# The First Cells and Endosymbiosis

Biology I · Life, Its Origin and Its Diversity · https://tryals.app/learn/biology-i/the-first-cells-and-endosymbiosis

## Two Domains, Then a Third

The first organisms were **prokaryotes**: no nucleus, no membrane-bound organelles, DNA in a single circular chromosome. They fall into two domains that are genuinely distinct, **Bacteria** and **Archaea**, differing in membrane lipids, cell-wall chemistry and transcription machinery. Archaea are, in several respects, closer to eukaryotes than to bacteria.

Prokaryotes reproduce by **binary fission**, an efficient doubling: one cell becomes 2, then 4, then 8. After $n$ divisions a single cell has become $2^n$, so 20 divisions produce over a million. That exponential is why bacterial populations can respond to opportunity so fast.

They also exchange genes **horizontally** — by conjugation, transformation or transduction — rather than only passing them down. This is why antibiotic resistance can spread between unrelated species, and why the early tree of life is better drawn as a web than as a branching tree.

**Eukaryotes** appear later and are far larger and more complex. The **endosymbiotic theory**, established by Lynn Margulis, explains their organelles: mitochondria and chloroplasts descend from free-living bacteria engulfed and retained rather than digested. The evidence is strong and independent:

- Both have **double membranes**, as expected from engulfment
- Both retain their **own circular DNA**
- Both have **70S ribosomes**, the bacterial type, not the eukaryotic 80S
- Both divide by binary fission, independently of the host cell
- Both are inhibited by antibiotics that target bacteria

Endosymbionts lose most of their genome over time, transferring genes to the host nucleus. A free-living bacterium carries a few thousand genes; a modern mitochondrion retains only around 37.

**Viruses** sit outside all of this. They have genetic material and a protein coat but no metabolism and no ribosomes, and they replicate only inside a host cell, which is why whether they are alive at all remains genuinely contested.

> **Common pitfall:** treating the double membrane as proof on its own. It is suggestive, but the case rests on the *convergence* of independent lines, bacterial ribosomes, circular DNA, independent division and antibiotic sensitivity all pointing the same way.

## Practice questions

7 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. An endosymbiont surrenders most of its genome, which is why it can no longer live alone. A free-living bacterium carries about 3000 genes and a modern mitochondrion about 37. Set the percentage that has been lost or transferred.

**Answer:** 98.8 (within ±0.6)

**Why:** Only $37/3000 = 1.2\%$ of the genes remain, so about **98.8 %** have been lost or moved to the nucleus. An endosymbiont surrenders nearly its whole genome, which is why a mitochondrion can no longer live alone.

Page: https://tryals.app/practice/biology-i/the-first-cells-and-endosymbiosis/an-endosymbiont-surrenders-most-of-its-genome-which-is-why-it-can-no

### 2. Mitochondria contain their own DNA and divide independently of the cell that hosts them.

**Answer:** True

**Why:** True, mitochondria keep a small circular genome and divide by fission on their own schedule, both hallmarks of their bacterial ancestry. It is also why mitochondrial DNA is inherited maternally and used to trace lineages.

Page: https://tryals.app/practice/biology-i/the-first-cells-and-endosymbiosis/mitochondria-contain-their-own-dna-and-divide-independently-of-the

### 3. Match each piece of evidence for endosymbiosis to what it indicates.

**Answer:**

- Double membrane → Consistent with engulfment by a host cell
- Circular DNA → The chromosome form used by bacteria
- 70S ribosomes → The bacterial ribosome type, not the eukaryotic one
- Sensitivity to antibacterial antibiotics → Machinery still resembling a bacterium’s

**Why:** Each observation on its own admits alternative explanations; together they converge on one conclusion. That convergence is what turned endosymbiosis from a speculation into the standard account.

Page: https://tryals.app/practice/biology-i/the-first-cells-and-endosymbiosis/match-each-piece-of-evidence-for-endosymbiosis-to-what-it-indicates

### 4. Which statements about viruses are correct?

A. They contain genetic material and a protein coat
B. They replicate only inside a host cell
C. They reproduce by binary fission
D. They lack ribosomes of their own

**Answer:** A. They contain genetic material and a protein coat; B. They replicate only inside a host cell; D. They lack ribosomes of their own

**Why:** Viruses have genes and a coat but no ribosomes and no metabolism, so they can only replicate by commandeering a host. Binary fission is prokaryotic reproduction and requires machinery viruses simply lack.

Page: https://tryals.app/practice/biology-i/the-first-cells-and-endosymbiosis/which-statements-about-viruses-are-correct

### 5. Complete the account of how genes move between prokaryotes.

**Answer:** Prokaryotes pass genes to their offspring **vertically**, but they also exchange them **horizontally** between unrelated cells. This is why **antibiotic** resistance can spread across species boundaries, and why the earliest part of the tree of life is better drawn as a **web**.

**Why:** Vertical transfer builds a tree; horizontal transfer draws lines across it, producing a web. This is not a minor complication, it is why antibiotic resistance genes travel so readily between species that never interbreed.

Page: https://tryals.app/practice/biology-i/the-first-cells-and-endosymbiosis/complete-the-account-of-how-genes-move-between-prokaryotes

### 6. Early phylogenies assumed inheritance was strictly vertical, yet horizontal gene transfer is pervasive among prokaryotes. What does this distinction imply for reconstructing the earliest evolutionary relationships?

A. Early lineages cannot be classified into distinct domains like Bacteria and Archaea
B. Gene trees for different loci will frequently conflict rather than trace one trunk
C. Vertical descent became significant only after the evolution of eukaryotic lineages
D. Organismal lineages merged continuously until all biochemical differences vanished

**Answer:** B. Gene trees for different loci will frequently conflict rather than trace one trunk

**Why:** Because horizontal gene transfer moves sequences between unrelated contemporary lineages, different genes tell divergent evolutionary histories. This yields a reticulated web rather than a simple bifurcating tree, though fundamental domain biochemistry remains distinct.

Page: https://tryals.app/practice/biology-i/the-first-cells-and-endosymbiosis/early-phylogenies-assumed-inheritance-was-strictly-vertical-yet

### 7. Arrange these events in the order they occurred in the history of life.

**Answer:**

1. The first prokaryotic cells appear
2. Photosynthesis begins to oxygenate the atmosphere
3. A bacterium is engulfed and becomes the mitochondrion
4. A photosynthetic bacterium is engulfed and becomes the chloroplast

**Why:** Prokaryotes come first, then oxygenic photosynthesis transforms the atmosphere, then the mitochondrial endosymbiosis, and only later the chloroplast. The order is fixed by the observation that every chloroplast-bearing cell also has mitochondria, but not the reverse.

Page: https://tryals.app/practice/biology-i/the-first-cells-and-endosymbiosis/arrange-these-events-in-the-order-they-occurred-in-the-history-of