each individual has 2 copies of each gene - Each individual has 2 alleles of a gene - There are many alleles of a gene within a population
Define homozygous
Two alleles of a gene are different Individual is a homozygote
Define heterozygous
Two alleles of a gene are different Individual is a heterozygote
Define hemizygous
Only one alleles of a gene on the X chromosome - males only
Define dominant
The dominant allele in a heterozygote determines the phenotype
Define recessive
The non-dominant allele in a heterozygote
Give a flow diagram to help identify type of inheritance
See image
What are the different symbols used when drawing a pedigree?
See image
Describe autosomal recessive diseases
- Heterozygotes unaffected - Males and females equally affected - Two heterozygotes have 25% chance of having affected offspring - Two homozygous individuals will have affected offspring only - Disease seems to come out of nowhere; can skip generations - Both parents of affected individual must be at least heterozygous carriers - For example: Cystic Fibrosis
Describe autosomal dominant diseases
- Heterozygotes affected - Males and females equally affected - Disease rarely found in homozygous state - Every affected individual has 50% chance of having affected offspring - Every affected individual will have at least one affected parent - Disease cannot skip generations - For example: Huntington’s disease
Describe X-linked recessive diseases
- Hemizygous males and homozygous females affected - Disease more common in males - Heterozygous female carrier had 50% chance of having affected sons - Affected males cannot give trait to son - Daughters of affected males are heterozygous - Every affected male will have at least a heterozygous carrier mother - Every affected female will have at least a carrier mother and an affected father - For example: Haemophilia A
Give an example of codominance
Human ABO blood groups - Human isoglutamin gene codes for glycoproteins on the surface of red blood cells - Gene I: 3 alleles A, B, O or Ia, Ib, Io - Alleles A and B are dominant over allele O - Neither A or B is dominant over the other; they are codominant - Blood Group A: IaIa/IaIo - Blood Group B: IbIb/IbIo - Blood Group AB: IaIb - Blood Group O: IoIo
Give an example of complementation
Albinism Recessive Two genes: A1 and A2 Alleles: A1, a1 and A2, a2
Why does complementation occur?
More than one gene can be involved in producing a phenotype
Describe linkage and recombination
- Genes on the same chromosome are “linked” - Genes on different chromosomes are “not linked” - Linked genes do not show independent assortment at meiosis - Recombination frequency between two linked genes is dependent on the distance between the genes - Genes close together are “tightly linked” - Genes far apart on the same chromosome almost behave as unlinked genes
Give an overview of meiosis II
See image
